[PATCH v3 00/10] drm: Add support for low-color frame buffer formats

Geert Uytterhoeven
2022-07-08T18:21:37Z

Hi all,

A long outstanding issue with the DRM subsystem has been the lack of
support for low-color displays, as used typically on older desktop
systems, and on small embedded displays.

This patch series adds support for color-indexed frame buffer formats
with 2, 4, and 16 colors. It has been tested on ARAnyM using a
work-in-progress Atari DRM driver supporting 2, 4, 16, 256, and 65536
colors, with text console operation, fbtest, and modetest.

Overview:
- Patch 1 introduces a helper, to be used by later patches in the
series,
- Patch 2 introduces a flag to indicate color-indexed formats,
- Patches 3 and 4 correct calculations of bits per pixel for sub-byte
pixel formats,
- Patches 5 and 6 introduce the new C[124] formats,
- Patch 7 fixes an untested code path,
- Patch 8 documents the use of "red" for light-on-dark displays,
- Patches 9 and 10 add more fourcc codes for light-on-dark and
dark-on-light frame buffer formats, which may be useful for e.g. the
ssd130x and repaper drivers.

Changes compared to v2[1]:
- Add Reviewed-by,
- Document fill order,
- Fix FB_VISUAL_TRUECOLOR,
- Replace light-on-dark/dark-on-light by direct/inverse relationship
between channel value and brightness.

Changes compared to v1[2]:
- Reshuffle patches,
- New patch "[PATCH v2 02/10] drm/fourcc: Add
drm_format_info.is_color_indexed flag",
- Improve pixel descriptions,
- Require depth to match bpp in drm_mode_legacy_fb_format(),
- Set .is_color_indexed flag.
- Use drm_format_info_bpp() helper instead of deprecated .depth field
or format-dependent calculations,
- Use new .is_color_indexed field instead of checking against a list
of formats,
- Add Acked-by,
- Replace FIXME by TODO comment,
- New patch "[PATCH v2 08/10] [RFC] drm/fourcc: Document that
single-channel "red" can be any color",
- Add rationale for adding new formats,
- Add D[248] for completeness.

Notes:
- This is the first patch series in a series of 3:
- To make high-color modes work on big-endian, you also need [3],
- To make mode selection on the command line work for Atari video
modes, you need [4].
- There is also a related series of 3 patch series for modetest:
- Using modetest with low-color formats (C[124]) requires [5],
- Using modetest with high-color formats (RG16, XR24) requires
[6],
- Using modetest with video mode names containing dashes requires
[7].
- As this was used on emulated hardware only, and I do not have Atari
hardware, I do not have performance figures to compare with fbdev.
I hope to do proper measuring with an Amiga DRM driver, eventually.
- While the Atari DRM driver is not fit for submission yet, you can
find it at [8], if you are adventurous.

Thanks for your comments!

[1] "[PATCH v2 00/10] drm: Add support for low-color frame buffer
formats"
https://lore.kernel.org/r/cover.1646683502.git.geert@linux-m68k.org/
[2] "[PATCH 0/8] drm: Add support for low-color frame buffer formats"
https://lore.kernel.org/r/20220215165226.2738568-1-geert@linux-m68k.org/
[3] "[PATCH 0/3] drm: Endianness fixes"
https://lore.kernel.org/r/cover.1657300532.git.geert@linux-m68k.org
[4] "[PATCH 0/5] drm/modes: Command line mode selection fixes and
improvements"
https://lore.kernel.org/r/cover.1657301107.git.geert@linux-m68k.org
[5] "[PATCH libdrm v2 00/10] Add support for low-color frame buffer
formats"
https://lore.kernel.org/r/cover.1657302034.git.geert@linux-m68k.org
[6] "[PATCH libdrm v2 00/10] Big-endian fixes"
https://lore.kernel.org/r/cover.1657302103.git.geert@linux-m68k.org
[7] "[PATCH libdrm] modetest: Add support for named modes containing
dashes"
[8] https://git.kernel.org/pub/scm/linux/kernel/git/geert/linux-m68k.git/log/?h=atari-drm-wip

Geert Uytterhoeven (10):
drm/fourcc: Add drm_format_info_bpp() helper
drm/fourcc: Add drm_format_info.is_color_indexed flag
drm/client: Use actual bpp when allocating frame buffers
drm/framebuffer: Use actual bpp for DRM_IOCTL_MODE_GETFB
drm/fourcc: Add DRM_FORMAT_C[124]
drm/fb-helper: Add support for DRM_FORMAT_C[124]
drm/gem-fb-helper: Use actual bpp for size calculations
drm/fourcc: Clarify the meaning of single-channel "red"
[RFC] drm/fourcc: Add DRM_FORMAT_R[124]
[RFC] drm/fourcc: Add DRM_FORMAT_D[1248]

drivers/gpu/drm/drm_client.c | 4 +-
drivers/gpu/drm/drm_fb_helper.c | 101 ++++++++++++++-----
drivers/gpu/drm/drm_fourcc.c | 55 +++++++++-
drivers/gpu/drm/drm_framebuffer.c | 2 +-
drivers/gpu/drm/drm_gem_framebuffer_helper.c | 12 +--
include/drm/drm_fourcc.h | 4 +
include/uapi/drm/drm_fourcc.h | 32 +++++-
7 files changed, 167 insertions(+), 43 deletions(-)

--
2.25.1

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