[PATCH] drm/fourcc: document modifier uniqueness requirements

[Simon Ser]

There have suggestions to bake pitch alignment, address alignement,
contiguous memory or other placement (hidden VRAM, GTT/BAR, etc)
constraints into modifiers. Last time this was brought up it seemed
like the consensus was to not allow this. Document this in drm_fourcc.h.

There are several reasons for this.

- Encoding all of these constraints in the modifiers would explode the
  search space pretty quickly (we only have 64 bits to work with).
- Modifiers need to be unambiguous: a buffer can only have a single
  modifier.
- Modifier users aren't expected to parse modifiers.

Signed-off-by: Simon Ser <contact at emersion.fr>
Cc: Daniel Vetter <daniel at ffwll.ch>
Cc: Daniel Stone <daniel at fooishbar.org>
Cc: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
Cc: Dave Airlie <airlied at gmail.com>
Cc: Marek Olšák <maraeo at gmail.com>
---
 include/uapi/drm/drm_fourcc.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
index 490143500a50..97eb0f1cf9f8 100644
--- a/include/uapi/drm/drm_fourcc.h
+++ b/include/uapi/drm/drm_fourcc.h
@@ -58,6 +58,17 @@ extern "C" {
  * may preserve meaning - such as number of planes - from the fourcc code,
  * whereas others may not.
  *
+ * Modifiers must uniquely encode buffer layout. In other words, a buffer must
+ * match only a single modifier. A modifier must not be a subset of layouts of
+ * another modifier. For instance, it's incorrect to encode pitch alignment in
+ * a modifier: a buffer may match a 64-pixel aligned modifier and a 32-pixel
+ * aligned modifier. That said, modifiers can have implicit minimal
+ * requirements.
+ *
+ * Users see modifiers as opaque tokens they can check for equality and
+ * intersect. Users musn't need to know to reason about the modifier value
+ * (i.e. users are not expected to extract information out of the modifier).
+ *
  * Vendors should document their modifier usage in as much detail as
  * possible, to ensure maximum compatibility across devices, drivers and
  * applications.
-- 
2.26.2