[PATCH 2/2] drm/amd/display: Remove wait for hw/flip done in atomic check

[Kazlauskas, Nicholas]

On 11/22/18 2:39 PM, Grodzovsky, Andrey wrote:
> 
> 
> On 11/22/2018 12:34 PM, Nicholas Kazlauskas wrote:
>> [Why]
>> Atomic check can't truly be non-blocking if amdgpu_dm is waiting for
>> hw_done and flip_done in atomic check. This introduces waits when
>> any previous non-blocking commits queued work on a worker thread and
>> a new atomic commit attempts to do another full update/modeset.
>>
>> [How]
>> Drop the waits and move the global lock acqusition into atomic check.
>>
>> This is fine to do since commit tail waits for these dependencies
>> before calling amdgpu_dm_atomic_commit_tail.
> 
> Note that drm_atomic_helper_wait_for_dependencies waits only on all
> preceeding commits that touch the same CRTC
> while our custom do_aquire_global_lock wait on ALL previous commits in
> the system - even on other CRTCS. As far as I can
> remember that was important because we have global dc_state context
> which must be protected for access/modifications while
> looks like DRM assumes unrelated CRTCs can be modified concurrently.
> Try to verify that latest display code will not be broken by this
> relaxation of synchronization.
> 
> Andrey

This is a good point.

What we should really be doing instead here is locking anything that can 
modify dc->current_state from within atomic commit tail. Serializing 
commits into a queue could work too.

Either should fix non-blocking support.

Nicholas Kazlauskas

> 
>>
>> This is only safe as long as DC never queries anything from within
>> current_state when doing validation in atomic_check. This is the
>> case as of writing, but any future uses of dc->current_state from
>> within atomic_check should be considered incorrect.
>>
>> Cc: Harry Wentland <harry.wentland at amd.com>
>> Cc: Leo Li <sunpeng.li at amd.com>
>> Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas at amd.com>
>> ---
>>    .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 58 ++-----------------
>>    1 file changed, 6 insertions(+), 52 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 3ae438d9849f..fe21bb86b66a 100644
>> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>> @@ -5078,57 +5078,6 @@ void dm_restore_drm_connector_state(struct drm_device *dev,
>>    		dm_force_atomic_commit(&aconnector->base);
>>    }
>>    
>> -/*
>> - * Grabs all modesetting locks to serialize against any blocking commits,
>> - * Waits for completion of all non blocking commits.
>> - */
>> -static int do_aquire_global_lock(struct drm_device *dev,
>> -				 struct drm_atomic_state *state)
>> -{
>> -	struct drm_crtc *crtc;
>> -	struct drm_crtc_commit *commit;
>> -	long ret;
>> -
>> -	/*
>> -	 * Adding all modeset locks to aquire_ctx will
>> -	 * ensure that when the framework release it the
>> -	 * extra locks we are locking here will get released to
>> -	 */
>> -	ret = drm_modeset_lock_all_ctx(dev, state->acquire_ctx);
>> -	if (ret)
>> -		return ret;
>> -
>> -	list_for_each_entry(crtc, &dev->mode_config.crtc_list, head) {
>> -		spin_lock(&crtc->commit_lock);
>> -		commit = list_first_entry_or_null(&crtc->commit_list,
>> -				struct drm_crtc_commit, commit_entry);
>> -		if (commit)
>> -			drm_crtc_commit_get(commit);
>> -		spin_unlock(&crtc->commit_lock);
>> -
>> -		if (!commit)
>> -			continue;
>> -
>> -		/*
>> -		 * Make sure all pending HW programming completed and
>> -		 * page flips done
>> -		 */
>> -		ret = wait_for_completion_interruptible_timeout(&commit->hw_done, 10*HZ);
>> -
>> -		if (ret > 0)
>> -			ret = wait_for_completion_interruptible_timeout(
>> -					&commit->flip_done, 10*HZ);
>> -
>> -		if (ret == 0)
>> -			DRM_ERROR("[CRTC:%d:%s] hw_done or flip_done "
>> -				  "timed out\n", crtc->base.id, crtc->name);
>> -
>> -		drm_crtc_commit_put(commit);
>> -	}
>> -
>> -	return ret < 0 ? ret : 0;
>> -}
>> -
>>    void set_freesync_on_stream(struct amdgpu_display_manager *dm,
>>    			    struct dm_crtc_state *new_crtc_state,
>>    			    struct dm_connector_state *new_con_state,
>> @@ -5793,7 +5742,12 @@ static int amdgpu_dm_atomic_check(struct drm_device *dev,
>>    		if (ret)
>>    			goto fail;
>>    
>> -		ret = do_aquire_global_lock(dev, state);
>> +		/*
>> +		 * This should be replaced with finer locking on the
>> +		 * on the appropriate resources when possible.
>> +		 * For now it's safer to grab everything.
>> +		 */
>> +		ret = drm_modeset_lock_all_ctx(dev, state->acquire_ctx);
>>    		if (ret)
>>    			goto fail;
>>    
> 
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