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Merge tag 'docs-4.11' of git://git.lwn.net/linux

Browse Source 
Pull documentation updates from Jonathan Corbet:
 "A slightly quieter cycle for documentation this time around.

  Three more DocBook template files have been converted to RST; only 21
  to go. There are various build improvements and the usual array of
  documentation improvements and fixes"

* tag 'docs-4.11' of git://git.lwn.net/linux: (44 commits)
  docs / driver-api: Fix structure references in device_link.rst
  PM / docs: Fix structure references in device.rst
  Add a target to check broken external links in the Documentation
  Documentation: Fix linux-api list typo
  Documentation: DocBook/Makefile comment typo
  Improve sparse documentation
  Documentation: make Makefile.sphinx no-ops quieter
  Documentation: DMA-ISA-LPC.txt
  Documentation: input: fix path to input code definitions
  docs: Remove the copyright year from conf.py
  docs: Fix a warning in the Korean HOWTO.rst translation
  PM / sleep / docs: Convert PM notifiers document to reST
  PM / core / docs: Convert sleep states API document to reST
  PM / core: Update kerneldoc comments in pm.h
  doc-rst: Fix recursive make invocation from macros
  doc-rst: Delete output of failed dot-SVG conversion
  doc-rst: Break shell command sequences on failure
  Documentation/sphinx: make targets independent of Sphinx work for HAVE_SPHINX=0
  doc-rst: fixed cleandoc target when used with O=dir
  Documentation/sphinx: prevent generation of .pyc files in the source tree
  ...
This commit is contained in:
Linus Torvalds
2017-02-22 18:51:29 -08:00
parent fd7e9a8834 bd8562626c
commit c1aac62f36
56 changed files with 3288 additions and 3510 deletions
Download Patch File Download Diff File Expand all files Collapse all files
Documentation/DMA-ISA-LPC.txt
+1 -1
View File
@@ -42,7 +42,7 @@ requirements you pass the flag GFP_DMA to kmalloc.
Unfortunately the memory available for ISA DMA is scarce so unless you
allocate the memory during boot-up it's a good idea to also pass
__GFP_REPEAT and __GFP_NOWARN to make the allocater try a bit harder.
__GFP_REPEAT and __GFP_NOWARN to make the allocator try a bit harder.
(This scarcity also means that you should allocate the buffer as
early as possible and not release it until the driver is unloaded.)
Documentation/DocBook/Makefile
+3 -2
View File
@@ -13,7 +13,7 @@ DOCBOOKS := z8530book.xml \
gadget.xml libata.xml mtdnand.xml librs.xml rapidio.xml \
genericirq.xml s390-drivers.xml scsi.xml \
sh.xml regulator.xml w1.xml \
writing_musb_glue_layer.xml iio.xml
writing_musb_glue_layer.xml
ifeq ($(DOCBOOKS),)
@@ -71,6 +71,7 @@ installmandocs: mandocs
# no-op for the DocBook toolchain
epubdocs:
latexdocs:
linkcheckdocs:
###
#External programs used
@@ -272,6 +273,6 @@ cleandocs:
$(Q)rm -rf $(call objectify, $(clean-dirs))
# Declare the contents of the .PHONY variable as phony. We keep that
# information in a variable se we can use it in if_changed and friends.
# information in a variable so we can use it in if_changed and friends.
.PHONY: $(PHONY)
Documentation/DocBook/deviceiobook.tmpl
-323
View File
@@ -1,323 +0,0 @@
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
<book id="DoingIO">
<bookinfo>
<title>Bus-Independent Device Accesses</title>
<authorgroup>
<author>
<firstname>Matthew</firstname>
<surname>Wilcox</surname>
<affiliation>
<address>
<email>matthew@wil.cx</email>
</address>
</affiliation>
</author>
</authorgroup>
<authorgroup>
<author>
<firstname>Alan</firstname>
<surname>Cox</surname>
<affiliation>
<address>
<email>alan@lxorguk.ukuu.org.uk</email>
</address>
</affiliation>
</author>
</authorgroup>
<copyright>
<year>2001</year>
<holder>Matthew Wilcox</holder>
</copyright>
<legalnotice>
<para>
This documentation is free software; you can redistribute
it and/or modify it under the terms of the GNU General Public
License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later
version.
</para>
<para>
This program is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the implied
warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.
</para>
<para>
You should have received a copy of the GNU General Public
License along with this program; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
MA 02111-1307 USA
</para>
<para>
For more details see the file COPYING in the source
distribution of Linux.
</para>
</legalnotice>
</bookinfo>
<toc></toc>
<chapter id="intro">
<title>Introduction</title>
<para>
Linux provides an API which abstracts performing IO across all busses
and devices, allowing device drivers to be written independently of
bus type.
</para>
</chapter>
<chapter id="bugs">
<title>Known Bugs And Assumptions</title>
<para>
None.
</para>
</chapter>
<chapter id="mmio">
<title>Memory Mapped IO</title>
<sect1 id="getting_access_to_the_device">
<title>Getting Access to the Device</title>
<para>
The most widely supported form of IO is memory mapped IO.
That is, a part of the CPU's address space is interpreted
not as accesses to memory, but as accesses to a device. Some
architectures define devices to be at a fixed address, but most
have some method of discovering devices. The PCI bus walk is a
good example of such a scheme. This document does not cover how
to receive such an address, but assumes you are starting with one.
Physical addresses are of type unsigned long.
</para>
<para>
This address should not be used directly. Instead, to get an
address suitable for passing to the accessor functions described
below, you should call <function>ioremap</function>.
An address suitable for accessing the device will be returned to you.
</para>
<para>
After you've finished using the device (say, in your module's
exit routine), call <function>iounmap</function> in order to return
the address space to the kernel. Most architectures allocate new
address space each time you call <function>ioremap</function>, and
they can run out unless you call <function>iounmap</function>.
</para>
</sect1>
<sect1 id="accessing_the_device">
<title>Accessing the device</title>
<para>
The part of the interface most used by drivers is reading and
writing memory-mapped registers on the device. Linux provides
interfaces to read and write 8-bit, 16-bit, 32-bit and 64-bit
quantities. Due to a historical accident, these are named byte,
word, long and quad accesses. Both read and write accesses are
supported; there is no prefetch support at this time.
</para>
<para>
The functions are named <function>readb</function>,
<function>readw</function>, <function>readl</function>,
<function>readq</function>, <function>readb_relaxed</function>,
<function>readw_relaxed</function>, <function>readl_relaxed</function>,
<function>readq_relaxed</function>, <function>writeb</function>,
<function>writew</function>, <function>writel</function> and
<function>writeq</function>.
</para>
<para>
Some devices (such as framebuffers) would like to use larger
transfers than 8 bytes at a time. For these devices, the
<function>memcpy_toio</function>, <function>memcpy_fromio</function>
and <function>memset_io</function> functions are provided.
Do not use memset or memcpy on IO addresses; they
are not guaranteed to copy data in order.
</para>
<para>
The read and write functions are defined to be ordered. That is the
compiler is not permitted to reorder the I/O sequence. When the
ordering can be compiler optimised, you can use <function>
__readb</function> and friends to indicate the relaxed ordering. Use
this with care.
</para>
<para>
While the basic functions are defined to be synchronous with respect
to each other and ordered with respect to each other the busses the
devices sit on may themselves have asynchronicity. In particular many
authors are burned by the fact that PCI bus writes are posted
asynchronously. A driver author must issue a read from the same
device to ensure that writes have occurred in the specific cases the
author cares. This kind of property cannot be hidden from driver
writers in the API. In some cases, the read used to flush the device
may be expected to fail (if the card is resetting, for example). In
that case, the read should be done from config space, which is
guaranteed to soft-fail if the card doesn't respond.
</para>
<para>
The following is an example of flushing a write to a device when
the driver would like to ensure the write's effects are visible prior
to continuing execution.
</para>
<programlisting>
static inline void
qla1280_disable_intrs(struct scsi_qla_host *ha)
{
struct device_reg *reg;
reg = ha->iobase;
/* disable risc and host interrupts */
WRT_REG_WORD(&amp;reg->ictrl, 0);
/*
* The following read will ensure that the above write
* has been received by the device before we return from this
* function.
*/
RD_REG_WORD(&amp;reg->ictrl);
ha->flags.ints_enabled = 0;
}
</programlisting>
<para>
In addition to write posting, on some large multiprocessing systems
(e.g. SGI Challenge, Origin and Altix machines) posted writes won't
be strongly ordered coming from different CPUs. Thus it's important
to properly protect parts of your driver that do memory-mapped writes
with locks and use the <function>mmiowb</function> to make sure they
arrive in the order intended. Issuing a regular <function>readX
</function> will also ensure write ordering, but should only be used
when the driver has to be sure that the write has actually arrived
at the device (not that it's simply ordered with respect to other
writes), since a full <function>readX</function> is a relatively
expensive operation.
</para>
<para>
Generally, one should use <function>mmiowb</function> prior to
releasing a spinlock that protects regions using <function>writeb
</function> or similar functions that aren't surrounded by <function>
readb</function> calls, which will ensure ordering and flushing. The
following pseudocode illustrates what might occur if write ordering
isn't guaranteed via <function>mmiowb</function> or one of the
<function>readX</function> functions.
