[PATCH v2] drm: drm_fourcc: Add generic alias for 16_16_TILE modifier

[Daniel Vetter]

On Fri, Jun 26, 2020 at 05:48:00PM +0100, Brian Starkey wrote:
> In cases such as DRM_FORMAT_MOD_SAMSUNG_16_16_TILE, the modifier
> describes a generic pixel re-ordering which can be applicable to
> multiple vendors.
> 
> Define an alias: DRM_FORMAT_MOD_GENERIC_16_16_TILE, which can be
> used to describe this layout in a vendor-neutral way, and add a
> comment about the expected usage of such "generic" modifiers.
> 
> Changes in v2:
>  - Move note about future cases to comment (Daniel)
> 
> Signed-off-by: Brian Starkey <brian.starkey at arm.com>

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

> ---
>  include/uapi/drm/drm_fourcc.h | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> index 299f9ac4840b..cb3db4a21012 100644
> --- a/include/uapi/drm/drm_fourcc.h
> +++ b/include/uapi/drm/drm_fourcc.h
> @@ -345,8 +345,33 @@ extern "C" {
>   * When adding a new token please document the layout with a code comment,
>   * similar to the fourcc codes above. drm_fourcc.h is considered the
>   * authoritative source for all of these.
> + *
> + * Generic modifier names:
> + *
> + * DRM_FORMAT_MOD_GENERIC_* definitions are used to provide vendor-neutral names
> + * for layouts which are common across multiple vendors. To preserve
> + * compatibility, in cases where a vendor-specific definition already exists and
> + * a generic name for it is desired, the common name is a purely symbolic alias
> + * and must use the same numerical value as the original definition.
> + *
> + * Note that generic names should only be used for modifiers which describe
> + * generic layouts (such as pixel re-ordering), which may have
> + * independently-developed support across multiple vendors.
> + *
> + * In future cases where a generic layout is identified before merging with a
> + * vendor-specific modifier, a new 'GENERIC' vendor or modifier using vendor
> + * 'NONE' could be considered. This should only be for obvious, exceptional
> + * cases to avoid polluting the 'GENERIC' namespace with modifiers which only
> + * apply to a single vendor.
> + *
> + * Generic names should not be used for cases where multiple hardware vendors
> + * have implementations of the same standardised compression scheme (such as
> + * AFBC). In those cases, all implementations should use the same format
> + * modifier(s), reflecting the vendor of the standard.
>   */
>  
> +#define DRM_FORMAT_MOD_GENERIC_16_16_TILE DRM_FORMAT_MOD_SAMSUNG_16_16_TILE
> +
>  /*
>   * Invalid Modifier
>   *
> -- 
> 2.17.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch