Hi, To my suprise I have discovered that the MIPI DBI specification does only list RGB111 as a pixel format for the Serial Interface (Type C). For the parallel interface type (Type A and B) it lists: RGB332, RGB444, RGB565, RGB666 and RGB888. I have never read the specification closely enough to discover this always assuming that the datasheets for the various MIPI DBI compatible controllers I've looked at over the years did follow the specification when they supported RGB565 and RGB666 on the serial interface. So it is quite clear that the industry has chosen to extend the standard and provide support for more pixel formats over the serial interface. drm_mipi_dbi and its predecessor fbtft support only RGB565 over SPI since RGB666 uses 3 bytes per pixel instead of 2 bytes, severly impacting the framerate. The reason I started to look at this is that there seem to be an increase in cheap SPI displays that is based on the ILI9488 controller. The datasheet for this controller states that it supports RGB565, but experience[1] shows that this is not true and that the controller only supports RGB666 over SPI. I have known for some time that the ILI9486 controller does not support RGB565 over SPI, it only supports RGB111 and RGB666. Some display breakoutboard manufacturers have solved this by putting a shift register in front of the parallel bus on this controller in order to support RGB565. This requires some custom code when writing to the SPI bus as shown in the tiny/ili9486.c driver. The downside is that these displays are really slow due to the slow shift registers used. This patchset documents the defacto industry standard wrt pixel formats over SPI and adds support for RGB666 in the panel-mipi-dbi driver. There have been two previous attempts to add a DRM driver for ili9488[2][3]. The panel-mipi-dbi driver is a generic MIPI DBI driver supporting controller initialization via a firmware file and with the help of this patchset it will support ILI9488 based SPI displays. [1] https://github.com/notro/panel-mipi-dbi/issues/2#issuecomment-2016857690 [2] https://lore.kernel.org/dri-devel/cover.1592055494.git.kamlesh.gurudasani@xxxxxxxxx/ [3] https://lore.kernel.org/dri-devel/20221018164532.1705215-1-tommaso.merciai@xxxxxxxxxxxxxxxxxxxx/ Signed-off-by: Noralf Trønnes <noralf@xxxxxxxxxxx> --- Changes in v2: - binding: Use 'default: r5g6b5' (Rob) - Link to v1: https://lore.kernel.org/r/20240507-panel-mipi-dbi-rgb666-v1-0-6799234afa3e@xxxxxxxxxxx --- Noralf Trønnes (5): dt-bindings: display: panel: mipi-dbi-spi: Add a pixel format property drm/mipi-dbi: Remove mipi_dbi_machine_little_endian() drm/mipi-dbi: Make bits per word configurable for pixel transfers drm/mipi-dbi: Add support for DRM_FORMAT_RGB888 drm/tiny: panel-mipi-dbi: Support the pixel format property .../bindings/display/panel/panel-mipi-dbi-spi.yaml | 30 +++++++++ drivers/gpu/drm/drm_mipi_dbi.c | 76 +++++++++++++++------- drivers/gpu/drm/tiny/panel-mipi-dbi.c | 55 +++++++++++++++- include/drm/drm_mipi_dbi.h | 10 +++ 4 files changed, 147 insertions(+), 24 deletions(-) --- base-commit: 0209df3b4731516fe77638bfc52ba2e9629c67cd change-id: 20240405-panel-mipi-dbi-rgb666-4e033787d6c9 Best regards, -- Noralf Trønnes <noralf@xxxxxxxxxxx>