</para>
<programlisting>
CPU A: spin_lock_irqsave(&amp;dev_lock, flags)
CPU A: ...
CPU A: writel(newval, ring_ptr);
CPU A: spin_unlock_irqrestore(&amp;dev_lock, flags)
...
CPU B: spin_lock_irqsave(&amp;dev_lock, flags)
CPU B: writel(newval2, ring_ptr);
CPU B: ...
CPU B: spin_unlock_irqrestore(&amp;dev_lock, flags)
</programlisting>
<para>
In the case above, newval2 could be written to ring_ptr before
newval. Fixing it is easy though:
</para>
<programlisting>
CPU A: spin_lock_irqsave(&amp;dev_lock, flags)
CPU A: ...
CPU A: writel(newval, ring_ptr);
CPU A: mmiowb(); /* ensure no other writes beat us to the device */
CPU A: spin_unlock_irqrestore(&amp;dev_lock, flags)
...
CPU B: spin_lock_irqsave(&amp;dev_lock, flags)
CPU B: writel(newval2, ring_ptr);
CPU B: ...
CPU B: mmiowb();
CPU B: spin_unlock_irqrestore(&amp;dev_lock, flags)
</programlisting>
<para>
See tg3.c for a real world example of how to use <function>mmiowb
</function>
</para>
<para>
PCI ordering rules also guarantee that PIO read responses arrive
after any outstanding DMA writes from that bus, since for some devices
the result of a <function>readb</function> call may signal to the
driver that a DMA transaction is complete. In many cases, however,
the driver may want to indicate that the next
<function>readb</function> call has no relation to any previous DMA
writes performed by the device. The driver can use
<function>readb_relaxed</function> for these cases, although only
some platforms will honor the relaxed semantics. Using the relaxed
read functions will provide significant performance benefits on
platforms that support it. The qla2xxx driver provides examples
of how to use <function>readX_relaxed</function>. In many cases,
a majority of the driver's <function>readX</function> calls can
safely be converted to <function>readX_relaxed</function> calls, since
only a few will indicate or depend on DMA completion.
</para>
</sect1>
</chapter>
<chapter id="port_space_accesses">
<title>Port Space Accesses</title>
<sect1 id="port_space_explained">
<title>Port Space Explained</title>
<para>
Another form of IO commonly supported is Port Space. This is a
range of addresses separate to the normal memory address space.
Access to these addresses is generally not as fast as accesses
to the memory mapped addresses, and it also has a potentially
smaller address space.
</para>
<para>
Unlike memory mapped IO, no preparation is required
to access port space.
</para>
</sect1>
<sect1 id="accessing_port_space">
<title>Accessing Port Space</title>
<para>
Accesses to this space are provided through a set of functions
which allow 8-bit, 16-bit and 32-bit accesses; also
known as byte, word and long. These functions are
<function>inb</function>, <function>inw</function>,
<function>inl</function>, <function>outb</function>,
<function>outw</function> and <function>outl</function>.
</para>
<para>
Some variants are provided for these functions. Some devices
require that accesses to their ports are slowed down. This
functionality is provided by appending a <function>_p</function>
to the end of the function. There are also equivalents to memcpy.
The <function>ins</function> and <function>outs</function>
functions copy bytes, words or longs to the given port.
</para>
</sect1>
</chapter>
<chapter id="pubfunctions">
<title>Public Functions Provided</title>
!Iarch/x86/include/asm/io.h
!Elib/pci_iomap.c
</chapter>
</book>
Documentation/DocBook/iio.tmpl
-697
View File
File diff suppressed because it is too large Load Diff
Documentation/DocBook/regulator.tmpl
-304
View File
@@ -1,304 +0,0 @@
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
<book id="regulator-api">
<bookinfo>
<title>Voltage and current regulator API</title>
<authorgroup>
<author>
<firstname>Liam</firstname>
<surname>Girdwood</surname>
<affiliation>
<address>
<email>lrg@slimlogic.co.uk</email>
</address>
</affiliation>
</author>
<author>
<firstname>Mark</firstname>
<surname>Brown</surname>
<affiliation>
<orgname>Wolfson Microelectronics</orgname>
<address>
<email>broonie@opensource.wolfsonmicro.com</email>
</address>
</affiliation>
</author>
</authorgroup>
<copyright>
<year>2007-2008</year>
<holder>Wolfson Microelectronics</holder>
</copyright>
<copyright>
<year>2008</year>
<holder>Liam Girdwood</holder>
</copyright>
<legalnotice>
<para>
This documentation is free software; you can redistribute
it and/or modify it under the terms of the GNU General Public
License version 2 as published by the Free Software Foundation.
</para>
<para>
This program is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the implied
warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.
</para>
<para>
You should have received a copy of the GNU General Public
License along with this program; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
MA 02111-1307 USA
</para>
<para>
For more details see the file COPYING in the source
distribution of Linux.
</para>
</legalnotice>
</bookinfo>
<toc></toc>
<chapter id="intro">
<title>Introduction</title>
<para>
This framework is designed to provide a standard kernel
interface to control voltage and current regulators.
</para>
<para>
The intention is to allow systems to dynamically control
regulator power output in order to save power and prolong
battery life. This applies to both voltage regulators (where
voltage output is controllable) and current sinks (where current
limit is controllable).
</para>
<para>
Note that additional (and currently more complete) documentation
is available in the Linux kernel source under
<filename>Documentation/power/regulator</filename>.
</para>
<sect1 id="glossary">
<title>Glossary</title>
<para>
The regulator API uses a number of terms which may not be
familiar:
</para>
<glossary>
<glossentry>
<glossterm>Regulator</glossterm>
<glossdef>
<para>
Electronic device that supplies power to other devices. Most
regulators can enable and disable their output and some can also
control their output voltage or current.
</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>Consumer</glossterm>
<glossdef>
<para>
Electronic device which consumes power provided by a regulator.
These may either be static, requiring only a fixed supply, or
dynamic, requiring active management of the regulator at
runtime.
</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>Power Domain</glossterm>
<glossdef>
<para>
The electronic circuit supplied by a given regulator, including
the regulator and all consumer devices. The configuration of
the regulator is shared between all the components in the
circuit.
</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>Power Management Integrated Circuit</glossterm>
<acronym>PMIC</acronym>
<glossdef>
<para>
An IC which contains numerous regulators and often also other
subsystems. In an embedded system the primary PMIC is often
equivalent to a combination of the PSU and southbridge in a
desktop system.
</para>
</glossdef>
</glossentry>
</glossary>
</sect1>
</chapter>
<chapter id="consumer">
<title>Consumer driver interface</title>
<para>
This offers a similar API to the kernel clock framework.
Consumer drivers use <link
linkend='API-regulator-get'>get</link> and <link
linkend='API-regulator-put'>put</link> operations to acquire and
release regulators. Functions are
provided to <link linkend='API-regulator-enable'>enable</link>
and <link linkend='API-regulator-disable'>disable</link> the
regulator and to get and set the runtime parameters of the
regulator.
</para>
<para>
When requesting regulators consumers use symbolic names for their
supplies, such as "Vcc", which are mapped into actual regulator
devices by the machine interface.
</para>
<para>
A stub version of this API is provided when the regulator
framework is not in use in order to minimise the need to use
ifdefs.
</para>
<sect1 id="consumer-enable">
<title>Enabling and disabling</title>
<para>
The regulator API provides reference counted enabling and
disabling of regulators. Consumer devices use the <function><link
linkend='API-regulator-enable'>regulator_enable</link></function>
and <function><link
linkend='API-regulator-disable'>regulator_disable</link>
</function> functions to enable and disable regulators. Calls
to the two functions must be balanced.
</para>
<para>
Note that since multiple consumers may be using a regulator and
machine constraints may not allow the regulator to be disabled
there is no guarantee that calling
<function>regulator_disable</function> will actually cause the
supply provided by the regulator to be disabled. Consumer
drivers should assume that the regulator may be enabled at all
times.
</para>
</sect1>
<sect1 id="consumer-config">
<title>Configuration</title>
<para>
Some consumer devices may need to be able to dynamically
configure their supplies. For example, MMC drivers may need to
select the correct operating voltage for their cards. This may
be done while the regulator is enabled or disabled.
</para>
<para>
The <function><link
linkend='API-regulator-set-voltage'>regulator_set_voltage</link>
</function> and <function><link
linkend='API-regulator-set-current-limit'
>regulator_set_current_limit</link>
</function> functions provide the primary interface for this.
Both take ranges of voltages and currents, supporting drivers
that do not require a specific value (eg, CPU frequency scaling
normally permits the CPU to use a wider range of supply
voltages at lower frequencies but does not require that the
supply voltage be lowered). Where an exact value is required
both minimum and maximum values should be identical.
</para>
</sect1>
<sect1 id="consumer-callback">
<title>Callbacks</title>
<para>
Callbacks may also be <link
linkend='API-regulator-register-notifier'>registered</link>
for events such as regulation failures.
</para>
</sect1>
</chapter>
<chapter id="driver">
<title>Regulator driver interface</title>
<para>
Drivers for regulator chips <link
linkend='API-regulator-register'>register</link> the regulators
with the regulator core, providing operations structures to the
core. A <link
linkend='API-regulator-notifier-call-chain'>notifier</link> interface
allows error conditions to be reported to the core.
