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

[Thomas Zimmermann <tzimmermann@suse.de>]

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.

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)