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 <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Reviewed-by: Brian Starkey <[email protected]>
Cc: Daniel Stone <[email protected]>
Cc: Bas Nieuwenhuizen <[email protected]>
Cc: Dave Airlie <[email protected]>
Cc: Marek Olšák <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: Neil Armstrong <[email protected]>
Cc: Michel Dänzer <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/MGwgeXojKNdNXjCxuMhRlwcJM4vdYph_WJcMeGPPGMcRKtHV41XAXlh2tCc-pPJZCAhS3gwbWMWTd8f03NBA2ZYKfr0QxLhcPivpopr5c6M=@emersion.fr

0 files changed, 0 insertions, 0 deletions