Fix this kernel test robot warning:
drivers/video/fbdev/core/fbcon.c: In function 'fbcon_init':
drivers/video/fbdev/core/fbcon.c:1028:6: warning: variable 'cap' set but not used [-Wunused-but-set-variable]
The cap variable is only used when CONFIG_FRAMEBUFFER_CONSOLE_LEGACY_ACCELERATION
is enabled. Drop the temporary variable and use info->flags instead.
Fixes: 87ab9f6b74 ("Revert "fbcon: Disable accelerated scrolling")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/YgFB4xqI+As196FR@p100
In function do_fb_ioctl(), the "arg" is the type of unsigned long,
and in "case FBIOBLANK:" this argument is casted into an int before
passig to fb_blank(). In fb_blank(), the comparision
if (blank > FB_BLANK_POWERDOWN) would be bypass if the original
"arg" is a large number, which is possible because it comes from
the user input. Fix this by adding the check before the function
call.
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Helge Deller <deller@gmx.de>
Signed-off-by: Yizhuo Zhai <yzhai003@ucr.edu>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220202235811.1621017-1-yzhai003@ucr.edu
Add a config option CONFIG_FRAMEBUFFER_CONSOLE_LEGACY_ACCELERATION to
enable bitblt and fillrect hardware acceleration in the framebuffer
console. If disabled, such acceleration will not be used, even if it is
supported by the graphics hardware driver.
If you plan to use DRM as your main graphics output system, you should
disable this option since it will prevent compiling in code which isn't
used later on when DRM takes over.
For all other configurations, e.g. if none of your graphic cards support
DRM (yet), DRM isn't available for your architecture, or you can't be
sure that the graphic card in the target system will support DRM, you
most likely want to enable this option.
In the non-accelerated case (e.g. when DRM is used), the inlined
fb_scrollmode() function is hardcoded to return SCROLL_REDRAW and as such the
compiler is able to optimize much unneccesary code away.
In this v3 patch version I additionally changed the GETVYRES() and GETVXRES()
macros to take a pointer to the fbcon_display struct. This fixes the build when
console rotation is enabled and helps the compiler again to optimize out code.
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220202135531.92183-4-deller@gmx.de
This reverts commit 39aead8373.
Revert the first (of 2) commits which disabled scrolling acceleration in
fbcon/fbdev. It introduced a regression for fbdev-supported graphic cards
because of the performance penalty by doing screen scrolling by software
instead of using the existing graphic card 2D hardware acceleration.
Console scrolling acceleration was disabled by dropping code which
checked at runtime the driver hardware capabilities for the
BINFO_HWACCEL_COPYAREA or FBINFO_HWACCEL_FILLRECT flags and if set, it
enabled scrollmode SCROLL_MOVE which uses hardware acceleration to move
screen contents. After dropping those checks scrollmode was hard-wired
to SCROLL_REDRAW instead, which forces all graphic cards to redraw every
character at the new screen position when scrolling.
This change effectively disabled all hardware-based scrolling acceleration for
ALL drivers, because now all kind of 2D hardware acceleration (bitblt,
fillrect) in the drivers isn't used any longer.
The original commit message mentions that only 3 DRM drivers (nouveau, omapdrm
and gma500) used hardware acceleration in the past and thus code for checking
and using scrolling acceleration is obsolete.
This statement is NOT TRUE, because beside the DRM drivers there are around 35
other fbdev drivers which depend on fbdev/fbcon and still provide hardware
acceleration for fbdev/fbcon.
The original commit message also states that syzbot found lots of bugs in fbcon
and thus it's "often the solution to just delete code and remove features".
This is true, and the bugs - which actually affected all users of fbcon,
including DRM - were fixed, or code was dropped like e.g. the support for
software scrollback in vgacon (commit 973c096f6a).
So to further analyze which bugs were found by syzbot, I've looked through all
patches in drivers/video which were tagged with syzbot or syzkaller back to
year 2005. The vast majority fixed the reported issues on a higher level, e.g.
when screen is to be resized, or when font size is to be changed. The few ones
which touched driver code fixed a real driver bug, e.g. by adding a check.
But NONE of those patches touched code of either the SCROLL_MOVE or the
SCROLL_REDRAW case.
That means, there was no real reason why SCROLL_MOVE had to be ripped-out and
just SCROLL_REDRAW had to be used instead. The only reason I can imagine so far
was that SCROLL_MOVE wasn't used by DRM and as such it was assumed that it
could go away. That argument completely missed the fact that SCROLL_MOVE is
still heavily used by fbdev (non-DRM) drivers.
Some people mention that using memcpy() instead of the hardware acceleration is
pretty much the same speed. But that's not true, at least not for older graphic
cards and machines where we see speed decreases by factor 10 and more and thus
this change leads to console responsiveness way worse than before.
That's why the original commit is to be reverted. By reverting we
reintroduce hardware-based scrolling acceleration and fix the
performance regression for fbdev drivers.
There isn't any impact on DRM when reverting those patches.
Signed-off-by: Helge Deller <deller@gmx.de>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Sven Schnelle <svens@stackframe.org>
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220202135531.92183-3-deller@gmx.de
This reverts commit b3ec8cdf45.
Revert the second (of 2) commits which disabled scrolling acceleration
in fbcon/fbdev. It introduced a regression for fbdev-supported graphic
cards because of the performance penalty by doing screen scrolling by
software instead of using the existing graphic card 2D hardware
acceleration.
Console scrolling acceleration was disabled by dropping code which
checked at runtime the driver hardware capabilities for the
BINFO_HWACCEL_COPYAREA or FBINFO_HWACCEL_FILLRECT flags and if set, it
enabled scrollmode SCROLL_MOVE which uses hardware acceleration to move
screen contents. After dropping those checks scrollmode was hard-wired
to SCROLL_REDRAW instead, which forces all graphic cards to redraw every
character at the new screen position when scrolling.
This change effectively disabled all hardware-based scrolling acceleration for
ALL drivers, because now all kind of 2D hardware acceleration (bitblt,
fillrect) in the drivers isn't used any longer.
The original commit message mentions that only 3 DRM drivers (nouveau, omapdrm
and gma500) used hardware acceleration in the past and thus code for checking
and using scrolling acceleration is obsolete.
This statement is NOT TRUE, because beside the DRM drivers there are around 35
other fbdev drivers which depend on fbdev/fbcon and still provide hardware
acceleration for fbdev/fbcon.
The original commit message also states that syzbot found lots of bugs in fbcon
and thus it's "often the solution to just delete code and remove features".
This is true, and the bugs - which actually affected all users of fbcon,
including DRM - were fixed, or code was dropped like e.g. the support for
software scrollback in vgacon (commit 973c096f6a).
So to further analyze which bugs were found by syzbot, I've looked through all
patches in drivers/video which were tagged with syzbot or syzkaller back to
year 2005. The vast majority fixed the reported issues on a higher level, e.g.
when screen is to be resized, or when font size is to be changed. The few ones
which touched driver code fixed a real driver bug, e.g. by adding a check.
But NONE of those patches touched code of either the SCROLL_MOVE or the
SCROLL_REDRAW case.
That means, there was no real reason why SCROLL_MOVE had to be ripped-out and
just SCROLL_REDRAW had to be used instead. The only reason I can imagine so far
was that SCROLL_MOVE wasn't used by DRM and as such it was assumed that it
could go away. That argument completely missed the fact that SCROLL_MOVE is
still heavily used by fbdev (non-DRM) drivers.
Some people mention that using memcpy() instead of the hardware acceleration is
pretty much the same speed. But that's not true, at least not for older graphic
cards and machines where we see speed decreases by factor 10 and more and thus
this change leads to console responsiveness way worse than before.
That's why the original commit is to be reverted. By reverting we
reintroduce hardware-based scrolling acceleration and fix the
performance regression for fbdev drivers.
