[PATCH v2] drm/doc: document front-buffer rendering

[Simona Vetter]

On Mon, Apr 14, 2025 at 04:28:55PM +0300, Ville Syrjälä wrote:
> On Mon, Apr 14, 2025 at 08:56:56AM +0000, Simon Ser wrote:
> > Explain how to perform front-buffer rendering.
> > 
> > v2: apply Pekka's rewrite
> > 
> > Signed-off-by: Simon Ser <contact at emersion.fr>
> > Cc: Pekka Paalanen <pekka.paalanen at collabora.com>
> > Cc: Simona Vetter <simona.vetter at ffwll.ch>
> > ---
> >  drivers/gpu/drm/drm_blend.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_blend.c b/drivers/gpu/drm/drm_blend.c
> > index 6e74de833466..4e83f372ea51 100644
> > --- a/drivers/gpu/drm/drm_blend.c
> > +++ b/drivers/gpu/drm/drm_blend.c
> > @@ -75,6 +75,12 @@
> >   * 	the currently visible vertical area of the &drm_crtc.
> >   * FB_ID:
> >   * 	Mode object ID of the &drm_framebuffer this plane should scan out.
> > + *
> > + *	When a KMS client is perfoming front-buffer rendering, it should set
> > + *	FB_ID to the same front-buffer FB on each atomic commit. This implies
> > + *	to the driver that it needs to re-read the same FB again. Otherwise
> > + *	drivers which do not employ continuously repeated scanout cycles might
> > + *	not update the screen.
> 
> Should probably add a caveat that this needs to be a sync commit/flip.
> The way the async flip was specified for atomic explicitly requires the
> driver to ignore the plane when the fb doesn't change.
> 
> Also use dirtyfb instead if you don't want to get throttled to the
> vrefresh rate. Tthough I think with some drivers you might get
> throttled even with dirtyfb...

Half the userspace wants throttling, the other half wants it gone so
glxgears looks better. I don't think you want to recommend the dirtyfb
ioctl for that reason at all, because it's just a ymmv ioctl in what
exactly you get in terms of blocking.

It's funny.
-Sima

> 
> >   * CRTC_ID:
> >   * 	Mode object ID of the &drm_crtc this plane should be connected to.
> >   *
> > -- 
> > 2.49.0
> > 
> 
> -- 
> Ville Syrjälä
> Intel

-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch