Sorry for commenting on the obsolete v1 - that'll teach me for reading
my backlog chronologically.
On Thu, May 28, 2020 at 02:38:36PM +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.
>
> v2: add paragraph about aliases (Daniel)
>
> v3: fix unrelated changes sent with the patch
>
> 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>
> ---
> include/uapi/drm/drm_fourcc.h | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> index 490143500a50..f41fcb1ed63d 100644
> --- a/include/uapi/drm/drm_fourcc.h
> +++ b/include/uapi/drm/drm_fourcc.h
> @@ -58,6 +58,21 @@ 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.
As I mentioned, this isn't true for AFBC (at least, it shouldn't be),
so I don't think we can use that as an example.
We define a "canonical" ordering in AFBC for exactly this reason - the
ordering can differ and not all decoders support all orderings, so we
need to pick one for best compatibility.
Thanks,
-Brian
> + *
> + * 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
>
>
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