There isn't any impact on DRM when reverting those patches.
Signed-off-by: Helge Deller <deller@gmx.de>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Sven Schnelle <svens@stackframe.org>
Cc: stable@vger.kernel.org # v5.16+
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220202135531.92183-2-deller@gmx.de
Pull drm updates from Dave Airlie:
"Highlights are support for privacy screens found in new laptops, a
bunch of nomodeset refactoring, and i915 enables ADL-P systems by
default, while starting to add RPL-S support.
vmwgfx adds GEM and support for OpenGL 4.3 features in userspace.
Lots of internal refactorings around dma reservations, and lots of
driver refactoring as well.
Summary:
core:
- add privacy screen support
- move nomodeset option into drm subsystem
- clean up nomodeset handling in drivers
- make drm_irq.c legacy
- fix stack_depot name conflicts
- remove DMA_BUF_SET_NAME ioctl restrictions
- sysfs: send hotplug event
- replace several DRM_* logging macros with drm_*
- move hashtable to legacy code
- add error return from gem_create_object
- cma-helper: improve interfaces, drop CONFIG_DRM_KMS_CMA_HELPER
- kernel.h related include cleanups
- support XRGB2101010 source buffers
ttm:
- don't include drm hashtable
- stop pruning fences after wait
- documentation updates
dma-buf:
- add dma_resv selftest
- add debugfs helpers
- remove dma_resv_get_excl_unlocked
- documentation
- make fences mandatory in dma_resv_add_excl_fence
dp:
- add link training delay helpers
gem:
- link shmem/cma helpers into separate modules
- use dma_resv iteratior
- import dma-buf namespace into gem helper modules
scheduler:
- fence grab fix
- lockdep fixes
bridge:
- switch to managed MIPI DSI helpers
- register and attach during probe fixes
- convert to YAML in several places.
panel:
- add bunch of new panesl
simpledrm:
- support FB_DAMAGE_CLIPS
- support virtual screen sizes
- add Apple M1 support
amdgpu:
- enable seamless boot for DCN 3.01
- runtime PM fixes
- use drm_kms_helper_connector_hotplug_event
- get all fences at once
- use generic drm fb helpers
- PSR/DPCD/LTTPR/DSC/PM/RAS/OLED/SRIOV fixes
- add smart trace buffer (STB) for supported GPUs
- display debugfs entries
- new SMU debug option
- Documentation update
amdkfd:
- IP discovery enumeration refactor
- interface between driver fixes
- SVM fixes
- kfd uapi header to define some sysfs bitfields.
i915:
- support VESA panel backlights
- enable ADL-P by default
- add eDP privacy screen support
- add Raptor Lake S (RPL-S) support
- DG2 page table support
- lots of GuC/HuC fw refactoring
- refactored i915->gt interfaces
- CD clock squashing support
- enable 10-bit gamma support
- update ADL-P DMC fw to v2.14
- enable runtime PM autosuspend by default
- ADL-P DSI support
- per-lane DP drive settings for ICL+
- add support for pipe C/D DMC firmware
- Atomic gamma LUT updates
- remove CCS FB stride restrictions on ADL-P
- VRR platform support for display 11
- add support for display audio codec keepalive
- lots of display refactoring
- fix runtime PM handling during PXP suspend
- improved eviction performance with async TTM moves
- async VMA unbinding improvements
- VMA locking refactoring
- improved error capture robustness
- use per device iommu checks
- drop bits stealing from i915_sw_fence function ptr
- remove dma_resv_prune
- add IC cache invalidation on DG2
nouveau:
- crc fixes
- validate LUTs in atomic check
- set HDMI AVI RGB quant to full
tegra:
- buffer objects reworks for dma-buf compat
- NVDEC driver uAPI support
- power management improvements
etnaviv:
- IOMMU enabled system support
- fix > 4GB command buffer mapping
- close a DoS vector
- fix spurious GPU resets
ast:
- fix i2c initialization
rcar-du:
- DSI output support
exynos:
- replace legacy gpio interface
- implement generic GEM object mmap
msm:
- dpu plane state cleanup in prep for multirect
- dpu debugfs cleanups
- dp support for sc7280
- a506 support
- removal of struct_mutex
- remove old eDP sub-driver
anx7625:
- support MIPI DSI input
- support HDMI audio
- fix reading EDID
lvds:
- fix bridge DT bindings
megachips:
- probe both bridges before registering
dw-hdmi:
- allow interlace on bridge
ps8640:
- enable runtime PM
- support aux-bus
tx358768:
- enable reference clock
- add pulse mode support
ti-sn65dsi86:
- use regmap bulk write
- add PWM support
etnaviv:
- get all fences at once
gma500:
- gem object cleanups
kmb:
- enable fb console
radeon:
- use dma_resv_wait_timeout
rockchip:
- add DSP hold timeout
- suspend/resume fixes
- PLL clock fixes
- implement mmap in GEM object functions
- use generic fbdev emulation
sun4i:
- use CMA helpers without vmap support
vc4:
- fix HDMI-CEC hang with display is off
- power on HDMI controller while disabling
- support 4K@60Hz modes
- support 10-bit YUV 4:2:0 output
vmwgfx:
- fix leak on probe errors
- fail probing on broken hosts
- new placement for MOB page tables
- hide internal BOs from userspace
- implement GEM support
- implement GL 4.3 support
virtio:
- overflow fixes
xen:
- implement mmap as GEM object function
omapdrm:
- fix scatterlist export
- support virtual planes
mediatek:
- MT8192 support
- CMDQ refinement"
* tag 'drm-next-2022-01-07' of git://anongit.freedesktop.org/drm/drm: (1241 commits)
drm/amdgpu: no DC support for headless chips
drm/amd/display: fix dereference before NULL check
drm/amdgpu: always reset the asic in suspend (v2)
drm/amdgpu: put SMU into proper state on runpm suspending for BOCO capable platform
drm/amd/display: Fix the uninitialized variable in enable_stream_features()
drm/amdgpu: fix runpm documentation
amdgpu/pm: Make sysfs pm attributes as read-only for VFs
drm/amdgpu: save error count in RAS poison handler
drm/amdgpu: drop redundant semicolon
drm/amd/display: get and restore link res map
drm/amd/display: support dynamic HPO DP link encoder allocation
drm/amd/display: access hpo dp link encoder only through link resource
drm/amd/display: populate link res in both detection and validation
drm/amd/display: define link res and make it accessible to all link interfaces
drm/amd/display: 3.2.167
drm/amd/display: [FW Promotion] Release 0.0.98
drm/amd/display: Undo ODM combine
drm/amd/display: Add reg defs for DCN303
drm/amd/display: Changed pipe split policy to allow for multi-display pipe split
drm/amd/display: Set optimize_pwr_state for DCN31
...
drm-misc-next for v5.15-rc1:
UAPI Changes:
- Remove sysfs stats for dma-buf attachments, as it causes a performance regression.
Previous merge is not in a rc kernel yet, so no userspace regression possible.
Cross-subsystem Changes:
- Sanitize user input in kyro's viewport ioctl.
- Use refcount_t in fb_info->count
- Assorted fixes to dma-buf.
- Extend x86 efifb handling to all archs.
- Fix neofb divide by 0.
- Document corpro,gm7123 bridge dt bindings.
Core Changes:
- Slightly rework drm master handling.
- Cleanup vgaarb handling.
- Assorted fixes.
Driver Changes:
- Add support for ws2401 panel.
- Assorted fixes to stm, ast, bochs.
- Demidlayer ingenic irq.
Signed-off-by: Dave Airlie <airlied@redhat.com>
From: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2d0d2fe8-01fc-e216-c3fd-38db9e69944e@linux.intel.com
Commit b9d79e4ca4 ("fbmem: Mark proc_fb_seq_ops as __maybe_unused")
places the '__maybe_unused' in an entirely incorrect location between
the "struct" keyword and the structure name.
