Re: [PATCH v2 4/6] drm: allow DRM_MODE_PAGE_FLIP_ASYNC for atomic commits

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Tue, 30 Aug 2022 17:29:35 +0000
Simon Ser <contact@emersion.fr> wrote:

> If the driver supports it, allow user-space to supply the
> DRM_MODE_PAGE_FLIP_ASYNC flag to request an async page-flip.
> Set drm_crtc_state.async_flip accordingly.
> 
> Document that drivers will reject atomic commits if an async
> flip isn't possible. This allows user-space to fall back to
> something else. For instance, Xorg falls back to a blit.
> Another option is to wait as close to the next vblank as
> possible before performing the page-flip to reduce latency.
> 
> v2: document new uAPI
> 
> Signed-off-by: Simon Ser <contact@emersion.fr>
> Co-developed-by: André Almeida <andrealmeid@igalia.com>
> Signed-off-by: André Almeida <andrealmeid@igalia.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Joshua Ashton <joshua@froggi.es>
> Cc: Melissa Wen <mwen@igalia.com>
> Cc: Alex Deucher <alexander.deucher@amd.com>
> Cc: Harry Wentland <hwentlan@amd.com>
> Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  drivers/gpu/drm/drm_atomic_uapi.c | 28 +++++++++++++++++++++++++---
>  include/uapi/drm/drm_mode.h       |  4 ++++
>  2 files changed, 29 insertions(+), 3 deletions(-)

...

> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index 86a292c3185a..cce1a1bea645 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -942,6 +942,10 @@ struct hdr_output_metadata {
>   * Request that the page-flip is performed as soon as possible, ie. with no
>   * delay due to waiting for vblank. This may cause tearing to be visible on
>   * the screen.
> + *
> + * When used with atomic uAPI, the driver will return an error if the hardware
> + * doesn't support performing an asynchronous page-flip for this update.
> + * User-space should handle this, e.g. by falling back to a regular page-flip.
>   */
>  #define DRM_MODE_PAGE_FLIP_ASYNC 0x02
>  #define DRM_MODE_PAGE_FLIP_TARGET_ABSOLUTE 0x4

Hi Simon,

recalling what Ville explained that enabling async flips might require
one more sync flip first, how is that supposed to work?

A TEST_ONLY commit is not allowed to change hardware state, and a
failing real commit is not allowed to change hardware state either
(right?), therefore a failing async flip cannot prepare the next flip
to be async, meaning async will never work.


Thanks,
pq

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