[PATCH] drm: don't block fb changes for async plane updates

[Helen Koike]

Hi,

Correcting the tag, just for the record, is v5 of the patch.

On 6/10/19 10:36 AM, Helen Koike wrote:
> commit 89a4aac0ab0e6f5eea10d7bf4869dd15c3de2cd4 upstream.
> 
> 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 at vger.kernel.org> # v4.14+
> Fixes: 25dc194b34dd ("drm: Block fb changes for async plane updates")
> Suggested-by: Boris Brezillon <boris.brezillon at collabora.com>
> Signed-off-by: Helen Koike <helen.koike at collabora.com>
> Reviewed-by: Boris Brezillon <boris.brezillon at collabora.com>
> Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas at amd.com>
> Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>
> Link: https://patchwork.freedesktop.org/patch/msgid/20190603165610.24614-6-helen.koike@collabora.com
> ---
> 
> Hi,
> 
> This patch failed to apply on kernel stable v4.14, I'm re-sending it
> fixing the conflict.

fyi, this patch was already applied in other stable branches.

Thanks
Helen

> 
> Thanks
> Helen
> 
>  drivers/gpu/drm/drm_atomic_helper.c      | 10 ++++++++++
>  include/drm/drm_modeset_helper_vtables.h |  8 ++++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index d05ed0521e20..331478bd2ff8 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -1462,6 +1462,8 @@ 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)
> @@ -1472,8 +1474,16 @@ 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 *old_fb = plane->state->fb;
> +
>  		funcs = plane->helper_private;
>  		funcs->atomic_async_update(plane, plane_state);
> +
> +		/*
> +		 * 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 c55cf3ff6847..2f6d0f3ae985 100644
> --- a/include/drm/drm_modeset_helper_vtables.h
> +++ b/include/drm/drm_modeset_helper_vtables.h
> @@ -1159,6 +1159,14 @@ struct drm_plane_helper_funcs {
>  	 * current one with the new plane configurations in the new
>  	 * plane_state.
>  	 *
> +	 * Drivers should also swap the framebuffers between current plane
> +	 * state (&drm_plane.state) and new_state.
> +	 * 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
>