Re: [PATCH 3/3] drm/fourcc: Add documentation around drm_format_info

Pekka Paalanen
2024-04-17T11:44:11Z

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On Wed, 17 Apr 2024 00:30:58 +0200
Louis Chauvet <louis.chauvet@bootlin.com> wrote:

> Le 15/04/24 - 15:00, Pekka Paalanen a écrit :
> > On Tue, 09 Apr 2024 12:04:07 +0200
> > Louis Chauvet <louis.chauvet@bootlin.com> wrote:
> >
> > > Let's provide more details about the drm_format_info structure because
> > > its content may not be straightforward for someone not used to video
> > > formats and drm internals.
> > >
> > > Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
> > > ---
> > > include/drm/drm_fourcc.h | 45 ++++++++++++++++++++++++++++++++++++++-------
> > > 1 file changed, 38 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/include/drm/drm_fourcc.h b/include/drm/drm_fourcc.h
> > > index ccf91daa4307..66cc30e28f79 100644
> > > --- a/include/drm/drm_fourcc.h
> > > +++ b/include/drm/drm_fourcc.h
> > > @@ -58,6 +58,44 @@ struct drm_mode_fb_cmd2;
> > >
> > > /**
> > > * struct drm_format_info - information about a DRM format
> > > + *
> > > + * A drm_format_info describes how planes and pixels are stored in memory.
> > > + *
> > > + * Some format like YUV can have multiple planes, counted in @num_planes. It
> > > + * means that a full pixel can be stored in multiple non-continuous buffers.
> > > + * For example, NV12 is a YUV format using two planes: one for the Y values and
> > > + * one for the UV values.
> > > + *
> > > + * On each plane, the "pixel" unit can be different in case of subsampling. For
> > > + * example with the NV12 format, a pixel in the UV plane is used for four pixels
> > > + * in the Y plane.
> > > + * The fields @hsub and @vsub are the relation between the size of the main
> > > + * plane and the size of the subsampled planes in pixels:
> > > + * plane[0] width = hsub * plane[1] width
> > > + * plane[0] height = vsub * plane[1] height
> >
> > This makes it sound like plane[1] would be the one determining the
> > image size. It is plane[0] that determines the image size (I don't know
> > of a format that would have it otherwise), and vsub and hsub are used
> > as divisors. It's in their name, too: horizontal/vertical sub-sampling.
> >
> > This is important for images with odd dimensions. If plane[1]
> > determined the image size, it would be impossible to have odd sized
> > NV12 images, for instance.
> >
> > Odd dimensions also imply something about rounding the size of the
> > sub-sampled planes. I guess the rounding is up, not down?
>
> I will change the equation to:
>
> plane[1] = plane[0] / hsub (round up)
>
> Can a DRM maintainer confirm the rounding up?
>
> > > + *
> > > + * In some formats, pixels are not independent in memory. It can be a packed
> >
> > "Independent in memory" sounds to me like it describes sub-sampling:
> > some pixel components are shared between multiple pixels. Here you seem
> > to refer to just packing: one pixel's data may take a fractional number
> > of bytes.
>
> * In some formats, pixels are not individually addressable. It ...
>
> > > + * representation to store more pixels per byte (for example P030 uses 4 bytes
> > > + * for three 10 bit pixels). It can also be used to represent tiled formats,
> >
> > s/tiled/block/
> >
> > Tiling is given by format modifiers rather than formats.
>
> Fixed in the v2.
>
> > > + * where a continuous buffer in memory can represent a rectangle of pixels (for
> > > + * example, in DRM_FORMAT_Y0L0, a buffer of 8 bytes represents a 2x2 pixel
> > > + * region of the picture).
> > > + * The field @char_per_block is the size of a block on a specific plane, in
> > > + * bytes.
> > > + * The fields @block_w and @block_h are the size of a block in pixels.
> > > + *
> > > + * The older format representation (which only uses @cpp, kept for historical
> >
> > Move the paren to: representation which only uses @cpp (kept
> >
> > so that the sentence is still understandable if one skips the
> > parenthesised part.
>
> Fixed in v2.
>
> > > + * reasons because there are a lot of places in drivers where it's used) is
> > > + * assuming that a block is always 1x1 pixel.
> > > + *
> > > + * To keep the compatibility with older format representations and treat block
> > > + * and non-block formats in the same way one should use:
> > > + * - @char_per_block to access the size of a block on a specific plane, in
> > > + * bytes.
> > > + * - drm_format_info_block_width() to access the width of a block of a
> > > + * specific plane, in pixels.
> > > + * - drm_format_info_block_height() to access the height of a block of a
> > > + * specific plane, in pixels.
> > > */
> > > struct drm_format_info {
> > > /** @format: 4CC format identifier (DRM_FORMAT_*) */
> > > @@ -97,13 +135,6 @@ struct drm_format_info {
> > > * formats for which the memory needed for a single pixel is not
> > > * byte aligned.
> > > *
> > > - * @cpp has been kept for historical reasons because there are
> > > - * a lot of places in drivers where it's used. In drm core for
> > > - * generic code paths the preferred way is to use
> > > - * @char_per_block, drm_format_info_block_width() and
> > > - * drm_format_info_block_height() which allows handling both
> > > - * block and non-block formats in the same way.
> > > - *
> > > * For formats that are intended to be used only with non-linear
> > > * modifiers both @cpp and @char_per_block must be 0 in the
> > > * generic format table. Drivers could supply accurate
> > >
> >
> > Other than that, sounds fine to me.
> >
> > Perhaps one thing to clarify is that chroma sub-sampling and blocks are
> > two different things. Chroma sub-sampling is about the resolution of
> > the chroma (image). Blocks are about packing multiple pixels' components
> > into a contiguous addressable block of memory. Blocks could appear
> > inside a separate sub-sampled UV plane, for example.
>
> Is this clear? i will add it just before "In some formats,
> pixels...
>
> * Chroma subsamping (hsub/vsub) must not be confused with pixel blocks. The
> * first describe the relation between the resolution of each color components
> * (for YUV format, the relation between the "y" resolution and the "uv"
> * resolution), the second describe the way to pack multiple pixels into one
> * contiguous block of memory (for example, DRM_FORMAT_Y0L0, one block is 2x2
> * pixels).

A different example would be better, e.g. DRM_FORMAT_R2, because chroma
sub-sampling too can share U and V for a 2x2 set of pixels. R2 is in
the RGB family, so chroma sub-sampling does not even apply.

Yes, sounds fine.

Thanks,
pq

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