[PATCH v4] drm/fourcc: document modifier uniqueness requirements

[Brian Starkey]

Hi Simon,

On Fri, Jun 19, 2020 at 11:12:25AM +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 (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)
> 
> 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>
> Cc: Alex Deucher <alexdeucher at gmail.com>
> Cc: Neil Armstrong <narmstrong at baylibre.com>
> Cc: Michel Dänzer <michel at daenzer.net>
> Cc: Brian Starkey <brian.starkey at arm.com>
> ---
>  include/uapi/drm/drm_fourcc.h | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> index 490143500a50..4d3f819dca0b 100644
> --- a/include/uapi/drm/drm_fourcc.h
> +++ b/include/uapi/drm/drm_fourcc.h
> @@ -58,6 +58,28 @@ 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.

I still don't agree with this sentence. ARGB and ABGR have different
compressed layouts in AFBC.

Thanks,
-Brian

> + *
> + * 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
> 
>