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 (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)
Signed-off-by: Simon Ser <contact@xxxxxxxxxxx>
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Daniel Stone <daniel@xxxxxxxxxxxxx>
Cc: Bas Nieuwenhuizen <bas@xxxxxxxxxxxxxxxxxxx>
Cc: Dave Airlie <airlied@xxxxxxxxx>
Cc: Marek Olšák <maraeo@xxxxxxxxx>
Cc: Alex Deucher <alexdeucher@xxxxxxxxx>
Cc: Neil Armstrong <narmstrong@xxxxxxxxxxxx>
Cc: Michel Dänzer <michel@xxxxxxxxxxx>
Cc: Brian Starkey <brian.starkey@xxxxxxx>
---
include/uapi/drm/drm_fourcc.h | 28 ++++++++++++++++++++++++++++
1 file changed, 28 insertions(+)
diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
index 490143500a50..8296197189bf 100644
--- a/include/uapi/drm/drm_fourcc.h
+++ b/include/uapi/drm/drm_fourcc.h
@@ -58,6 +58,34 @@ 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. An example
+ * is AFBC, where both ARGB and ABGR have the exact same compressed layout.
+ *
+ * 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.
--
2.27.0
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