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

[Pekka Paalanen]

On Tue, 09 Apr 2024 12:04:07 +0200
Louis Chauvet <louis.chauvet@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> ---
>  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?

> + *
> + * 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.

> + * 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.

> + * 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.

> + * 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.


Thanks,
pq

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