On Tue, 15 Feb 2022 17:52:19 +0100
Geert Uytterhoeven <geert@linux-m68k.org> wrote:

> Introduce fourcc codes for color-indexed frame buffer formats with two,
> four, and sixteen color, and provide a suitable mapping from bit per
> pixel and depth to fourcc codes.
> 
> As the number of bits per pixel is less than eight, these rely on proper
> block handling for the calculation of bits per pixel and pitch.
> 
> Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
> ---
> Do we want to keep the rounding down if depth < bpp, or insist on depth
> == bpp? I don't think the rounding down will still be needed after
> "[PATCH 4/8] drm/client: Use actual bpp when allocating frame buffers".
> ---
>  drivers/gpu/drm/drm_fourcc.c  | 18 ++++++++++++++++++
>  include/uapi/drm/drm_fourcc.h |  3 +++
>  2 files changed, 21 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> index 07741b678798b0f1..60ce63d728b8e308 100644
> --- a/drivers/gpu/drm/drm_fourcc.c
> +++ b/drivers/gpu/drm/drm_fourcc.c
> @@ -46,6 +46,18 @@ uint32_t drm_mode_legacy_fb_format(uint32_t bpp, uint32_t depth)
>  	case 8:
>  		if (depth == 8)
>  			fmt = DRM_FORMAT_C8;
> +		fallthrough;
> +	case 4:
> +		if (depth == 4)
> +			fmt = DRM_FORMAT_C4;
> +		fallthrough;
> +	case 2:
> +		if (depth == 2)
> +			fmt = DRM_FORMAT_C2;
> +		fallthrough;
> +	case 1:
> +		if (depth == 1)
> +			fmt = DRM_FORMAT_C1;
>  		break;
>  
>  	case 16:
> @@ -132,6 +144,12 @@ EXPORT_SYMBOL(drm_driver_legacy_fb_format);
>  const struct drm_format_info *__drm_format_info(u32 format)
>  {
>  	static const struct drm_format_info formats[] = {
> +		{ .format = DRM_FORMAT_C1,		.depth = 1,  .num_planes = 1,
> +		  .char_per_block = { 1, }, .block_w = { 8, }, .block_h = { 1, }, .hsub = 1, .vsub = 1 },
> +		{ .format = DRM_FORMAT_C2,		.depth = 2,  .num_planes = 1,
> +		  .char_per_block = { 1, }, .block_w = { 4, }, .block_h = { 1, }, .hsub = 1, .vsub = 1 },
> +		{ .format = DRM_FORMAT_C4,		.depth = 4,  .num_planes = 1,
> +		  .char_per_block = { 1, }, .block_w = { 2, }, .block_h = { 1, }, .hsub = 1, .vsub = 1 },
>  		{ .format = DRM_FORMAT_C8,		.depth = 8,  .num_planes = 1, .cpp = { 1, 0, 0 }, .hsub = 1, .vsub = 1 },
>  		{ .format = DRM_FORMAT_R8,		.depth = 8,  .num_planes = 1, .cpp = { 1, 0, 0 }, .hsub = 1, .vsub = 1 },
>  		{ .format = DRM_FORMAT_R10,		.depth = 10, .num_planes = 1, .cpp = { 2, 0, 0 }, .hsub = 1, .vsub = 1 },
> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> index fc0c1454d2757d5d..3f09174670b3cce6 100644
> --- a/include/uapi/drm/drm_fourcc.h
> +++ b/include/uapi/drm/drm_fourcc.h
> @@ -99,6 +99,9 @@ extern "C" {
>  #define DRM_FORMAT_INVALID	0
>  
>  /* color index */
> +#define DRM_FORMAT_C1		fourcc_code('C', '1', ' ', ' ') /* [0] C */
> +#define DRM_FORMAT_C2		fourcc_code('C', '2', ' ', ' ') /* [1:0] C */
> +#define DRM_FORMAT_C4		fourcc_code('C', '4', ' ', ' ') /* [3:0] C */

Hi Geert,

generally this looks fine to me though I'm not familiar with the
code. The thing I'm missing here is a more precise description of the
new pixel formats.

>  #define DRM_FORMAT_C8		fourcc_code('C', '8', ' ', ' ') /* [7:0] C */

This description of C8 is a little vague maybe, but presumably one
pixel being one byte, the address of pixel x is just &bytes[x].

C4, C2 and C1 should also specify the pixel order within the byte.
There is some precedent of that in with some YUV formats in this file. 

Maybe something like: 

C2 /* [7:0] c0:c1:c2:c3 2:2:2:2 four pixels per byte */

or the other way around, which ever your ordering is?


Thanks,
pq