[PATCH v3] drm/fourcc: document modifier uniqueness requirements

[Marek Olšák]

On most hardware, there is a minimum pitch alignment for linear and any
greater multiple of the alignment is fine.

On Navi, the pitch in bytes for linear must be align(width * bpp / 8, 256).
That's because the hw computes the pitch from the width and doesn't allow
setting a custom pitch. For that reason, multi-GPU sharing might not be
possible if the other GPU doesn't align the pitch in exactly the same way.

Marek

On Thu, May 28, 2020 at 10:38 AM Simon Ser <contact at emersion.fr> 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 at emersion.fr>
> Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Daniel Stone <daniel at fooishbar.org>
> Cc: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
> Cc: Dave Airlie <airlied at gmail.com>
> Cc: Marek Olšák <maraeo at gmail.com>
> ---
>  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.
> + *
> + * 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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