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

[Thomas Zimmermann <tzimmermann@suse.de>]

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?

> 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)