Re: [PATCH] drm/amd/display: wait for fence without holding reservation lock

["Kazlauskas, Nicholas" <Nicholas.Kazlauskas-5C7GfCeVMHo@public.gmane.org>]

On 4/2/19 10:41 AM, Kazlauskas, Nicholas wrote:
> On 4/2/19 10:28 AM, Christian König wrote:
>> Don't block others while waiting for the fences to finish, concurrent
>> submission is perfectly valid in this case and holding the lock can
>> prevent killed applications from terminating.
>>
>> Signed-off-by: Christian König <christian.koenig@amd.com>
> 
> Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
> 
> I do recall having the discussion about this a while back with you and
> Andrey.
> 
> I highly doubt that the flags would ever really change here (if they're
> even capable) between the wait and the lock so I think this is fine.
> 
> I think there's a bigger problem with that and how they could
> potentially change between atomic check and atomic commit tail anyway
> (and doing something bad after we think it's passed validation).
> 
> But this is mostly a theoretical concern I think rather than a practical
> one.
> 
> Thanks.
> 
> Nicholas Kazlauskas

...and waiting for the fence isn't really analogous to ensuring that 
they can't change in the first place I suppose.

This looks fine.

Nicholas Kazlauskas


> 
> 
>> ---
>>    drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 13 ++++++++-----
>>    1 file changed, 8 insertions(+), 5 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 697af51d403a..9426e701a7b6 100644
>> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>> @@ -5129,23 +5129,26 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
>>    			continue;
>>    		}
>>    
>> +		abo = gem_to_amdgpu_bo(fb->obj[0]);
>> +
>> +		/* Wait for all fences on this FB */
>> +		r = reservation_object_wait_timeout_rcu(abo->tbo.resv, true,
>> +							false,
>> +							MAX_SCHEDULE_TIMEOUT);
>> +		WARN_ON(r < 0);
>> +
>>    		/*
>>    		 * TODO This might fail and hence better not used, wait
>>    		 * explicitly on fences instead
>>    		 * and in general should be called for
>>    		 * blocking commit to as per framework helpers
>>    		 */
>> -		abo = gem_to_amdgpu_bo(fb->obj[0]);
>>    		r = amdgpu_bo_reserve(abo, true);
>>    		if (unlikely(r != 0)) {
>>    			DRM_ERROR("failed to reserve buffer before flip\n");
>>    			WARN_ON(1);
>>    		}
>>    
>> -		/* Wait for all fences on this FB */
>> -		WARN_ON(reservation_object_wait_timeout_rcu(abo->tbo.resv, true, false,
>> -									    MAX_SCHEDULE_TIMEOUT) < 0);
>> -
>>    		amdgpu_bo_get_tiling_flags(abo, &tiling_flags);
>>    
>>    		amdgpu_bo_unreserve(abo);
>>
> 
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