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

[Brian Starkey]

Hi,

On Sun, Dec 15, 2024 at 03:53:14PM +0000, Marek Olšák wrote:
> The comment explains the problem with DRM_FORMAT_MOD_LINEAR.
> 
> Signed-off-by: Marek Olšák <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)

Why do we want this to be a modifier? All (?) of the other modifiers
describe properties which the producer and consumer need to know in
order to correctly fill/interpret the data.

Framebuffers already have a pitch property which tells the
producer/consumer how to do that for linear buffers.

Modifiers are meant to describe framebuffers, and this pitch alignment
requirement isn't really a framebuffer property - it's a device
constraint. It feels out of place to overload modifiers with it.

I'm not saying we don't need a way to describe constraints to
allocators, but I question if modifiers the right mechanism to
communicate them?

Cheers,
-Brian