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

[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
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,