Re: [PATCH v4 02/11] drm/fourcc: Add DRM_FORMAT_XV15/XV20

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Tomi,

Thank you for the patch.

On Wed, Mar 26, 2025 at 03:22:45PM +0200, Tomi Valkeinen wrote:
> Add two new pixel formats:
> 
> DRM_FORMAT_XV15 ("XV15")
> DRM_FORMAT_XV20 ("XV20")
> 
> The formats are 2 plane 10 bit per component YCbCr, with the XV15 2x2
> subsampled whereas XV20 is 2x1 subsampled.
> 
> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> ---
>  drivers/gpu/drm/drm_fourcc.c  | 6 ++++++
>  include/uapi/drm/drm_fourcc.h | 8 ++++++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> index 2f5781f5dcda..e101d1b99aeb 100644
> --- a/drivers/gpu/drm/drm_fourcc.c
> +++ b/drivers/gpu/drm/drm_fourcc.c
> @@ -346,6 +346,12 @@ const struct drm_format_info *__drm_format_info(u32 format)
>  		{ .format = DRM_FORMAT_P030,            .depth = 0,  .num_planes = 2,
>  		  .char_per_block = { 4, 8, 0 }, .block_w = { 3, 3, 0 }, .block_h = { 1, 1, 0 },
>  		  .hsub = 2, .vsub = 2, .is_yuv = true},
> +		{ .format = DRM_FORMAT_XV15,		.depth = 0,  .num_planes = 2,
> +		  .char_per_block = { 4, 8, 0 }, .block_w = { 3, 3, 0 }, .block_h = { 1, 1, 0 },
> +		  .hsub = 2, .vsub = 2, .is_yuv = true },
> +		{ .format = DRM_FORMAT_XV20,		.depth = 0,  .num_planes = 2,
> +		  .char_per_block = { 4, 8, 0 }, .block_w = { 3, 3, 0 }, .block_h = { 1, 1, 0 },
> +		  .hsub = 2, .vsub = 1, .is_yuv = true },

It appears we can never have too much (or enough) documentation, as
reading the format info documentation leaves me with unanswered
questions.

Looking at drm_format_info_min_pitch():

uint64_t drm_format_info_min_pitch(const struct drm_format_info *info,
				   int plane, unsigned int buffer_width)
{
	if (!info || plane < 0 || plane >= info->num_planes)
		return 0;

	return DIV_ROUND_UP_ULL((u64)buffer_width * info->char_per_block[plane],
			    drm_format_info_block_width(info, plane) *
			    drm_format_info_block_height(info, plane));
}

For the first plane, the function will return `buffer_width * 4 / 3`
(rouding up), which I think is right. For the second plane, it will
return `buffer_width * 8 / 3`, which I believe is wrong as the format is
subsampled by a factor 2 horizontally. It seems that either
char_per_block and block_w need to take horizontal subsampling into
account (and therefore be 8 and 6 for the second plane), or
drm_format_info_min_pitch() should consider .hsub. Or there's something
else I'm missing :-)

>  	};
>  
>  	unsigned int i;
> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> index 81202a50dc9e..1247b814bd66 100644
> --- a/include/uapi/drm/drm_fourcc.h
> +++ b/include/uapi/drm/drm_fourcc.h
> @@ -304,6 +304,14 @@ extern "C" {
>  #define DRM_FORMAT_RGB565_A8	fourcc_code('R', '5', 'A', '8')
>  #define DRM_FORMAT_BGR565_A8	fourcc_code('B', '5', 'A', '8')
>  
> +/*
> + * 2 plane 10 bit per component YCrCb
> + * index 0 = Y plane, [31:0] x:Y2:Y1:Y0 2:10:10:10 little endian
> + * index 1 = Cb:Cr plane, [63:0] x:Cr2:Cb2:Cr1:x:Cb1:Cr0:Cb0 2:10:10:10:2:10:10:10 little endian

I believe this is right, but I have a hard time validating it, as I
think the corresponding figures in UG1085 are incorrect (they show a
8bpp format as far as I can tell). Do I assume correctly that you've
tested the formats ?

> + */
> +#define DRM_FORMAT_XV15		fourcc_code('X', 'V', '1', '5') /* 2x2 subsampled Cr:Cb plane 2:10:10:10 */
> +#define DRM_FORMAT_XV20		fourcc_code('X', 'V', '2', '0') /* 2x1 subsampled Cr:Cb plane 2:10:10:10 */
> +
>  /*
>   * 2 plane YCbCr
>   * index 0 = Y plane, [7:0] Y

-- 
Regards,

Laurent Pinchart