It's a wonder that gcc accepts that silently, but clang quite reasonably
warns about it:
drivers/video/fbdev/core/fbmem.c:736:21: warning: attribute declaration must precede definition [-Wignored-attributes]
static const struct __maybe_unused seq_operations proc_fb_seq_ops = {
^
Fix it.
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Pull drm fixes from Dave Airlie:
"Bit later than usual, I queued them all up on Friday then promptly
forgot to write the pull request email. This is mainly amdgpu fixes,
with some radeon/msm/fbdev and one i915 gvt fix thrown in.
amdgpu:
- MPO hang workaround
- Fix for concurrent VM flushes on vega/navi
- dcefclk is not adjustable on navi1x and newer
- MST HPD debugfs fix
- Suspend/resumes fixes
- Register VGA clients late in case driver fails to load
- Fix GEM leak in user framebuffer create
- Add support for polaris12 with 32 bit memory interface
- Fix duplicate cursor issue when using overlay
- Fix corruption with tiled surfaces on VCN3
- Add BO size and stride check to fix BO size verification
radeon:
- Fix off-by-one in power state parsing
- Fix possible memory leak in power state parsing
msm:
- NULL ptr dereference fix
fbdev:
- procfs disabled warning fix
i915:
- gvt: Fix a possible division by zero in vgpu display rate
calculation"
* tag 'drm-next-2021-05-10' of git://anongit.freedesktop.org/drm/drm:
drm/amdgpu: Use device specific BO size & stride check.
drm/amdgpu: Init GFX10_ADDR_CONFIG for VCN v3 in DPG mode.
drm/amd/pm: initialize variable
drm/radeon: Avoid power table parsing memory leaks
drm/radeon: Fix off-by-one power_state index heap overwrite
drm/amd/display: Fix two cursor duplication when using overlay
drm/amdgpu: add new MC firmware for Polaris12 32bit ASIC
fbmem: Mark proc_fb_seq_ops as __maybe_unused
drm/msm/dpu: Delete bonkers code
drm/i915/gvt: Prevent divided by zero when calculating refresh rate
amdgpu: fix GEM obj leak in amdgpu_display_user_framebuffer_create
drm/amdgpu: Register VGA clients after init can no longer fail
drm/amdgpu: Handling of amdgpu_device_resume return value for graceful teardown
drm/amdgpu: fix r initial values
drm/amd/display: fix wrong statement in mst hpd debugfs
amdgpu/pm: set pp_dpm_dcefclk to readonly on NAVI10 and newer gpus
amdgpu/pm: Prevent force of DCEFCLK on NAVI10 and SIENNA_CICHLID
drm/amdgpu: fix concurrent VM flushes on Vega/Navi v2
drm/amd/display: Reject non-zero src_y and src_x for video planes
Pull drm updates from Dave Airlie:
"The usual lots of work all over the place.
i915 has gotten some Alderlake work and prelim DG1 code, along with a
major locking rework over the GEM code, and brings back the property
of timing out long running jobs using a watchdog. amdgpu has some
Alderbran support (new GPU), freesync HDMI support along with a lot
other fixes.
Outside of the drm, there is a new printf specifier added which should
have all the correct acks/sobs:
- printk fourcc modifier support added %p4cc
Summary:
core:
- drm_crtc_commit_wait
- atomic plane state helpers reworked for full state
- dma-buf heaps API rework
- edid: rework and improvements for displayid
dp-mst:
- better topology logging
bridge:
- Chipone ICN6211
- Lontium LT8912B
- anx7625 regulator support
panel:
- fix lt9611 4k panels handling
simple-kms:
- add plane state helpers
ttm:
- debugfs support
- removal of unused sysfs
- ignore signaled moved fences
- ioremap buffer according to mem caching
i915:
- Alderlake S enablement
- Conversion to dma_resv_locking
- Bring back watchdog timeout support
- legacy ioctl cleanups
- add GEM TDDO and RFC process
- DG1 LMEM preparation work
- intel_display.c refactoring
- Gen9/TGL PCH combination support
- eDP MSO Support
- multiple PSR instance support
- Link training debug updates
- Disable PSR2 support on JSL/EHL
- DDR5/LPDDR5 support for bw calcs
- LSPCON limited to gen9/10 platforms
- HSW/BDW async flip/VTd corruption workaround
- SAGV watermark fixes
- SNB hard hang on ring resume fix
- Limit imported dma-buf size
- move to use new tasklet API
- refactor KBL/TGL/ADL-S display/gt steppings
- refactoring legacy DP/HDMI, FB plane code out
amdgpu:
- uapi: add ioctl to query video capabilities
- Iniital AMD Freesync HDMI support
- Initial Adebaran support
- 10bpc dithering improvements
- DCN secure display support
- Drop legacy IO BAR requirements
- PCIE/S0ix/RAS/Prime/Reset fixes
- Display ASSR support
- SMU gfx busy queues for RV/PCO
- Initial LTTPR display work
amdkfd:
- MMU notifier fixes
- APU fixes
radeon:
- debugfs cleanps
- fw error handling ifix
- Flexible array cleanups
msm:
- big DSI phy/pll cleanup
- sc7280 initial support
- commong bandwidth scaling path
- shrinker locking contention fixes
- unpin/swap support for GEM objcets
ast:
- cursor plane handling reworked
tegra:
- don't register DP AUX channels before connectors
zynqmp:
- fix OOB struct padding memset
gma500:
- drop ttm and medfield support
exynos:
- request_irq cleanup function
mediatek:
- fine tune line time for EOTp
- MT8192 dpi support
- atomic crtc config updates
- don't support HDMI connector creation
mxsdb:
- imx8mm support
panfrost:
- MMU IRQ handling rework
qxl:
- locking fixes
- resource deallocation changes
sun4i:
- add alpha properties to UI/VI layers
vc4:
- RPi4 CEC support
vmwgfx:
- doc cleanups
arc:
- moved to drm/tiny"
* tag 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm: (1390 commits)
drm/ttm: Don't count pages in SG BOs against pages_limit
drm/ttm: fix return value check
drm/bridge: lt8912b: fix incorrect handling of of_* return values
drm: bridge: fix LONTIUM use of mipi_dsi_() functions
drm: bridge: fix ANX7625 use of mipi_dsi_() functions
drm/amdgpu: page retire over debugfs mechanism
drm/radeon: Fix a missing check bug in radeon_dp_mst_detect()
drm/amd/display: Fix the Wunused-function warning
drm/radeon/r600: Fix variables that are not used after assignment
drm/amdgpu/smu7: fix CAC setting on TOPAZ
drm/amd/display: Update DCN302 SR Exit Latency
drm/amdgpu: enable ras eeprom on aldebaran
drm/amdgpu: RAS harvest on driver load
drm/amdgpu: add ras aldebaran ras eeprom driver
drm/amd/pm: increase time out value when sending msg to SMU
drm/amdgpu: add DMUB outbox event IRQ source define/complete/debug flag
drm/amd/pm: add the callback to get vbios bootup values for vangogh
drm/radeon: Fix size overflow
drm/amdgpu: Fix size overflow
drm/amdgpu: move mmhub ras_func init to ip specific file
...
msm-next pull request has a baseline with stuff from -fixes, roll
forward first.
Some simple conflicts in amdgpu, ttm and one in i915 where git gets
confused and tries to add the same function twice.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Helge Deller
Yizhuo Zhai
Linus Torvalds
Alex Deucher
Qing Wang
Claudio Suarez
Zheyu Ma
Tetsuo Handa
Dave Airlie
Xiyu Yang
Zhen Lei
Matthew Wilcox
Guenter Roeck
Daniel Vetter
Phillip Potter
Du Cheng