On Wed, May 27, 2020 at 10:55:34AM +0200, Daniel Vetter wrote:
On Wed, May 27, 2020 at 08:52:00AM +0000, Simon Ser wrote:
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@emersion.fr Cc: Daniel Vetter daniel@ffwll.ch Cc: Daniel Stone daniel@fooishbar.org Cc: Bas Nieuwenhuizen bas@basnieuwenhuizen.nl Cc: Dave Airlie airlied@gmail.com Cc: Marek Olšák maraeo@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.
I think we should also add the aliasing when the fourcc codes are involved, with afbc as example. Maybe:
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.
That's actually explicitly _not_ the case for AFBC, which was one of the things I was trying to document in afbc.rst.
With something like that added:
Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch
- 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).
Doesn't this conflict with "implicit minimal requirements" above?
Certainly for a bunch of different AFBC modifiers, the allocator would need to understand some fields in order to properly align-up the buffer size.
Thanks, -Brian
- Vendors should document their modifier usage in as much detail as
- possible, to ensure maximum compatibility across devices, drivers and
- applications.
-- 2.26.2
-- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel