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Sarah Sharp
8b3d45705e
When a device signals remote wakeup on a roothub, and the suspend change
bit is set, the host controller driver must not give control back to the
USB core until the port goes back into the active state.
EHCI accomplishes this by waiting in the get port status function until
the PORT_RESUME bit is cleared:
/* stop resume signaling */
temp &= ~(PORT_RWC_BITS | PORT_SUSPEND | PORT_RESUME);
ehci_writel(ehci, temp, status_reg);
clear_bit(wIndex, &ehci->resuming_ports);
retval = ehci_handshake(ehci, status_reg,
PORT_RESUME, 0, 2000 /* 2msec */);
Similarly, the xHCI host should wait until the port goes into U0, before
passing control up to the USB core. When the port transitions from the
RExit state to U0, the xHCI driver will get a port status change event.
We need to wait for that event before passing control up to the USB
core.
After the port transitions to the active state, the USB core should time
a recovery interval before it talks to the device. The length of that
recovery interval is TRSMRCY, 10 ms, mentioned in the USB 2.0 spec,
section 7.1.7.7. The previous xHCI code (which did not wait for the
port to go into U0) would cause the USB core to violate that recovery
interval.
This bug caused numerous USB device disconnects on remote wakeup under
ChromeOS and a Lynx Point LP xHCI host that takes up to 20 ms to move
from RExit to U0. ChromeOS is very aggressive about power savings, and
sets the autosuspend_delay to 100 ms, and disables USB persist.
I attempted to replicate this bug with Ubuntu 12.04, but could not. I
used Ubuntu 12.04 on the same platform, with the same BIOS that the bug
was triggered on ChromeOS with. I also changed the USB sysfs settings
as described above, but still could not reproduce the bug under Ubuntu.
It may be that ChromeOS userspace triggers this bug through additional
settings.
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
To understand all the Linux-USB framework, you'll use these resources:
* This source code. This is necessarily an evolving work, and
includes kerneldoc that should help you get a current overview.
("make pdfdocs", and then look at "usb.pdf" for host side and
"gadget.pdf" for peripheral side.) Also, Documentation/usb has
more information.
* The USB 2.0 specification (from www.usb.org), with supplements
such as those for USB OTG and the various device classes.
The USB specification has a good overview chapter, and USB
peripherals conform to the widely known "Chapter 9".
* Chip specifications for USB controllers. Examples include
host controllers (on PCs, servers, and more); peripheral
controllers (in devices with Linux firmware, like printers or
cell phones); and hard-wired peripherals like Ethernet adapters.
* Specifications for other protocols implemented by USB peripheral
functions. Some are vendor-specific; others are vendor-neutral
but just standardized outside of the www.usb.org team.
Here is a list of what each subdirectory here is, and what is contained in
them.
core/ - This is for the core USB host code, including the
usbfs files and the hub class driver ("khubd").
host/ - This is for USB host controller drivers. This
includes UHCI, OHCI, EHCI, and others that might
be used with more specialized "embedded" systems.
gadget/ - This is for USB peripheral controller drivers and
the various gadget drivers which talk to them.
Individual USB driver directories. A new driver should be added to the
first subdirectory in the list below that it fits into.
image/ - This is for still image drivers, like scanners or
digital cameras.
../input/ - This is for any driver that uses the input subsystem,
like keyboard, mice, touchscreens, tablets, etc.
../media/ - This is for multimedia drivers, like video cameras,
radios, and any other drivers that talk to the v4l
subsystem.
../net/ - This is for network drivers.
serial/ - This is for USB to serial drivers.
storage/ - This is for USB mass-storage drivers.
class/ - This is for all USB device drivers that do not fit
into any of the above categories, and work for a range
of USB Class specified devices.
misc/ - This is for all USB device drivers that do not fit
into any of the above categories.