On Wed, Jun 24, 2020 at 1:08 PM Brian Starkey <brian.starkey@xxxxxxx> wrote:
>
> Hi,
>
> On Tue, Jun 23, 2020 at 03:25:08PM +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)
> >
> > 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.
>
> The new paragraph below looks good, but this sentence from the end of
> the paragraph above still needs to be removed:
>
> An example is AFBC, where both ARGB and ABGR have the exact same compressed layout.
I think that entire paragraph was meant to be deleted, the replacement
is the new one below. Otherwise we have 2x the exact same sentence :-)
-Daniel
>
> With that fixed:
>
> Reviewed-by: Brian Starkey <brian.starkey@xxxxxxx>
>
> Thanks,
> -Brian
>
> > + *
> > + * 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
> >
> >
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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