</para>
<para>
Registration should be triggered by explicit setup done by the
platform, supplying a <link
linkend='API-struct-regulator-init-data'>struct
regulator_init_data</link> for the regulator containing
<link linkend='machine-constraint'>constraint</link> and
<link linkend='machine-supply'>supply</link> information.
</para>
</chapter>
<chapter id="machine">
<title>Machine interface</title>
<para>
This interface provides a way to define how regulators are
connected to consumers on a given system and what the valid
operating parameters are for the system.
</para>
<sect1 id="machine-supply">
<title>Supplies</title>
<para>
Regulator supplies are specified using <link
linkend='API-struct-regulator-consumer-supply'>struct
regulator_consumer_supply</link>. This is done at
<link linkend='driver'>driver registration
time</link> as part of the machine constraints.
</para>
</sect1>
<sect1 id="machine-constraint">
<title>Constraints</title>
<para>
As well as defining the connections the machine interface
also provides constraints defining the operations that
clients are allowed to perform and the parameters that may be
set. This is required since generally regulator devices will
offer more flexibility than it is safe to use on a given
system, for example supporting higher supply voltages than the
consumers are rated for.
</para>
<para>
This is done at <link linkend='driver'>driver
registration time</link> by providing a <link
linkend='API-struct-regulation-constraints'>struct
regulation_constraints</link>.
</para>
<para>
The constraints may also specify an initial configuration for the
regulator in the constraints, which is particularly useful for
use with static consumers.
</para>
</sect1>
</chapter>
<chapter id="api">
<title>API reference</title>
<para>
Due to limitations of the kernel documentation framework and the
existing layout of the source code the entire regulator API is
documented here.
</para>
!Iinclude/linux/regulator/consumer.h
!Iinclude/linux/regulator/machine.h
!Iinclude/linux/regulator/driver.h
!Edrivers/regulator/core.c
</chapter>
</book>
Documentation/Makefile.sphinx
+23 -11
View File
@@ -43,7 +43,7 @@ ALLSPHINXOPTS = $(KERNELDOC_CONF) $(PAPEROPT_$(PAPER)) $(SPHINXOPTS)
I18NSPHINXOPTS = $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
# commands; the 'cmd' from scripts/Kbuild.include is not *loopable*
loop_cmd = $(echo-cmd) $(cmd_$(1))
loop_cmd = $(echo-cmd) $(cmd_$(1)) || exit;
# $2 sphinx builder e.g. "html"
# $3 name of the build subfolder / e.g. "media", used as:
@@ -54,7 +54,8 @@ loop_cmd = $(echo-cmd) $(cmd_$(1))
# e.g. "media" for the linux-tv book-set at ./Documentation/media
quiet_cmd_sphinx = SPHINX $@ --> file://$(abspath $(BUILDDIR)/$3/$4)
cmd_sphinx = $(MAKE) BUILDDIR=$(abspath $(BUILDDIR)) $(build)=Documentation/media $2;\
cmd_sphinx = $(MAKE) BUILDDIR=$(abspath $(BUILDDIR)) $(build)=Documentation/media $2 && \
PYTHONDONTWRITEBYTECODE=1 \
BUILDDIR=$(abspath $(BUILDDIR)) SPHINX_CONF=$(abspath $(srctree)/$(src)/$5/$(SPHINX_CONF)) \
$(SPHINXBUILD) \
-b $2 \
@@ -63,13 +64,16 @@ quiet_cmd_sphinx = SPHINX $@ --> file://$(abspath $(BUILDDIR)/$3/$4)
-D version=$(KERNELVERSION) -D release=$(KERNELRELEASE) \
$(ALLSPHINXOPTS) \
$(abspath $(srctree)/$(src)/$5) \
$(abspath $(BUILDDIR)/$3/$4);
$(abspath $(BUILDDIR)/$3/$4)
htmldocs:
@$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,html,$(var),,$(var)))
@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,html,$(var),,$(var)))
linkcheckdocs:
@$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,linkcheck,$(var),,$(var)))
latexdocs:
@$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,latex,$(var),latex,$(var)))
@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,latex,$(var),latex,$(var)))
ifeq ($(HAVE_PDFLATEX),0)
@@ -80,27 +84,34 @@ pdfdocs:
else # HAVE_PDFLATEX
pdfdocs: latexdocs
$(foreach var,$(SPHINXDIRS), $(MAKE) PDFLATEX=$(PDFLATEX) LATEXOPTS="$(LATEXOPTS)" -C $(BUILDDIR)/$(var)/latex;)
$(foreach var,$(SPHINXDIRS), $(MAKE) PDFLATEX=$(PDFLATEX) LATEXOPTS="$(LATEXOPTS)" -C $(BUILDDIR)/$(var)/latex || exit;)
endif # HAVE_PDFLATEX
epubdocs:
@$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,epub,$(var),epub,$(var)))
@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,epub,$(var),epub,$(var)))
xmldocs:
@$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,xml,$(var),xml,$(var)))
@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,xml,$(var),xml,$(var)))
endif # HAVE_SPHINX
# The following targets are independent of HAVE_SPHINX, and the rules should
# work or silently pass without Sphinx.
# no-ops for the Sphinx toolchain
sgmldocs:
@:
psdocs:
@:
mandocs:
@:
installmandocs:
@:
cleandocs:
$(Q)rm -rf $(BUILDDIR)
$(Q)$(MAKE) BUILDDIR=$(abspath $(BUILDDIR)) -C Documentation/media clean
endif # HAVE_SPHINX
$(Q)$(MAKE) BUILDDIR=$(abspath $(BUILDDIR)) $(build)=Documentation/media clean
dochelp:
@echo ' Linux kernel internal documentation in different formats (Sphinx):'
@@ -109,6 +120,7 @@ dochelp:
@echo ' pdfdocs - PDF'
@echo ' epubdocs - EPUB'
@echo ' xmldocs - XML'
@echo ' linkcheckdocs - check for broken external links (will connect to external hosts)'
@echo ' cleandocs - clean all generated files'
@echo
@echo ' make SPHINXDIRS="s1 s2" [target] Generate only docs of folder s1, s2'
Documentation/admin-guide/README.rst
+2 -2
View File
@@ -17,7 +17,7 @@ What is Linux?
loading, shared copy-on-write executables, proper memory management,
and multistack networking including IPv4 and IPv6.
It is distributed under the GNU General Public License - see the
It is distributed under the GNU General Public License v2 - see the
accompanying COPYING file for more details.
On what hardware does it run?
@@ -236,7 +236,7 @@ Configuring the kernel
- Having unnecessary drivers will make the kernel bigger, and can
under some circumstances lead to problems: probing for a
nonexistent controller card may confuse your other controllers
nonexistent controller card may confuse your other controllers.
- A kernel with math-emulation compiled in will still use the
coprocessor if one is present: the math emulation will just
Documentation/admin-guide/dynamic-debug-howto.rst
+2 -2
View File
@@ -93,9 +93,9 @@ Command Language Reference
At the lexical level, a command comprises a sequence of words separated
by spaces or tabs. So these are all equivalent::
nullarbor:~ # echo -c 'file svcsock.c line 1603 +p' >
nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
<debugfs>/dynamic_debug/control
nullarbor:~ # echo -c ' file svcsock.c line 1603 +p ' >
nullarbor:~ # echo -n ' file svcsock.c line 1603 +p ' >
<debugfs>/dynamic_debug/control
nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
<debugfs>/dynamic_debug/control
Documentation/block/pr.txt
+1 -1
View File
@@ -90,7 +90,7 @@ and thus removes any access restriction implied by it.
4. IOC_PR_PREEMPT
This ioctl command releases the existing reservation referred to by
old_key and replaces it with a a new reservation of type for the
old_key and replaces it with a new reservation of type for the
reservation key new_key.
Documentation/cgroup-v1/cpusets.txt
+1 -1
View File
@@ -615,7 +615,7 @@ to allocate a page of memory for that task.
If a cpuset has its 'cpuset.cpus' modified, then each task in that cpuset
will have its allowed CPU placement changed immediately. Similarly,
if a task's pid is written to another cpusets 'cpuset.tasks' file, then its
if a task's pid is written to another cpuset's 'tasks' file, then its
allowed CPU placement is changed immediately. If such a task had been
bound to some subset of its cpuset using the sched_setaffinity() call,
the task will be allowed to run on any CPU allowed in its new cpuset,
Documentation/conf.py
+1 -1
View File
@@ -58,7 +58,7 @@ master_doc = 'index'
# General information about the project.
project = 'The Linux Kernel'
copyright = '2016, The kernel development community'
copyright = 'The kernel development community'
author = 'The kernel development community'
# The version info for the project you're documenting, acts as replacement for
Documentation/core-api/cpu_hotplug.rst
+372
View File
@@ -0,0 +1,372 @@
=========================
CPU hotplug in the Kernel
=========================
:Date: December, 2016
:Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Rusty Russell <rusty@rustcorp.com.au>,
Srivatsa Vaddagiri <vatsa@in.ibm.com>,
Ashok Raj <ashok.raj@intel.com>,
Joel Schopp <jschopp@austin.ibm.com>
Introduction
============
Modern advances in system architectures have introduced advanced error
reporting and correction capabilities in processors. There are couple OEMS that
support NUMA hardware which are hot pluggable as well, where physical node
insertion and removal require support for CPU hotplug.
Such advances require CPUs available to a kernel to be removed either for
provisioning reasons, or for RAS purposes to keep an offending CPU off
system execution path. Hence the need for CPU hotplug support in the
Linux kernel.
A more novel use of CPU-hotplug support is its use today in suspend resume
support for SMP. Dual-core and HT support makes even a laptop run SMP kernels
which didn't support these methods.
Command Line Switches
=====================
``maxcpus=n``
Restrict boot time CPUs to *n*. Say if you have fourV CPUs, using
``maxcpus=2`` will only boot two. You can choose to bring the
other CPUs later online.
``nr_cpus=n``
Restrict the total amount CPUs the kernel will support. If the number
supplied here is lower than the number of physically available CPUs than
those CPUs can not be brought online later.
``additional_cpus=n``
Use this to limit hotpluggable CPUs. This option sets
``cpu_possible_mask = cpu_present_mask + additional_cpus``
This option is limited to the IA64 architecture.
``possible_cpus=n``
This option sets ``possible_cpus`` bits in ``cpu_possible_mask``.
This option is limited to the X86 and S390 architecture.
``cede_offline={"off","on"}``
Use this option to disable/enable putting offlined processors to an extended
``H_CEDE`` state on supported pseries platforms. If nothing is specified,
``cede_offline`` is set to "on".
This option is limited to the PowerPC architecture.
``cpu0_hotplug``
Allow to shutdown CPU0.
This option is limited to the X86 architecture.
CPU maps
========
``cpu_possible_mask``
Bitmap of possible CPUs that can ever be available in the
system. This is used to allocate some boot time memory for per_cpu variables
that aren't designed to grow/shrink as CPUs are made available or removed.
Once set during boot time discovery phase, the map is static, i.e no bits
are added or removed anytime. Trimming it accurately for your system needs
upfront can save some boot time memory.
``cpu_online_mask``
Bitmap of all CPUs currently online. Its set in ``__cpu_up()``
after a CPU is available for kernel scheduling and ready to receive
interrupts from devices. Its cleared when a CPU is brought down using
``__cpu_disable()``, before which all OS services including interrupts are
migrated to another target CPU.
``cpu_present_mask``
Bitmap of CPUs currently present in the system. Not all
of them may be online. When physical hotplug is processed by the relevant
subsystem (e.g ACPI) can change and new bit either be added or removed
from the map depending on the event is hot-add/hot-remove. There are currently
no locking rules as of now. Typical usage is to init topology during boot,
at which time hotplug is disabled.
You really don't need to manipulate any of the system CPU maps. They should
be read-only for most use. When setting up per-cpu resources almost always use
``cpu_possible_mask`` or ``for_each_possible_cpu()`` to iterate. To macro
``for_each_cpu()`` can be used to iterate over a custom CPU mask.
Never use anything other than ``cpumask_t`` to represent bitmap of CPUs.
Using CPU hotplug
=================
The kernel option *CONFIG_HOTPLUG_CPU* needs to be enabled. It is currently
available on multiple architectures including ARM, MIPS, PowerPC and X86. The
configuration is done via the sysfs interface: ::
$ ls -lh /sys/devices/system/cpu
total 0
drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu0
drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu1
drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu2
drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu3
drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu4
drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu5
drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu6
drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu7
drwxr-xr-x 2 root root 0 Dec 21 16:33 hotplug
-r--r--r-- 1 root root 4.0K Dec 21 16:33 offline
-r--r--r-- 1 root root 4.0K Dec 21 16:33 online
-r--r--r-- 1 root root 4.0K Dec 21 16:33 possible
-r--r--r-- 1 root root 4.0K Dec 21 16:33 present
The files *offline*, *online*, *possible*, *present* represent the CPU masks.
Each CPU folder contains an *online* file which controls the logical on (1) and
off (0) state. To logically shutdown CPU4: ::
$ echo 0 > /sys/devices/system/cpu/cpu4/online
smpboot: CPU 4 is now offline
Once the CPU is shutdown, it will be removed from */proc/interrupts*,
*/proc/cpuinfo* and should also not be shown visible by the *top* command. To
bring CPU4 back online: ::
$ echo 1 > /sys/devices/system/cpu/cpu4/online
smpboot: Booting Node 0 Processor 4 APIC 0x1
The CPU is usable again. This should work on all CPUs. CPU0 is often special
and excluded from CPU hotplug. On X86 the kernel option
*CONFIG_BOOTPARAM_HOTPLUG_CPU0* has to be enabled in order to be able to
shutdown CPU0. Alternatively the kernel command option *cpu0_hotplug* can be
used. Some known dependencies of CPU0:
* Resume from hibernate/suspend. Hibernate/suspend will fail if CPU0 is offline.
* PIC interrupts. CPU0 can't be removed if a PIC interrupt is detected.
Please let Fenghua Yu <fenghua.yu@intel.com> know if you find any dependencies
on CPU0.
The CPU hotplug coordination
============================
The offline case
----------------
Once a CPU has been logically shutdown the teardown callbacks of registered
hotplug states will be invoked, starting with ``CPUHP_ONLINE`` and terminating
at state ``CPUHP_OFFLINE``. This includes:
* If tasks are frozen due to a suspend operation then *cpuhp_tasks_frozen*
will be set to true.
* All processes are migrated away from this outgoing CPU to new CPUs.
The new CPU is chosen from each process' current cpuset, which may be
a subset of all online CPUs.
* All interrupts targeted to this CPU are migrated to a new CPU
* timers are also migrated to a new CPU
* Once all services are migrated, kernel calls an arch specific routine
``__cpu_disable()`` to perform arch specific cleanup.
Using the hotplug API
---------------------
It is possible to receive notifications once a CPU is offline or onlined. This
might be important to certain drivers which need to perform some kind of setup
or clean up functions based on the number of available CPUs: ::
#include <linux/cpuhotplug.h>
ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "X/Y:online",
Y_online, Y_prepare_down);
*X* is the subsystem and *Y* the particular driver. The *Y_online* callback
will be invoked during registration on all online CPUs. If an error
occurs during the online callback the *Y_prepare_down* callback will be
invoked on all CPUs on which the online callback was previously invoked.
After registration completed, the *Y_online* callback will be invoked
once a CPU is brought online and *Y_prepare_down* will be invoked when a
CPU is shutdown. All resources which were previously allocated in
*Y_online* should be released in *Y_prepare_down*.
The return value *ret* is negative if an error occurred during the
registration process. Otherwise a positive value is returned which
contains the allocated hotplug for dynamically allocated states
(*CPUHP_AP_ONLINE_DYN*). It will return zero for predefined states.
The callback can be remove by invoking ``cpuhp_remove_state()``. In case of a
dynamically allocated state (*CPUHP_AP_ONLINE_DYN*) use the returned state.
During the removal of a hotplug state the teardown callback will be invoked.
Multiple instances
~~~~~~~~~~~~~~~~~~
If a driver has multiple instances and each instance needs to perform the
callback independently then it is likely that a ''multi-state'' should be used.
First a multi-state state needs to be registered: ::
ret = cpuhp_setup_state_multi(CPUHP_AP_ONLINE_DYN, "X/Y:online,
Y_online, Y_prepare_down);
Y_hp_online = ret;
The ``cpuhp_setup_state_multi()`` behaves similar to ``cpuhp_setup_state()``
except it prepares the callbacks for a multi state and does not invoke
the callbacks. This is a one time setup.
Once a new instance is allocated, you need to register this new instance: ::
ret = cpuhp_state_add_instance(Y_hp_online, &d->node);
This function will add this instance to your previously allocated
*Y_hp_online* state and invoke the previously registered callback
(*Y_online*) on all online CPUs. The *node* element is a ``struct
hlist_node`` member of your per-instance data structure.
On removal of the instance: ::
cpuhp_state_remove_instance(Y_hp_online, &d->node)
should be invoked which will invoke the teardown callback on all online
CPUs.
Manual setup
~~~~~~~~~~~~
Usually it is handy to invoke setup and teardown callbacks on registration or
removal of a state because usually the operation needs to performed once a CPU
goes online (offline) and during initial setup (shutdown) of the driver. However
each registration and removal function is also available with a ``_nocalls``
suffix which does not invoke the provided callbacks if the invocation of the
callbacks is not desired. During the manual setup (or teardown) the functions
``get_online_cpus()`` and ``put_online_cpus()`` should be used to inhibit CPU
hotplug operations.
The ordering of the events
--------------------------
The hotplug states are defined in ``include/linux/cpuhotplug.h``:
* The states *CPUHP_OFFLINE* … *CPUHP_AP_OFFLINE* are invoked before the
CPU is up.
* The states *CPUHP_AP_OFFLINE* … *CPUHP_AP_ONLINE* are invoked
just the after the CPU has been brought up. The interrupts are off and
the scheduler is not yet active on this CPU. Starting with *CPUHP_AP_OFFLINE*
the callbacks are invoked on the target CPU.
* The states between *CPUHP_AP_ONLINE_DYN* and *CPUHP_AP_ONLINE_DYN_END* are
reserved for the dynamic allocation.
