[PATCH] drm/amd/display: Fix deadlock with display during hanged ring recovery.

[Michel Dänzer]

On 2019-02-14 4:54 p.m., Kazlauskas, Nicholas wrote:
> On 2/14/19 10:42 AM, Grodzovsky, Andrey wrote:
>> On 2/14/19 4:05 AM, Christian König wrote:
>>> Am 13.02.19 um 19:58 schrieb Andrey Grodzovsky:
>>>> When ring hang happens amdgpu_dm_commit_planes during flip is holding
>>>> the BO reserved and then stack waiting for fences to signal in
>>>> reservation_object_wait_timeout_rcu (which won't signal because there
>>>> was a hnag). Then when we try to shutdown display block during reset
>>>> recovery from drm_atomic_helper_suspend we also try to reserve the BO
>>>> from dm_plane_helper_cleanup_fb ending in deadlock.
>>>> Also remove useless WARN_ON
>>>
>>> Well it is good that you pointed this out, but there are more problems
>>> than just waiting wile the BO is reserved here.
>>>
>>>>
>>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
>>>> ---
>>>>    drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 19
>>>> +++++++++++++------
>>>>    1 file changed, 13 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>> b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>> index acc4ff8..f8dec36 100644
>>>> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>> @@ -4802,14 +4802,21 @@ static void amdgpu_dm_commit_planes(struct
>>>> drm_atomic_state *state,
>>>>                 */
>>>>                abo = gem_to_amdgpu_bo(fb->obj[0]);
>>>>                r = amdgpu_bo_reserve(abo, true);
>>>
>>> Well why do we reserve the BO in the first place? As the name
>>> indicates reservation_object_wait_timeout_rcu() just uses rcu to wait
>>> for the BO to be idle, no need to actually reserve it at all.
>>
>> That a good point, I honestly can't remember any more why it's here...
>> It does look unnecessary given that we are not validating the BO here.
> 
> I think we reserve it to grab the tiling flags to update the plane 
> address (and some other bits of the plane state with a patch I'm 
> currently working on). I'm guessing that we still need the reservation 
> so nothing else tries to grab it after it becomes idle, but I'm not 
> overly familiar with that bit.

The BO is already pinned at this point, isn't it? That's enough to
prevent it from moving, no reservation needed.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer