Re: [PATCH v6 6/6] drm/doc: Define KMS atomic state set

[André Almeida]

Hi Michel,

Em 17/08/2023 07:37, Michel Dänzer escreveu:
> On 8/15/23 20:57, André Almeida wrote:
> > From: Pekka Paalanen <pekka.paalanen@xxxxxxxxxxxxx>
> >
> > Specify how the atomic state is maintained between userspace and
> > kernel, plus the special case for async flips.
> >
> > Signed-off-by: Pekka Paalanen <pekka.paalanen@xxxxxxxxxxxxx>
> > Signed-off-by: André Almeida <andrealmeid@xxxxxxxxxx>
>
> [...]
>
> > +An atomic commit with the flag DRM_MODE_PAGE_FLIP_ASYNC is allowed to
> > +effectively change only the FB_ID property on any planes. No-operation changes
> > +are ignored as always. [...]
>
> During the hackfest in Brno, it was mentioned that a commit which re-sets the same FB_ID could actually have an effect with VRR: It could trigger scanout of the next frame before vertical blank has reached its maximum duration. Some kind of mechanism is required for this in order to allow user space to perform low frame rate compensation.

I believe the documentation already addresses that sending redundant 
information may not lead to the desired behavior during an async flip. 
Do you think adding a note about using the same FB_ID would be helpful?