Re: [PATCH] drm/fourcc: add LINEAR modifiers with an exact pitch alignment

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi


Am 14.01.25 um 14:46 schrieb Thomas Zimmermann:
> Hi
>
>
> Am 15.12.24 um 21:53 schrieb Marek Olšák:
>> The comment explains the problem with DRM_FORMAT_MOD_LINEAR.
>>
>> Signed-off-by: Marek Olšák <marek.olsak@amd.com>
>>
>> diff --git a/include/uapi/drm/drm_fourcc.h 
>> b/include/uapi/drm/drm_fourcc.h
>> index 78abd819fd62e..8ec4163429014 100644
>> --- a/include/uapi/drm/drm_fourcc.h
>> +++ b/include/uapi/drm/drm_fourcc.h
>> @@ -484,9 +484,27 @@ extern "C" {
>>   * modifier (e.g. not setting DRM_MODE_FB_MODIFIERS in the 
>> DRM_ADDFB2 ioctl),
>>   * which tells the driver to also take driver-internal information 
>> into account
>>   * and so might actually result in a tiled framebuffer.
>> + *
>> + * WARNING:
>> + * There are drivers out there that expose DRM_FORMAT_MOD_LINEAR, 
>> but only
>> + * support a certain pitch alignment and can't import images with 
>> this modifier
>> + * if the pitch alignment isn't exactly the one supported. They can 
>> however
>> + * allocate images with this modifier and other drivers can import 
>> them only
>> + * if they support the same pitch alignment. Thus, 
>> DRM_FORMAT_MOD_LINEAR is
>> + * fundamentically incompatible across devices and is the only 
>> modifier that
>> + * has a chance of not working. The PITCH_ALIGN modifiers should be 
>> used
>> + * instead.
>>   */
>>  #define DRM_FORMAT_MOD_LINEAR  fourcc_mod_code(NONE, 0)
>>
>> +/* Linear layout modifiers with an explicit pitch alignment in bytes.
>> + * Exposing this modifier requires that the pitch alignment is exactly
>> + * the number in the definition.
>> + */
>> +#define DRM_FORMAT_MOD_LINEAR_PITCH_ALIGN_64B fourcc_mod_code(NONE, 1)
>
> I recently went through drivers and tried to harmonize some 
> pitch-related code. There are essentially all cases from 2 to 256. See 
> my series at
>
>   https://patchwork.freedesktop.org/series/141168/
>
> Sometimes the driver computes the pitch requirements from the 
> bit-per-pixels. [1]
>
> Hardware also has alignment requirements for framebuffer width 
> (usually 8 pixels).

To clarify: given this and the overall discussion here, it seems to me 
that modifiers and descriptors really don't fit well to the problem.

Best regards
Thomas

>
> Best regards
> Thomas
>
> [1] 
> https://elixir.bootlin.com/linux/v6.12.6/source/drivers/gpu/drm/renesas/rcar-du/rcar_du_kms.c#L410
>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
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