drm/fourcc: document modifier uniqueness requirements

There have suggestions to bake pitch alignment, address alignment,
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 (except drivers).

v2: add paragraph about aliases (Daniel)

v3: fix unrelated changes sent with the patch

v4: disambiguate users between driver and higher-level programs (Brian,
Daniel)

v5: fix AFBC example (Brian, Daniel)

v6: remove duplicated paragraph (Daniel)

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Brian Starkey <brian.starkey@arm.com>
Cc: Daniel Stone <daniel@fooishbar.org>
Cc: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Marek Olšák <maraeo@gmail.com>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Michel Dänzer <michel@daenzer.net>
Link: https://patchwork.freedesktop.org/patch/msgid/MGwgeXojKNdNXjCxuMhRlwcJM4vdYph_WJcMeGPPGMcRKtHV41XAXlh2tCc-pPJZCAhS3gwbWMWTd8f03NBA2ZYKfr0QxLhcPivpopr5c6M=@emersion.fr

include/uapi/drm/drm_fourcc.h

@@ -58,6 +58,30 @@ 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.
+ *
+ * For modifiers where the combination of fourcc code and modifier can alias,
+ * a canonical pair needs to be defined and used by all drivers. Preferred
+ * combinations are also encouraged where all combinations might lead to
+ * confusion and unnecessarily reduced interoperability. An example for the
+ * latter is AFBC, where the ABGR layouts are preferred over ARGB layouts.
+ *
+ * There are two kinds of modifier users:
+ *
+ * - Kernel and user-space drivers: for drivers it's important that modifiers
+ *   don't alias, otherwise two drivers might support the same format but use
+ *   different aliases, preventing them from sharing buffers in an efficient
+ *   format.
+ * - Higher-level programs interfacing with KMS/GBM/EGL/Vulkan/etc: these users
+ *   see modifiers as opaque tokens they can check for equality and intersect.
+ *   These users musn't need to know to reason about the modifier value
+ *   (i.e. they 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.