* The states are invoked in the reverse order on CPU shutdown starting with
*CPUHP_ONLINE* and stopping at *CPUHP_OFFLINE*. Here the callbacks are
invoked on the CPU that will be shutdown until *CPUHP_AP_OFFLINE*.
A dynamically allocated state via *CPUHP_AP_ONLINE_DYN* is often enough.
However if an earlier invocation during the bring up or shutdown is required
then an explicit state should be acquired. An explicit state might also be
required if the hotplug event requires specific ordering in respect to
another hotplug event.
Testing of hotplug states
=========================
One way to verify whether a custom state is working as expected or not is to
shutdown a CPU and then put it online again. It is also possible to put the CPU
to certain state (for instance *CPUHP_AP_ONLINE*) and then go back to
*CPUHP_ONLINE*. This would simulate an error one state after *CPUHP_AP_ONLINE*
which would lead to rollback to the online state.
All registered states are enumerated in ``/sys/devices/system/cpu/hotplug/states``: ::
$ tail /sys/devices/system/cpu/hotplug/states
138: mm/vmscan:online
139: mm/vmstat:online
140: lib/percpu_cnt:online
141: acpi/cpu-drv:online
142: base/cacheinfo:online
143: virtio/net:online
144: x86/mce:online
145: printk:online
168: sched:active
169: online
To rollback CPU4 to ``lib/percpu_cnt:online`` and back online just issue: ::
$ cat /sys/devices/system/cpu/cpu4/hotplug/state
169
$ echo 140 > /sys/devices/system/cpu/cpu4/hotplug/target
$ cat /sys/devices/system/cpu/cpu4/hotplug/state
140
It is important to note that the teardown callbac of state 140 have been
invoked. And now get back online: ::
$ echo 169 > /sys/devices/system/cpu/cpu4/hotplug/target
$ cat /sys/devices/system/cpu/cpu4/hotplug/state
169
With trace events enabled, the individual steps are visible, too: ::
# TASK-PID CPU# TIMESTAMP FUNCTION
# | | | | |
bash-394 [001] 22.976: cpuhp_enter: cpu: 0004 target: 140 step: 169 (cpuhp_kick_ap_work)
cpuhp/4-31 [004] 22.977: cpuhp_enter: cpu: 0004 target: 140 step: 168 (sched_cpu_deactivate)
cpuhp/4-31 [004] 22.990: cpuhp_exit: cpu: 0004 state: 168 step: 168 ret: 0
cpuhp/4-31 [004] 22.991: cpuhp_enter: cpu: 0004 target: 140 step: 144 (mce_cpu_pre_down)
cpuhp/4-31 [004] 22.992: cpuhp_exit: cpu: 0004 state: 144 step: 144 ret: 0
cpuhp/4-31 [004] 22.993: cpuhp_multi_enter: cpu: 0004 target: 140 step: 143 (virtnet_cpu_down_prep)
cpuhp/4-31 [004] 22.994: cpuhp_exit: cpu: 0004 state: 143 step: 143 ret: 0
cpuhp/4-31 [004] 22.995: cpuhp_enter: cpu: 0004 target: 140 step: 142 (cacheinfo_cpu_pre_down)
cpuhp/4-31 [004] 22.996: cpuhp_exit: cpu: 0004 state: 142 step: 142 ret: 0
bash-394 [001] 22.997: cpuhp_exit: cpu: 0004 state: 140 step: 169 ret: 0
bash-394 [005] 95.540: cpuhp_enter: cpu: 0004 target: 169 step: 140 (cpuhp_kick_ap_work)
cpuhp/4-31 [004] 95.541: cpuhp_enter: cpu: 0004 target: 169 step: 141 (acpi_soft_cpu_online)
cpuhp/4-31 [004] 95.542: cpuhp_exit: cpu: 0004 state: 141 step: 141 ret: 0
cpuhp/4-31 [004] 95.543: cpuhp_enter: cpu: 0004 target: 169 step: 142 (cacheinfo_cpu_online)
cpuhp/4-31 [004] 95.544: cpuhp_exit: cpu: 0004 state: 142 step: 142 ret: 0
cpuhp/4-31 [004] 95.545: cpuhp_multi_enter: cpu: 0004 target: 169 step: 143 (virtnet_cpu_online)
cpuhp/4-31 [004] 95.546: cpuhp_exit: cpu: 0004 state: 143 step: 143 ret: 0
cpuhp/4-31 [004] 95.547: cpuhp_enter: cpu: 0004 target: 169 step: 144 (mce_cpu_online)
cpuhp/4-31 [004] 95.548: cpuhp_exit: cpu: 0004 state: 144 step: 144 ret: 0
cpuhp/4-31 [004] 95.549: cpuhp_enter: cpu: 0004 target: 169 step: 145 (console_cpu_notify)
cpuhp/4-31 [004] 95.550: cpuhp_exit: cpu: 0004 state: 145 step: 145 ret: 0
cpuhp/4-31 [004] 95.551: cpuhp_enter: cpu: 0004 target: 169 step: 168 (sched_cpu_activate)
cpuhp/4-31 [004] 95.552: cpuhp_exit: cpu: 0004 state: 168 step: 168 ret: 0
bash-394 [005] 95.553: cpuhp_exit: cpu: 0004 state: 169 step: 140 ret: 0
As it an be seen, CPU4 went down until timestamp 22.996 and then back up until
95.552. All invoked callbacks including their return codes are visible in the
trace.
Architecture's requirements
===========================
The following functions and configurations are required:
``CONFIG_HOTPLUG_CPU``
This entry needs to be enabled in Kconfig
``__cpu_up()``
Arch interface to bring up a CPU
``__cpu_disable()``
Arch interface to shutdown a CPU, no more interrupts can be handled by the
kernel after the routine returns. This includes the shutdown of the timer.
``__cpu_die()``
This actually supposed to ensure death of the CPU. Actually look at some
example code in other arch that implement CPU hotplug. The processor is taken
down from the ``idle()`` loop for that specific architecture. ``__cpu_die()``
typically waits for some per_cpu state to be set, to ensure the processor dead
routine is called to be sure positively.
User Space Notification
=======================
After CPU successfully onlined or offline udev events are sent. A udev rule like: ::
SUBSYSTEM=="cpu", DRIVERS=="processor", DEVPATH=="/devices/system/cpu/*", RUN+="the_hotplug_receiver.sh"
will receive all events. A script like: ::
#!/bin/sh
if [ "${ACTION}" = "offline" ]
then
echo "CPU ${DEVPATH##*/} offline"
elif [ "${ACTION}" = "online" ]
then
echo "CPU ${DEVPATH##*/} online"
fi
can process the event further.
Kernel Inline Documentations Reference
======================================
.. kernel-doc:: include/linux/cpuhotplug.h
Documentation/core-api/index.rst
+1
View File
@@ -13,6 +13,7 @@ Core utilities
assoc_array
atomic_ops
cpu_hotplug
local_ops
workqueue
Documentation/cpu-freq/user-guide.txt
+3 -1
View File
@@ -82,7 +82,9 @@ UltraSPARC-III
-------
Several "PowerBook" and "iBook2" notebooks are supported.
The following POWER processors are supported in powernv mode:
POWER8
POWER9
1.5 SuperH
----------
Documentation/cpu-hotplug.txt
-452
View File
@@ -1,452 +0,0 @@
CPU hotplug Support in Linux(tm) Kernel
Maintainers:
CPU Hotplug Core:
Rusty Russell <rusty@rustcorp.com.au>
Srivatsa Vaddagiri <vatsa@in.ibm.com>
i386:
Zwane Mwaikambo <zwanem@gmail.com>
ppc64:
Nathan Lynch <nathanl@austin.ibm.com>
Joel Schopp <jschopp@austin.ibm.com>
ia64/x86_64:
Ashok Raj <ashok.raj@intel.com>
s390:
Heiko Carstens <heiko.carstens@de.ibm.com>
Authors: Ashok Raj <ashok.raj@intel.com>
Lots of feedback: Nathan Lynch <nathanl@austin.ibm.com>,
Joel Schopp <jschopp@austin.ibm.com>
Introduction
Modern advances in system architectures have introduced advanced error
reporting and correction capabilities in processors. CPU architectures permit
partitioning support, where compute resources of a single CPU could be made
available to virtual machine environments. There are couple OEMS that
support NUMA hardware which are hot pluggable as well, where physical
node insertion and removal require support for CPU hotplug.
Such advances require CPUs available to a kernel to be removed either for
provisioning reasons, or for RAS purposes to keep an offending CPU off
system execution path. Hence the need for CPU hotplug support in the
Linux kernel.
A more novel use of CPU-hotplug support is its use today in suspend
resume support for SMP. Dual-core and HT support makes even
a laptop run SMP kernels which didn't support these methods. SMP support
for suspend/resume is a work in progress.
General Stuff about CPU Hotplug
--------------------------------
Command Line Switches
---------------------
maxcpus=n Restrict boot time cpus to n. Say if you have 4 cpus, using
maxcpus=2 will only boot 2. You can choose to bring the
other cpus later online, read FAQ's for more info.
additional_cpus=n (*) Use this to limit hotpluggable cpus. This option sets
cpu_possible_mask = cpu_present_mask + additional_cpus
cede_offline={"off","on"} Use this option to disable/enable putting offlined
processors to an extended H_CEDE state on
supported pseries platforms.
