Re: [PATCH v3 5/5] drm: don't block fb changes for async plane updates

[Sean Paul <sean@poorly.run>]

On Wed, Mar 13, 2019 at 09:20:26PM -0300, Helen Koike wrote:
> In the case of a normal sync update, the preparation of framebuffers (be
> it calling drm_atomic_helper_prepare_planes() or doing setups with
> drm_framebuffer_get()) are performed in the new_state and the respective
> cleanups are performed in the old_state.
> 
> In the case of async updates, the preparation is also done in the
> new_state but the cleanups are done in the new_state (because updates
> are performed in place, i.e. in the current state).
> 
> The current code blocks async udpates when the fb is changed, turning
> async updates into sync updates, slowing down cursor updates and
> introducing regressions in igt tests with errors of type:
> 
> "CRITICAL: completed 97 cursor updated in a period of 30 flips, we
> expect to complete approximately 15360 updates, with the threshold set
> at 7680"
> 
> Fb changes in async updates were prevented to avoid the following scenario:
> 
> - Async update, oldfb = NULL, newfb = fb1, prepare fb1, cleanup fb1
> - Async update, oldfb = fb1, newfb = fb2, prepare fb2, cleanup fb2
> - Non-async commit, oldfb = fb2, newfb = fb1, prepare fb1, cleanup fb2 (wrong)
> Where we have a single call to prepare fb2 but double cleanup call to fb2.
> 
> To solve the above problems, instead of blocking async fb changes, we
> place the old framebuffer in the new_state object, so when the code
> performs cleanups in the new_state it will cleanup the old_fb and we
> will have the following scenario instead:
> 
> - Async update, oldfb = NULL, newfb = fb1, prepare fb1, no cleanup
> - Async update, oldfb = fb1, newfb = fb2, prepare fb2, cleanup fb1
> - Non-async commit, oldfb = fb2, newfb = fb1, prepare fb1, cleanup fb2
> 
> Where calls to prepare/cleanup are balanced.
> 
> Cc: <stable@vger.kernel.org> # v4.14+

I'm not convinced this should be cc: stable, seems more in the improvement
category than a bug fix.

> Fixes: 25dc194b34dd ("drm: Block fb changes for async plane updates")
> Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
> Signed-off-by: Helen Koike <helen.koike@collabora.com>
> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
> Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
> 
> ---
> Hello,
> 
> I added a TODO in drm_atomic_helper_async_commit() regarding doing a
> full state swap(), Boris and Nicholas, let me know if this is ok and if
> I can keep your Reviewed-by tags)
> 
> As mentioned in the cover letter, I tested in almost all platforms with
> igt plane_cursor_legacy and kms_cursor_legacy and I didn't see any
> regressions. But I couldn't test on MSM and AMD because I don't have
> the hardware I would appreciate if anyone could help me testing those.
> 
> Thanks!
> Helen
> 
> Changes in v3:
> - Add Reviewed-by tags
> - Add TODO in drm_atomic_helper_async_commit()
> 
> Changes in v2:
> - Change the order of the patch in the series, add this as the last one.
> - Add documentation
> - s/ballanced/balanced
> 
>  drivers/gpu/drm/drm_atomic_helper.c      | 22 ++++++++++++----------
>  include/drm/drm_modeset_helper_vtables.h |  5 +++++
>  2 files changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index 2453678d1186..de5812c362b5 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -1608,15 +1608,6 @@ int drm_atomic_helper_async_check(struct drm_device *dev,
>  	    old_plane_state->crtc != new_plane_state->crtc)
>  		return -EINVAL;
>  
> -	/*
> -	 * FIXME: Since prepare_fb and cleanup_fb are always called on
> -	 * the new_plane_state for async updates we need to block framebuffer
> -	 * changes. This prevents use of a fb that's been cleaned up and
> -	 * double cleanups from occuring.
> -	 */
> -	if (old_plane_state->fb != new_plane_state->fb)
> -		return -EINVAL;
> -
>  	funcs = plane->helper_private;
>  	if (!funcs->atomic_async_update)
>  		return -EINVAL;
> @@ -1647,6 +1638,8 @@ EXPORT_SYMBOL(drm_atomic_helper_async_check);
>   * drm_atomic_async_check() succeeds. Async commits are not supposed to swap
>   * the states like normal sync commits, but just do in-place changes on the
>   * current state.
> + *
> + * TODO: Implement full swap instead of doing in-place changes.
>   */
>  void drm_atomic_helper_async_commit(struct drm_device *dev,
>  				    struct drm_atomic_state *state)
> @@ -1657,6 +1650,9 @@ void drm_atomic_helper_async_commit(struct drm_device *dev,
>  	int i;
>  
>  	for_each_new_plane_in_state(state, plane, plane_state, i) {
> +		struct drm_framebuffer *new_fb = plane_state->fb;
> +		struct drm_framebuffer *old_fb = plane->state->fb;
> +
>  		funcs = plane->helper_private;
>  		funcs->atomic_async_update(plane, plane_state);
>  
> @@ -1665,11 +1661,17 @@ void drm_atomic_helper_async_commit(struct drm_device *dev,
>  		 * plane->state in-place, make sure at least common
>  		 * properties have been properly updated.
>  		 */
> -		WARN_ON_ONCE(plane->state->fb != plane_state->fb);
> +		WARN_ON_ONCE(plane->state->fb != new_fb);
>  		WARN_ON_ONCE(plane->state->crtc_x != plane_state->crtc_x);
>  		WARN_ON_ONCE(plane->state->crtc_y != plane_state->crtc_y);
>  		WARN_ON_ONCE(plane->state->src_x != plane_state->src_x);
>  		WARN_ON_ONCE(plane->state->src_y != plane_state->src_y);
> +
> +		/*
> +		 * Make sure the FBs have been swapped so that cleanups in the
> +		 * new_state performs a cleanup in the old FB.
> +		 */
> +		WARN_ON_ONCE(plane_state->fb != old_fb);
>  	}
>  }
>  EXPORT_SYMBOL(drm_atomic_helper_async_commit);
> diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
> index cfb7be40bed7..ce582e8e8f2f 100644
> --- a/include/drm/drm_modeset_helper_vtables.h
> +++ b/include/drm/drm_modeset_helper_vtables.h
> @@ -1174,6 +1174,11 @@ struct drm_plane_helper_funcs {
>  	 * current one with the new plane configurations in the new
>  	 * plane_state.
>  	 *
> +	 * Drivers should also swap the framebuffers between plane state

Perhaps add "current" before plane state and then after add "(&drm_plane.state)"
so it's more clear what you're referring to here?

> +	 * and new_state. This is required because prepare and cleanup calls
> +	 * are performed on the new_state object, then to cleanup the old
> +	 * framebuffer, it needs to be placed inside the new_state object.

I'd change this bit to:

        * This is required since cleanup for async commits is performed on
        * the new state, rather than old state like for traditional commits.
        * Since we want to give up the reference on the current (old) fb instead
        * of our brand new one, swap them in the driver during the async commit.

> +	 *
>  	 * FIXME:
>  	 *  - It only works for single plane updates
>  	 *  - Async Pageflips are not supported yet
> -- 
> 2.20.1
> 

-- 
Sean Paul, Software Engineer, Google / Chromium OS