[PATCH] drm/amd/display: Add fast path for cursor plane updates

[Kazlauskas, Nicholas]

On 12/13/18 11:01 AM, Kazlauskas, Nicholas wrote:
> On 12/13/18 10:48 AM, Michel Dänzer wrote:
>> On 2018-12-05 8:59 p.m., Nicholas Kazlauskas wrote:
>>> [Why]
>>> Legacy cursor plane updates from drm helpers go through the full
>>> atomic codepath. A high volume of cursor updates through this slow
>>> code path can cause subsequent page-flips to skip vblank intervals
>>> since each individual update is slow.
>>>
>>> This problem is particularly noticeable for the compton compositor.
>>>
>>> [How]
>>> A fast path for cursor plane updates is added by using DRM asynchronous
>>> commit support provided by async_check and async_update. These don't do
>>> a full state/flip_done dependency stall and they don't block other
>>> commit work.
>>>
>>> However, DC still expects itself to be single-threaded for anything
>>> that can issue register writes. Screen corruption or hangs can occur
>>> if write sequences overlap. Every call that potentially perform
>>> register writes needs to be guarded for asynchronous updates to work.
>>> The dc_lock mutex was added for this.
>>>
>>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106175
>>>
>>> Cc: Leo Li <sunpeng.li at amd.com>
>>> Cc: Harry Wentland <harry.wentland at amd.com>
>>> Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas at amd.com>
>>
>> Looks like this change introduced (or at least exposed) a reference
>> counting bug resulting in use-after-free when Xorg shuts down[0]. See
>> the attached dmesg excerpt (note that I wrapped the !bo->pin_count check
>> in amdgpu_bo_unpin in WARN_ON_ONCE).
>>
>>
>> [0] Only with
>> https://gitlab.freedesktop.org/xorg/driver/xf86-video-amdgpu/commit/0d60233d26ec70d4e1faa343b438e33829c6d5e4
>> , i.e. alternating between two BOs for the HW cursor, instead of always
>> using the same one.
>>
>>
> 
> Thanks for the heads up, I don't think I had that patch in my
> xf86-video-amdgpu when testing the desktop stack.
> 
> The async atomic helper does the:
> 
> drm_atomic_helper_prepare_planes
> drm_atomic_helper_async_commit
> drm_atomic_helper_cleanup_planes
> 
> ...sequence correctly from what I can tell, so maybe it's something with
> dm_plane_helper_prepare_fb or dm_plane_helper_cleanup_fb itself.
> 
> One case where unref could be called (not following a ref) is during
> drm_atomic_helper_prepare_planes - if prepare_fb fails then cleanup_fb
> gets called regardless, and we only ref the fb if prepare_fb is in the
> success path.
> 
> Nicholas Kazlauskas
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> 

The prepare_fb/cleanup_fb calls are actually fine since cleanup_fb only 
gets called on planes that had prepare_fb succeed in all cases as far as 
I can tell.

I think the bug here might be forgetting to set the plane->state to the 
new_state. The cleanup fb callback decides whether to call it on the old 
plane state or new plane state depending on if the commit was aborted or 
not. I think every fast plane update might be treated as aborted in this 
case.

Nicholas Kazlauskas