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

[Grodzovsky, Andrey]

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 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;
>