[RFC] drm/atomic: Abuse legacy cursor update flag for legacy gamma update too

[Daniel Vetter]

On Tue, Feb 13, 2018 at 10:12:01AM +0100, Maarten Lankhorst wrote:
> Programs like redshift set the legacy gamma for X.org every 5 seconds.
> Because atomic commits wait for vblank completion, we get a frame drop
> every 5 seconds because of the legacy gamma update.
> 
> Work around this by setting the legacy_cursor_update flag, to force
> legacy gamma updates not to be synced against vblank.
> 
> Reported-by: Tholin #intel-gfx
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> ---
>  drivers/gpu/drm/drm_atomic_helper.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index ae3cbfe9e01c..f37ab26ef4d2 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -3806,6 +3806,9 @@ int drm_atomic_helper_legacy_gamma_set(struct drm_crtc *crtc,
>  	if (!state)
>  		return -ENOMEM;
>  
> +	/* Don't wait for vblank after updating gamma. */
> +	state->legacy_cursor_update = true;

The legacy_cursor_update hack is a fairly serious hack, and I'd kinda want
to avoid inflicting it on everyone. The cursor hack is a lot less minimal
(because few drivers have cursors, and we'd have the async plane update
stuff from Gustavo at least). Do we really need this?

Making sure that no one can trample over async gamma table updates,
especially when we eventually go to a vblank worker for them, sounds like
serious amounts of pain. I'd prefer we teach -modesetting to just push the
gamma update with the next frame as an atomic commit, or in the
blockhandler (to avoid delaying it).
-Daniel

> +
>  	blob = drm_property_create_blob(dev,
>  					sizeof(struct drm_color_lut) * size,
>  					NULL);
> -- 
> 2.16.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch