Hi Pekka,

On Thu, Feb 17, 2022 at 11:10 AM Pekka Paalanen [off-list ref] wrote:

On Tue, 15 Feb 2022 17:52:26 +0100
Geert Uytterhoeven [off-list ref] wrote:

quoted

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@linux-m68k.org>

quoted

--- 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 */
+

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

Right, will do.

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.

C1 is color-indexed, which can be any two colors.
R1 is light-on-dark.
D1 is dark-on-light.

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?

That's an option.  The repaper driver does that:

    drm_fb_xrgb8888_to_gray8(buf, 0, cma_obj->vaddr, fb, &clip);
    repaper_gray8_to_mono_reversed(buf, fb->width, fb->height);

Can drm_framebuffer objects be backed by graphics memory, i.e.
can they be displayed without copying?

quoted

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

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds