Re: [PATCH] drm/dumb-buffers: document that it's only for linear FB

[Thomas Zimmermann]

Am 25.02.26 um 08:38 schrieb Icenowy Zheng:
> 在 2026-02-25三的 08:26 +0100，Thomas Zimmermann写道：
> > Hi,
> >
> > Am 25.02.26 um 07:13 schrieb Icenowy Zheng:
> > > The ioctl interfaces for dumb buffers currently only properly
> > > support
> > > linear buffers.
> > >
> > > Mention this in the documentation snippet of dumb-buffers source
> > > code,
> > > which is referenced by drm-kms.rst and will end up in the built
> > > kernel
> > > documentation.
> > >
> > > Also mention the existence of current drivers abusing dumb buffers
> > > for
> > > AFBC to reduce confusion about this.
> > >
> > > Signed-off-by: Icenowy Zheng <zhengxingda@xxxxxxxxxxx>
> > > ---
> > >    drivers/gpu/drm/drm_dumb_buffers.c | 7 ++++++-
> > We documented the meaning of the color bits and the behavior of the
> > dumb-buffer interface at [1]. If anything is missing, it should be
> > added
> > there.
> Yes, I saw this piece of document; however it's part of the interface
> document instead of a concept document, and the whole existence of the

What is a concept document?

> document snippet I am changing can be considered a duplicate of the
> interface document.
>
> Thanks
> Icenowy
>
> > Best regards
> > Thomas
> >
> > [1]
> > https://elixir.bootlin.com/linux/v6.19/source/include/uapi/drm/drm_mode.h#L1200
> >
> > >    1 file changed, 6 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_dumb_buffers.c
> > > b/drivers/gpu/drm/drm_dumb_buffers.c
> > > index e2b62e5fb891b..06f74460adf62 100644
> > > --- a/drivers/gpu/drm/drm_dumb_buffers.c
> > > +++ b/drivers/gpu/drm/drm_dumb_buffers.c
> > > @@ -57,7 +57,12 @@
> > >     *
> > >     * Note that dumb objects may not be used for gpu acceleration,
> > > as has been
> > >     * attempted on some ARM embedded platforms. Such drivers really
> > > must have
> > > - * a hardware-specific ioctl to allocate suitable buffer objects.
> > > + * a hardware-specific ioctl to allocate suitable buffer objects.
> > > They are
> > > + * also currently meant for only linear buffers, and using them
> > > with any
> > > + * modifier other than DRM_FORMAT_MOD_LINEAR is undefined
> > > behavior. There
> > > + * exist some KMS drivers abusing dumb objects for AFBC
> > > framebuffers, but this
> > > + * behavior is discouraged, only exists as a hack now and
> > > shouldn't be
> > > + * replicated.
> > >     */
> > >    
> > >    static int drm_mode_align_dumb(struct drm_mode_create_dumb *args,

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)