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

[Kazlauskas, Nicholas]

On 2/14/19 11:47 AM, Grodzovsky, Andrey wrote:
> 
> On 2/14/19 11:07 AM, Michel Dänzer wrote:
>> 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.
> 
> 
> Yes, agree.
> 
> Andrey
> 
> 
>>
>>

I'm not worried about the buffer moving so much as I am about userspace 
changing the tiling flags by calling the IOCTL. Can that happen when 
it's pinned?

Nicholas Kazlauskas