linux-kernel - Re: [PATCH 8/8] drm/fourcc: Add DRM_FORMAT

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Date:   Thu, 17 Feb 2022 12:10:33 +0200
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, Helge Deller <deller@....de>,
        Javier Martinez Canillas <javierm@...hat.com>,
        linux-fbdev@...r.kernel.org, linux-m68k@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/8] drm/fourcc: Add DRM_FORMAT_D1

On Tue, 15 Feb 2022 17:52:26 +0100
Geert Uytterhoeven <geert@...ux-m68k.org> wrote:

> Introduce a fourcc code for a single-channel frame buffer format with two
> darkness levels.  This can be used for two-level dark-on-light displays.
> 
> As the number of bits per pixel is less than eight, this relies on
> proper block handling for the calculation of bits per pixel and pitch.
> 
> Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> ---
>  drivers/gpu/drm/drm_fourcc.c  | 2 ++
>  include/uapi/drm/drm_fourcc.h | 3 +++
>  2 files changed, 5 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> index c12e48ecb1ab8aad..d00ce5d8d1fb9dd3 100644
> --- a/drivers/gpu/drm/drm_fourcc.c
> +++ b/drivers/gpu/drm/drm_fourcc.c
> @@ -151,6 +151,8 @@ const struct drm_format_info *__drm_format_info(u32 format)
>  		{ .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_D1,		.depth = 1,  .num_planes = 1,
> +		  .char_per_block = { 1, }, .block_w = { 8, }, .block_h = { 1, }, .hsub = 1, .vsub = 1 },
>  		{ .format = DRM_FORMAT_R1,		.depth = 1,  .num_planes = 1,
>  		  .char_per_block = { 1, }, .block_w = { 8, }, .block_h = { 1, }, .hsub = 1, .vsub = 1 },
>  		{ .format = DRM_FORMAT_R2,		.depth = 2,  .num_planes = 1,
> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> index 8605a1acc6813e6c..c15c6efcc65e5827 100644
> --- a/include/uapi/drm/drm_fourcc.h
> +++ b/include/uapi/drm/drm_fourcc.h
> @@ -104,6 +104,9 @@ extern "C" {
>  #define DRM_FORMAT_C4		fourcc_code('C', '4', ' ', ' ') /* [3:0] C */
>  #define DRM_FORMAT_C8		fourcc_code('C', '8', ' ', ' ') /* [7:0] C */
>  
> +/* 1 bpp Darkness */
> +#define DRM_FORMAT_D1		fourcc_code('D', '1', ' ', ' ') /* [0] D */
> +

Hi Geert,

the same comment here as for C1 and R1 formats, need to specify pixel
ordering inside a byte.

I think it would also be good to explain the rationale why C1 and R1
are not suitable for this case and we need yet another 1-bit format in
the commit message.

For posterity, of course. I roughly remember the discussions.

I also wonder if anyone would actually use D1. Should it be added
anyway? There is no rule that a pixel format must be used inside the
kernel AFAIK, but is there even a prospective userspace wanting this?

Exposing R1 and inverting bits while copying to hardware might be
enough?


Thanks,
pq

>  /* 1 bpp Red */
>  #define DRM_FORMAT_R1		fourcc_code('R', '1', ' ', ' ') /* [0] R */
>  


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