Re: [PATCH 1/5] drm: don't block fb changes for async plane updates

["Kazlauskas, Nicholas" <Nicholas.Kazlauskas@amd.com>]

On 3/4/19 9:49 AM, 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 ballanced.
> 
> Cc: <stable@vger.kernel.org> # v4.14+: 25dc194b34dd: drm: Block fb changes for async plane updates
> 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>
> 
> ---
> Hello,
> 
> As mentioned in the cover letter,
> I tested on the rockchip and on i915 (with a patch I am still working on for
> replacing cursors by async update), with igt plane_cursor_legacy and
> kms_cursor_legacy and I didn't see any regressions.
> I couldn't test on MSM and AMD because I don't have the hardware (and I am
> having some issues testing on vc4) and I would appreciate if anyone could help
> me testing those.
> 
> I also think it would be a better solution if, instead of having async
> to do in-place updates in the current state, the async path should be
> equivalent to a syncronous update, i.e., modifying new_state and
> performing a flip
> IMHO, the only difference between sync and async should be that async update
> doesn't wait for vblank and applies the changes immeditally to the hw,
> but the code path could be almost the same.
> But for now I think this solution is ok (swaping new_fb/old_fb), and
> then we can adjust things little by little, what do you think?
> 
> Thanks!
> Helen
> 
>   drivers/gpu/drm/drm_atomic_helper.c | 20 ++++++++++----------
>   1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index 540a77a2ade9..e7eb96f1efc2 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;
> @@ -1657,6 +1648,9 @@ 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 +1659,17 @@ 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);

I personally think this approach is fine and the WARN_ON s are good for 
catching drivers that want to use these in the future.

I do think it would be good to add something to the function docs that 
explains this requirement and the issue that it addresses. It's a little 
unintuitive to require that the old fb is placed into the new state, but 
it makes sense as a workaround to this problem.

Nicholas Kazlauskas

>   	}
>   }
>   EXPORT_SYMBOL(drm_atomic_helper_async_commit);
> 

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