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

[Michel Dänzer]

On 2024-12-15 21:53, Marek Olšák wrote:
> The comment explains the problem with DRM_FORMAT_MOD_LINEAR.
>    
> Signed-off-by: Marek Olšák <marek.olsak at amd.com <mailto:marek.olsak at 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)

It's not clear what you mean by "requires that the pitch alignment is exactly the number in the definition", since a pitch which is aligned to 256 bytes is also aligned to 128 & 64 bytes. Do you mean the pitch must be exactly the width rounded up to the next / smallest possible multiple of the specified number of bytes?


-- 
Earthling Michel Dänzer       \        GNOME / Xwayland / Mesa developer
https://redhat.com             \               Libre software enthusiast