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

[Brian Starkey]

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.

Signed-off-by: Brian Starkey <brian.starkey at arm.com>
---

This is based off a suggestion from Daniel here:
https://lore.kernel.org/dri-devel/20200529115328.GC1630358@phenom.ffwll.local/

If there are future cases where a "generic" modifier is identified
before merging with a specific vendor code, perhaps we could use
`fourcc_mod_code(NONE, n)` or add a DRM_FORMAT_MOD_VENDOR_GENERIC.

However, I also think we shouldn't try and "guess" what might be generic
up-front and end up polluting the generic namespace. It seems fine to
just alias vendor-specific definitions unless it's very clear-cut.

I typed a version which was s/GENERIC/COMMON/, other suggestions
welcome.

Cheers,
-Brian

 include/uapi/drm/drm_fourcc.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
index 299f9ac4840b..ef78dc9c3fb0 100644
--- a/include/uapi/drm/drm_fourcc.h
+++ b/include/uapi/drm/drm_fourcc.h
@@ -345,8 +345,27 @@ 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.
+ *
+ * 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