drm: drm_fourcc: Add generic alias for 16_16_TILE modifier

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@arm.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200626164800.11595-1-brian.starkey@arm.com

include/uapi/drm/drm_fourcc.h

@@ -346,8 +346,33 @@ 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
  *