Re: [PATCH 2/5] drm/atomic: Remove waits in drm_atomic_helper_commit_cleanup_done

Daniel Vetter
2017-08-30T12:28:43Z

On Wed, Aug 30, 2017 at 02:17:49PM +0200, Maarten Lankhorst wrote:
> When commit synchronization through drm_crtc_commit was first
> introduced, we tried to solve the problem of the flip_done
> needing a reference count by blocking in cleanup_done.
>
> This has been changed by commit 24835e442f28 ("drm: reference count
> event->completion") which made the waits here no longer needed.

This is wrong. Your next patch makes this no longer necessary.
>
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
> drivers/gpu/drm/drm_atomic_helper.c | 17 -----------------
> 1 file changed, 17 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index 4e53aae9a1fb..11d0e94a2181 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -1885,7 +1885,6 @@ void drm_atomic_helper_commit_cleanup_done(struct drm_atomic_state *old_state)
> struct drm_crtc_state *new_crtc_state;
> struct drm_crtc_commit *commit;
> int i;
> - long ret;
>
> for_each_new_crtc_in_state(old_state, crtc, new_crtc_state, i) {
> commit = old_state->crtcs[i].commit;
> @@ -1895,22 +1894,6 @@ void drm_atomic_helper_commit_cleanup_done(struct drm_atomic_state *old_state)
> complete_all(&commit->cleanup_done);
> WARN_ON(!try_wait_for_completion(&commit->hw_done));
>
> - /* commit_list borrows our reference, need to remove before we
> - * clean up our drm_atomic_state. But only after it actually
> - * completed, otherwise subsequent commits won't stall properly. */

This comment here is the clue: "... otherwise subsequent commits wont
stall properly". When we remove the drm_crtc_commit from commit_list, the
next commit won't be able to properly stall for flip_done. Hence we must
wait for flip_done before removing it.

But with your new patch we wait for flip_done using
old_crtc_state->commit, and this is indeed no longer necessary.

With the commit message and patch ordering fixed up:

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> - if (try_wait_for_completion(&commit->flip_done))
> - goto del_commit;
> -
> - /* We must wait for the vblank event to signal our completion
> - * before releasing our reference, since the vblank work does
> - * not hold a reference of its own. */
> - ret = wait_for_completion_timeout(&commit->flip_done,
> - 10*HZ);
> - if (ret == 0)
> - DRM_ERROR("[CRTC:%d:%s] flip_done timed out\n",
> - crtc->base.id, crtc->name);
> -
> -del_commit:
> spin_lock(&crtc->commit_lock);
> list_del(&commit->commit_entry);
> spin_unlock(&crtc->commit_lock);
> --
> 2.11.0
>
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--
Daniel Vetter
Software Engineer, Intel Corporation