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

[Icenowy Zheng <zhengxingda@iscas.ac.cn>]

在 2026-02-25三的 08:47 +0100，Thomas Zimmermann写道：
> 
> 
> 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@iscas.ac.cn>
> > > > ---
> > > >    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?

Well I am patching this document snippet because it becomes part of the
document at [1] (by being referenced in the .rst file).

[1] https://docs.kernel.org/gpu/drm-kms.html#dumb-buffer-objects

> 
> > 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,