If nothing is specified,
cede_offline is set to "on".
(*) Option valid only for following architectures
- ia64
ia64 uses the number of disabled local apics in ACPI tables MADT to
determine the number of potentially hot-pluggable cpus. The implementation
should only rely on this to count the # of cpus, but *MUST* not rely
on the apicid values in those tables for disabled apics. In the event
BIOS doesn't mark such hot-pluggable cpus as disabled entries, one could
use this parameter "additional_cpus=x" to represent those cpus in the
cpu_possible_mask.
possible_cpus=n [s390,x86_64] use this to set hotpluggable cpus.
This option sets possible_cpus bits in
cpu_possible_mask. Thus keeping the numbers of bits set
constant even if the machine gets rebooted.
CPU maps and such
-----------------
[More on cpumaps and primitive to manipulate, please check
include/linux/cpumask.h that has more descriptive text.]
cpu_possible_mask: Bitmap of possible CPUs that can ever be available in the
system. This is used to allocate some boot time memory for per_cpu variables
that aren't designed to grow/shrink as CPUs are made available or removed.
Once set during boot time discovery phase, the map is static, i.e no bits
are added or removed anytime. Trimming it accurately for your system needs
upfront can save some boot time memory. See below for how we use heuristics
in x86_64 case to keep this under check.
cpu_online_mask: Bitmap of all CPUs currently online. It's set in __cpu_up()
after a CPU is available for kernel scheduling and ready to receive
interrupts from devices. It's cleared when a CPU is brought down using
__cpu_disable(), before which all OS services including interrupts are
migrated to another target CPU.
cpu_present_mask: Bitmap of CPUs currently present in the system. Not all
of them may be online. When physical hotplug is processed by the relevant
subsystem (e.g ACPI) can change and new bit either be added or removed
from the map depending on the event is hot-add/hot-remove. There are currently
no locking rules as of now. Typical usage is to init topology during boot,
at which time hotplug is disabled.
You really dont need to manipulate any of the system cpu maps. They should
be read-only for most use. When setting up per-cpu resources almost always use
cpu_possible_mask/for_each_possible_cpu() to iterate.
Never use anything other than cpumask_t to represent bitmap of CPUs.
#include <linux/cpumask.h>
for_each_possible_cpu - Iterate over cpu_possible_mask
for_each_online_cpu - Iterate over cpu_online_mask
for_each_present_cpu - Iterate over cpu_present_mask
for_each_cpu(x,mask) - Iterate over some random collection of cpu mask.
#include <linux/cpu.h>
get_online_cpus() and put_online_cpus():
The above calls are used to inhibit cpu hotplug operations. While the
cpu_hotplug.refcount is non zero, the cpu_online_mask will not change.
If you merely need to avoid cpus going away, you could also use
preempt_disable() and preempt_enable() for those sections.
Just remember the critical section cannot call any
function that can sleep or schedule this process away. The preempt_disable()
will work as long as stop_machine_run() is used to take a cpu down.
CPU Hotplug - Frequently Asked Questions.
Q: How to enable my kernel to support CPU hotplug?
A: When doing make defconfig, Enable CPU hotplug support
"Processor type and Features" -> Support for Hotpluggable CPUs
Make sure that you have CONFIG_SMP turned on as well.
You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
as well.
Q: What architectures support CPU hotplug?
A: As of 2.6.14, the following architectures support CPU hotplug.
i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
Q: How to test if hotplug is supported on the newly built kernel?
A: You should now notice an entry in sysfs.
Check if sysfs is mounted, using the "mount" command. You should notice
an entry as shown below in the output.
....
none on /sys type sysfs (rw)
....
If this is not mounted, do the following.
#mkdir /sys
#mount -t sysfs sys /sys
Now you should see entries for all present cpu, the following is an example
in a 8-way system.
#pwd
#/sys/devices/system/cpu
#ls -l
total 0
drwxr-xr-x 10 root root 0 Sep 19 07:44 .
drwxr-xr-x 13 root root 0 Sep 19 07:45 ..
drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu0
drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu1
drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu2
drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu3
drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu4
drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu5
drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu6
drwxr-xr-x 3 root root 0 Sep 19 07:48 cpu7
Under each directory you would find an "online" file which is the control
file to logically online/offline a processor.
Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
A: The usage of hot-add/remove may not be very consistently used in the code.
CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
To support physical addition/removal, one would need some BIOS hooks and
the platform should have something like an attention button in PCI hotplug.
CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
Q: How do I logically offline a CPU?
A: Do the following.
#echo 0 > /sys/devices/system/cpu/cpuX/online
Once the logical offline is successful, check
#cat /proc/interrupts
You should now not see the CPU that you removed. Also online file will report
the state as 0 when a CPU is offline and 1 when it's online.
#To display the current cpu state.
#cat /sys/devices/system/cpu/cpuX/online
Q: Why can't I remove CPU0 on some systems?
A: Some architectures may have some special dependency on a certain CPU.
For e.g in IA64 platforms we have ability to send platform interrupts to the
OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
specifications, we didn't have a way to change the target CPU. Hence if the
current ACPI version doesn't support such re-direction, we disable that CPU
by making it not-removable.
In such cases you will also notice that the online file is missing under cpu0.
Q: Is CPU0 removable on X86?
A: Yes. If kernel is compiled with CONFIG_BOOTPARAM_HOTPLUG_CPU0=y, CPU0 is
removable by default. Otherwise, CPU0 is also removable by kernel option
cpu0_hotplug.
But some features depend on CPU0. Two known dependencies are:
1. Resume from hibernate/suspend depends on CPU0. Hibernate/suspend will fail if
CPU0 is offline and you need to online CPU0 before hibernate/suspend can
continue.
2. PIC interrupts also depend on CPU0. CPU0 can't be removed if a PIC interrupt
is detected.
It's said poweroff/reboot may depend on CPU0 on some machines although I haven't
seen any poweroff/reboot failure so far after CPU0 is offline on a few tested
machines.
Please let me know if you know or see any other dependencies of CPU0.
If the dependencies are under your control, you can turn on CPU0 hotplug feature
either by CONFIG_BOOTPARAM_HOTPLUG_CPU0 or by kernel parameter cpu0_hotplug.
--Fenghua Yu <fenghua.yu@intel.com>
Q: How do I find out if a particular CPU is not removable?
A: Depending on the implementation, some architectures may show this by the
absence of the "online" file. This is done if it can be determined ahead of
time that this CPU cannot be removed.
In some situations, this can be a run time check, i.e if you try to remove the
last CPU, this will not be permitted. You can find such failures by
investigating the return value of the "echo" command.
Q: What happens when a CPU is being logically offlined?
A: The following happen, listed in no particular order :-)
- A notification is sent to in-kernel registered modules by sending an event
CPU_DOWN_PREPARE or CPU_DOWN_PREPARE_FROZEN, depending on whether or not the
CPU is being offlined while tasks are frozen due to a suspend operation in
progress
- All processes are migrated away from this outgoing CPU to new CPUs.
The new CPU is chosen from each process' current cpuset, which may be
a subset of all online CPUs.
- All interrupts targeted to this CPU are migrated to a new CPU
- timers/bottom half/task lets are also migrated to a new CPU
- Once all services are migrated, kernel calls an arch specific routine
__cpu_disable() to perform arch specific cleanup.
- Once this is successful, an event for successful cleanup is sent by an event
CPU_DEAD (or CPU_DEAD_FROZEN if tasks are frozen due to a suspend while the
CPU is being offlined).
"It is expected that each service cleans up when the CPU_DOWN_PREPARE
notifier is called, when CPU_DEAD is called it's expected there is nothing
running on behalf of this CPU that was offlined"
Q: If I have some kernel code that needs to be aware of CPU arrival and
departure, how to i arrange for proper notification?
A: This is what you would need in your kernel code to receive notifications.
#include <linux/cpu.h>
static int foobar_cpu_callback(struct notifier_block *nfb,
unsigned long action, void *hcpu)
{
unsigned int cpu = (unsigned long)hcpu;
switch (action) {
case CPU_ONLINE:
case CPU_ONLINE_FROZEN:
foobar_online_action(cpu);
break;
case CPU_DEAD:
case CPU_DEAD_FROZEN:
foobar_dead_action(cpu);
break;
}
return NOTIFY_OK;
}
static struct notifier_block foobar_cpu_notifier =
{
.notifier_call = foobar_cpu_callback,
};
You need to call register_cpu_notifier() from your init function.
Init functions could be of two types:
1. early init (init function called when only the boot processor is online).
2. late init (init function called _after_ all the CPUs are online).
For the first case, you should add the following to your init function
register_cpu_notifier(&foobar_cpu_notifier);
For the second case, you should add the following to your init function
register_hotcpu_notifier(&foobar_cpu_notifier);
You can fail PREPARE notifiers if something doesn't work to prepare resources.
This will stop the activity and send a following CANCELED event back.
CPU_DEAD should not be failed, its just a goodness indication, but bad
things will happen if a notifier in path sent a BAD notify code.
Q: I don't see my action being called for all CPUs already up and running?
A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
If you need to perform some action for each CPU already in the system, then
do this:
for_each_online_cpu(i) {
foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i);
}
However, if you want to register a hotplug callback, as well as perform
some initialization for CPUs that are already online, then do this:
Version 1: (Correct)
---------
cpu_notifier_register_begin();
for_each_online_cpu(i) {
foobar_cpu_callback(&foobar_cpu_notifier,
CPU_UP_PREPARE, i);
foobar_cpu_callback(&foobar_cpu_notifier,
CPU_ONLINE, i);
}
/* Note the use of the double underscored version of the API */
__register_cpu_notifier(&foobar_cpu_notifier);
cpu_notifier_register_done();
Note that the following code is *NOT* the right way to achieve this,
because it is prone to an ABBA deadlock between the cpu_add_remove_lock
and the cpu_hotplug.lock.
Version 2: (Wrong!)
---------
get_online_cpus();
for_each_online_cpu(i) {
foobar_cpu_callback(&foobar_cpu_notifier,
CPU_UP_PREPARE, i);
foobar_cpu_callback(&foobar_cpu_notifier,
CPU_ONLINE, i);
}
register_cpu_notifier(&foobar_cpu_notifier);
put_online_cpus();
So always use the first version shown above when you want to register
callbacks as well as initialize the already online CPUs.
Q: If I would like to develop CPU hotplug support for a new architecture,
what do I need at a minimum?
A: The following are what is required for CPU hotplug infrastructure to work
correctly.
- Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
- __cpu_up() - Arch interface to bring up a CPU
- __cpu_disable() - Arch interface to shutdown a CPU, no more interrupts
can be handled by the kernel after the routine
returns. Including local APIC timers etc are
shutdown.
- __cpu_die() - This actually supposed to ensure death of the CPU.
Actually look at some example code in other arch
that implement CPU hotplug. The processor is taken
down from the idle() loop for that specific
architecture. __cpu_die() typically waits for some
per_cpu state to be set, to ensure the processor
dead routine is called to be sure positively.
Q: I need to ensure that a particular CPU is not removed when there is some
work specific to this CPU in progress.
A: There are two ways. If your code can be run in interrupt context, use
smp_call_function_single(), otherwise use work_on_cpu(). Note that
work_on_cpu() is slow, and can fail due to out of memory:
int my_func_on_cpu(int cpu)
{
int err;
get_online_cpus();
if (!cpu_online(cpu))
err = -EINVAL;
else
#if NEEDS_BLOCKING
err = work_on_cpu(cpu, __my_func_on_cpu, NULL);
#else
smp_call_function_single(cpu, __my_func_on_cpu, &err,
true);
#endif
put_online_cpus();
return err;
}
Q: How do we determine how many CPUs are available for hotplug.
A: There is no clear spec defined way from ACPI that can give us that
information today. Based on some input from Natalie of Unisys,
that the ACPI MADT (Multiple APIC Description Tables) marks those possible
CPUs in a system with disabled status.
Andi implemented some simple heuristics that count the number of disabled
CPUs in MADT as hotpluggable CPUS. In the case there are no disabled CPUS
we assume 1/2 the number of CPUs currently present can be hotplugged.
Caveat: ACPI MADT can only provide 256 entries in systems with only ACPI 2.0c
or earlier ACPI version supported, because the apicid field in MADT is only
8 bits. From ACPI 3.0, this limitation was removed since the apicid field
was extended to 32 bits with x2APIC introduced.
User Space Notification
Hotplug support for devices is common in Linux today. Its being used today to
support automatic configuration of network, usb and pci devices. A hotplug
event can be used to invoke an agent script to perform the configuration task.
You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
scripts.
#!/bin/bash
# $Id: cpu.agent
# Kernel hotplug params include:
#ACTION=%s [online or offline]
#DEVPATH=%s
#
cd /etc/hotplug
. ./hotplug.functions
case $ACTION in
online)
echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
;;
offline)
echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
;;
*)
debug_mesg CPU $ACTION event not supported
exit 1
;;
esac
Documentation/dev-tools/sparse.rst
+6
View File
@@ -103,3 +103,9 @@ have already built it.
The optional make variable CF can be used to pass arguments to sparse. The
build system passes -Wbitwise to sparse automatically.
Checking RCU annotations
~~~~~~~~~~~~~~~~~~~~~~~~
RCU annotations are not checked by default. To enable RCU annotation
checks, include -DCONFIG_SPARSE_RCU_POINTER in your CF flags.
Documentation/dontdiff
+5 -2
View File
@@ -116,9 +116,11 @@ crc32table.h*
cscope.*
defkeymap.c
devlist.h*
devicetable-offsets.h
dnotify_test
docproc
dslm
dtc
elf2ecoff
elfconfig.h*
evergreen_reg_safe.h
@@ -153,8 +155,8 @@ keywords.c
ksym.c*
ksym.h*
kxgettext
lex.c
lex.*.c
*lex.c
*lex.*.c
linux
logo_*.c
logo_*_clut224.c
@@ -215,6 +217,7 @@ series
setup
setup.bin
setup.elf
sortextable
sImage
sm_tbl*
split-include
Documentation/driver-api/device-io.rst
+201
View File
@@ -0,0 +1,201 @@
.. Copyright 2001 Matthew Wilcox
..
.. This documentation is free software; you can redistribute
.. it and/or modify it under the terms of the GNU General Public
.. License as published by the Free Software Foundation; either
.. version 2 of the License, or (at your option) any later
.. version.
===============================
Bus-Independent Device Accesses
===============================
:Author: Matthew Wilcox
:Author: Alan Cox
Introduction
============
Linux provides an API which abstracts performing IO across all busses
and devices, allowing device drivers to be written independently of bus
type.
Memory Mapped IO
================
Getting Access to the Device
----------------------------
The most widely supported form of IO is memory mapped IO. That is, a
part of the CPU's address space is interpreted not as accesses to
memory, but as accesses to a device. Some architectures define devices
to be at a fixed address, but most have some method of discovering
devices. The PCI bus walk is a good example of such a scheme. This
document does not cover how to receive such an address, but assumes you
are starting with one. Physical addresses are of type unsigned long.
This address should not be used directly. Instead, to get an address
suitable for passing to the accessor functions described below, you
should call :c:func:`ioremap()`. An address suitable for accessing
the device will be returned to you.
After you've finished using the device (say, in your module's exit
routine), call :c:func:`iounmap()` in order to return the address
space to the kernel. Most architectures allocate new address space each
time you call :c:func:`ioremap()`, and they can run out unless you
call :c:func:`iounmap()`.
Accessing the device
--------------------
The part of the interface most used by drivers is reading and writing
memory-mapped registers on the device. Linux provides interfaces to read
and write 8-bit, 16-bit, 32-bit and 64-bit quantities. Due to a
historical accident, these are named byte, word, long and quad accesses.
Both read and write accesses are supported; there is no prefetch support
at this time.
The functions are named readb(), readw(), readl(), readq(),
readb_relaxed(), readw_relaxed(), readl_relaxed(), readq_relaxed(),
writeb(), writew(), writel() and writeq().
Some devices (such as framebuffers) would like to use larger transfers than
8 bytes at a time. For these devices, the :c:func:`memcpy_toio()`,
:c:func:`memcpy_fromio()` and :c:func:`memset_io()` functions are
provided. Do not use memset or memcpy on IO addresses; they are not
guaranteed to copy data in order.
The read and write functions are defined to be ordered. That is the
compiler is not permitted to reorder the I/O sequence. When the ordering
can be compiler optimised, you can use __readb() and friends to
indicate the relaxed ordering. Use this with care.
While the basic functions are defined to be synchronous with respect to
each other and ordered with respect to each other the busses the devices
sit on may themselves have asynchronicity. In particular many authors
are burned by the fact that PCI bus writes are posted asynchronously. A
driver author must issue a read from the same device to ensure that
writes have occurred in the specific cases the author cares. This kind
of property cannot be hidden from driver writers in the API. In some
cases, the read used to flush the device may be expected to fail (if the
card is resetting, for example). In that case, the read should be done
from config space, which is guaranteed to soft-fail if the card doesn't
respond.
The following is an example of flushing a write to a device when the
driver would like to ensure the write's effects are visible prior to
continuing execution::
static inline void
qla1280_disable_intrs(struct scsi_qla_host *ha)
{
struct device_reg *reg;
reg = ha->iobase;
/* disable risc and host interrupts */
WRT_REG_WORD(&reg->ictrl, 0);
/*
* The following read will ensure that the above write
* has been received by the device before we return from this
* function.
*/
RD_REG_WORD(&reg->ictrl);
ha->flags.ints_enabled = 0;
}
In addition to write posting, on some large multiprocessing systems
(e.g. SGI Challenge, Origin and Altix machines) posted writes won't be
strongly ordered coming from different CPUs. Thus it's important to
properly protect parts of your driver that do memory-mapped writes with
locks and use the :c:func:`mmiowb()` to make sure they arrive in the
order intended. Issuing a regular readX() will also ensure write ordering,
but should only be used when the
driver has to be sure that the write has actually arrived at the device
(not that it's simply ordered with respect to other writes), since a
full readX() is a relatively expensive operation.
Generally, one should use :c:func:`mmiowb()` prior to releasing a spinlock
that protects regions using :c:func:`writeb()` or similar functions that
aren't surrounded by readb() calls, which will ensure ordering
and flushing. The following pseudocode illustrates what might occur if
write ordering isn't guaranteed via :c:func:`mmiowb()` or one of the
readX() functions::
CPU A: spin_lock_irqsave(&dev_lock, flags)
CPU A: ...
CPU A: writel(newval, ring_ptr);
CPU A: spin_unlock_irqrestore(&dev_lock, flags)
...
CPU B: spin_lock_irqsave(&dev_lock, flags)
CPU B: writel(newval2, ring_ptr);
CPU B: ...
CPU B: spin_unlock_irqrestore(&dev_lock, flags)
In the case above, newval2 could be written to ring_ptr before newval.
Fixing it is easy though::
CPU A: spin_lock_irqsave(&dev_lock, flags)
CPU A: ...
CPU A: writel(newval, ring_ptr);
CPU A: mmiowb(); /* ensure no other writes beat us to the device */
CPU A: spin_unlock_irqrestore(&dev_lock, flags)
...
CPU B: spin_lock_irqsave(&dev_lock, flags)
CPU B: writel(newval2, ring_ptr);
CPU B: ...
CPU B: mmiowb();
CPU B: spin_unlock_irqrestore(&dev_lock, flags)
See tg3.c for a real world example of how to use :c:func:`mmiowb()`
PCI ordering rules also guarantee that PIO read responses arrive after any
outstanding DMA writes from that bus, since for some devices the result of
a readb() call may signal to the driver that a DMA transaction is
complete. In many cases, however, the driver may want to indicate that the
next readb() call has no relation to any previous DMA writes
performed by the device. The driver can use readb_relaxed() for
these cases, although only some platforms will honor the relaxed
semantics. Using the relaxed read functions will provide significant
performance benefits on platforms that support it. The qla2xxx driver
provides examples of how to use readX_relaxed(). In many cases, a majority
of the driver's readX() calls can safely be converted to readX_relaxed()
calls, since only a few will indicate or depend on DMA completion.
Port Space Accesses
===================
Port Space Explained
--------------------
Another form of IO commonly supported is Port Space. This is a range of
addresses separate to the normal memory address space. Access to these
addresses is generally not as fast as accesses to the memory mapped
addresses, and it also has a potentially smaller address space.
Unlike memory mapped IO, no preparation is required to access port
space.
Accessing Port Space
--------------------
Accesses to this space are provided through a set of functions which
allow 8-bit, 16-bit and 32-bit accesses; also known as byte, word and
long. These functions are :c:func:`inb()`, :c:func:`inw()`,
:c:func:`inl()`, :c:func:`outb()`, :c:func:`outw()` and
:c:func:`outl()`.
Some variants are provided for these functions. Some devices require
that accesses to their ports are slowed down. This functionality is
provided by appending a ``_p`` to the end of the function.
There are also equivalents to memcpy. The :c:func:`ins()` and
:c:func:`outs()` functions copy bytes, words or longs to the given
port.
Public Functions Provided
=========================
.. kernel-doc:: arch/x86/include/asm/io.h
:internal:
.. kernel-doc:: lib/pci_iomap.c
:export:
Documentation/driver-api/device_link.rst
+10 -8
View File
@@ -1,3 +1,6 @@
.. |struct dev_pm_domain| replace:: :c:type:`struct dev_pm_domain <dev_pm_domain>`
.. |struct generic_pm_domain| replace:: :c:type:`struct generic_pm_domain <generic_pm_domain>`
============
Device links
============
@@ -120,12 +123,11 @@ Examples
is the same as if the MMU was the parent of the master device.
The fact that both devices share the same power domain would normally
suggest usage of a :c:type:`struct dev_pm_domain` or :c:type:`struct
generic_pm_domain`, however these are not independent devices that
happen to share a power switch, but rather the MMU device serves the
busmaster device and is useless without it. A device link creates a
synthetic hierarchical relationship between the devices and is thus
more apt.
suggest usage of a |struct dev_pm_domain| or |struct generic_pm_domain|,
however these are not independent devices that happen to share a power
switch, but rather the MMU device serves the busmaster device and is
useless without it. A device link creates a synthetic hierarchical
relationship between the devices and is thus more apt.
* A Thunderbolt host controller comprises a number of PCIe hotplug ports
and an NHI device to manage the PCIe switch. On resume from system sleep,
@@ -157,7 +159,7 @@ Examples
Alternatives
============
* A :c:type:`struct dev_pm_domain` can be used to override the bus,
* A |struct dev_pm_domain| can be used to override the bus,
class or device type callbacks. It is intended for devices sharing
a single on/off switch, however it does not guarantee a specific
suspend/resume ordering, this needs to be implemented separately.
@@ -166,7 +168,7 @@ Alternatives
suspended. Furthermore it cannot be used to enforce a specific shutdown
ordering or a driver presence dependency.
* A :c:type:`struct generic_pm_domain` is a lot more heavyweight than a
* A |struct generic_pm_domain| is a lot more heavyweight than a
device link and does not allow for shutdown ordering or driver presence
dependencies. It also cannot be used on ACPI systems.
Documentation/driver-api/iio/buffers.rst
+125
View File
@@ -0,0 +1,125 @@
=======
Buffers
=======
* struct :c:type:`iio_buffer` — general buffer structure
* :c:func:`iio_validate_scan_mask_onehot` — Validates that exactly one channel
is selected
* :c:func:`iio_buffer_get` — Grab a reference to the buffer
* :c:func:`iio_buffer_put` — Release the reference to the buffer
The Industrial I/O core offers a way for continuous data capture based on a
trigger source. Multiple data channels can be read at once from
:file:`/dev/iio:device{X}` character device node, thus reducing the CPU load.
IIO buffer sysfs interface
==========================
An IIO buffer has an associated attributes directory under
:file:`/sys/bus/iio/iio:device{X}/buffer/*`. Here are some of the existing
attributes:
* :file:`length`, the total number of data samples (capacity) that can be
stored by the buffer.
* :file:`enable`, activate buffer capture.
IIO buffer setup
================
The meta information associated with a channel reading placed in a buffer is
called a scan element . The important bits configuring scan elements are
exposed to userspace applications via the
:file:`/sys/bus/iio/iio:device{X}/scan_elements/*` directory. This file contains
attributes of the following form:
* :file:`enable`, used for enabling a channel. If and only if its attribute
is non *zero*, then a triggered capture will contain data samples for this
channel.
* :file:`type`, description of the scan element data storage within the buffer
and hence the form in which it is read from user space.
Format is [be|le]:[s|u]bits/storagebitsXrepeat[>>shift] .
* *be* or *le*, specifies big or little endian.
* *s* or *u*, specifies if signed (2's complement) or unsigned.
* *bits*, is the number of valid data bits.
* *storagebits*, is the number of bits (after padding) that it occupies in the
buffer.
* *shift*, if specified, is the shift that needs to be applied prior to
masking out unused bits.
* *repeat*, specifies the number of bits/storagebits repetitions. When the
repeat element is 0 or 1, then the repeat value is omitted.
For example, a driver for a 3-axis accelerometer with 12 bit resolution where
data is stored in two 8-bits registers as follows::
7 6 5 4 3 2 1 0
+---+---+---+---+---+---+---+---+
|D3 |D2 |D1 |D0 | X | X | X | X | (LOW byte, address 0x06)
+---+---+---+---+---+---+---+---+
7 6 5 4 3 2 1 0
+---+---+---+---+---+---+---+---+
|D11|D10|D9 |D8 |D7 |D6 |D5 |D4 | (HIGH byte, address 0x07)
+---+---+---+---+---+---+---+---+
will have the following scan element type for each axis::
$ cat /sys/bus/iio/devices/iio:device0/scan_elements/in_accel_y_type
le:s12/16>>4
A user space application will interpret data samples read from the buffer as
two byte little endian signed data, that needs a 4 bits right shift before
masking out the 12 valid bits of data.
For implementing buffer support a driver should initialize the following
fields in iio_chan_spec definition::
struct iio_chan_spec {
/* other members */
int scan_index
struct {
char sign;
u8 realbits;
u8 storagebits;
u8 shift;
u8 repeat;
enum iio_endian endianness;
} scan_type;
};
The driver implementing the accelerometer described above will have the
following channel definition::
struct struct iio_chan_spec accel_channels[] = {
{
.type = IIO_ACCEL,
.modified = 1,
.channel2 = IIO_MOD_X,
/* other stuff here */
.scan_index = 0,
.scan_type = {
.sign = 's',
.realbits = 12,
.storagebits = 16,
.shift = 4,
.endianness = IIO_LE,
},
}
/* similar for Y (with channel2 = IIO_MOD_Y, scan_index = 1)
* and Z (with channel2 = IIO_MOD_Z, scan_index = 2) axis
*/
}
Here **scan_index** defines the order in which the enabled channels are placed
inside the buffer. Channels with a lower **scan_index** will be placed before
channels with a higher index. Each channel needs to have a unique
**scan_index**.
Setting **scan_index** to -1 can be used to indicate that the specific channel
does not support buffered capture. In this case no entries will be created for
the channel in the scan_elements directory.
More details
============
.. kernel-doc:: include/linux/iio/buffer.h
.. kernel-doc:: drivers/iio/industrialio-buffer.c
:export:

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