drm/tiny: Add RAiO RA8875 display controller driver support

This series adds a new drm/tiny driver and matching dt-binding for the RAiO RA8875 SPI display controller. An fbtft driver for this chip currently exists at drivers/staging/fbtft/fb_ra8875.c, but fbtft is a legacy fbdev-based staging driver, and nothing in mainline DRM currently supports this controller.

This driver does not use drm_mipi_dbi. The RA8875 SPI protocol is structurally incompatible with both Type C options drm_mipi_dbi supports. Type C1 encodes command/data as a 9th bit within a 9-bit SPI word. The RA8875 uses a full prefix byte (0x80 = command write, 0x00 = data write, 0x40 = data read, 0xC0 = status read) sent as a separate 8-bit transaction. These are not the same thing and cannot be mapped to each other. Type C3 models two transfer types via a D/CX GPIO. The RA8875 has four transfer cycle types and no D/CX pin.

My ER-TFTM050-3 board has RST tied NC, and VDD is a fixed supply set via an on-board solder jumper rather than software-controllable, so I couldn't test reset-gpios/vdd-supply myself, but added them as optional/required properties respectively per review feedback for boards that do need them.

Tested on a Raspberry Pi 3B connected over SPI to an East Rising ER-TFTM050-3, a 5" 800x480 panel driven by the RA8875 in Linux 7.3.0-rc4.

Link to v1: https://lore.kernel.org/all/[email protected]/
Link to v2: https://lore.kernel.org/all/[email protected]/
Link to v3: https://lore.kernel.org/all/[email protected]/
Link to v4: https://lore.kernel.org/all/[email protected]/
Link to v5: https://lore.kernel.org/all/[email protected]/
---
Changes in v6:

Documentation/devicetree/bindings/display/panel/raio,ra8875.yaml:
- Drop Assisted-by tag.
- Remove sentence referencing driver polling from interrupt description.

drivers/gpu/drm/tiny/ra8875.c:
- use drm_atomic_helper_connector_create_state instead of drm_atomic_helper_connector_reset.

Changes in v5:

Documentation/devicetree/bindings/display/panel/raio,ra8875.yaml:
- Drop the "Add YAML binding schema..." paragraph from the commit message (Krzysztof)
- Add interrupts property for the INT# pin

drivers/gpu/drm/tiny/ra8875.c:
- Move bindings table to above ra8875_spi_driver definition

Changes in v4:

Documentation/devicetree/bindings/display/panel/raio,ra8875.yaml:
- Drop the sentence in the description saying the binding covers only the SPI-connected device (Conor).
- Drop unused panel label

drivers/gpu/drm/tiny/ra8875.c:
- Catch error value from ra8875_write_pixels
- Use consistent include header format.
- Return if fb is null in atomic_update
- Drop unnecessary extra byte in pixel data spi write.

drivers/gpu/drm/tiny/Kconfig:
- Add depends on OF, which is caught by intel 0-day[3]

Changes in v3:

Documentation/devicetree/bindings/display/panel/raio,ra8875.yaml:
- Add gpio.h include in example to pass dt_binding_check
- Remove spi-max-frequency as there is no reference in datasheet

drivers/gpu/drm/tiny/ra8875.c:
- Remove debug-only ra8875_read_reg function
- Chunk large SPI transfers to 16 rows per chunk
- Remove unnecessary includes and spi cs setups in probe
- Create ra8875_write_pixels function for clear_screen and fb_dirty
- Use 2 spi writes instead of 1 write with 2 transfers and cs_change in write_reg

Changes in v2:

Documentation/devicetree/bindings/display/panel/raio,ra8875.yaml (review by Krzysztof Kozlowski):
- Add hardware description to the binding
- Drop the display-timings property, and instead require it in the required block
- Add a required vdd-supply and an optional reset gpio property
- Rename the example node to a more generic "panel: display@0"

drivers/gpu/drm/tiny/Makefile and Kconfig:
- Remove TINYDRM prefix from the config name (Thomas Zimmermann [1])

drivers/gpu/drm/tiny/ra8875.c:
- Reformat device_id table to match current convention (Krzysztof Kozlowski)
- Drop unneeded linux/mod_devicetable.h include (Uwe Kleine-König)
- Use drm_dev_unplug() instead of drm_dev_unregister() in remove callback
- Move stack-allocated cmd/data buffers into ra8875_device struct
- Add mode_valid() callback to validate height/width against panel maximum
- Add optional reset gpio
- Use dev_err_probe() in probe function
- Remove connector detect callback
- Remove unnecessary |= on register write return value in hw_init
- Migrate from simple display pipe to manually-handled plane/crtc/encoder, following the simple-KMS-helper removal effort [2]
- Use defined constants instead of magic values where possible
- Use drm_atomic_helper_damage_merged() and drm_fb_swab() in plane update

[1] https://lore.kernel.org/all/[email protected]/
[2] https://lore.kernel.org/all/20260705-drm-simple-kms-removal-v1-0-b4e1ca053623@oss.qualcomm.com/
[3] https://lore.kernel.org/all/[email protected]/

Adam Azuddin (2):
dt-bindings: display: panel: Add RAiO RA8875 display controller drm/tiny: Add RAiO RA8875 display controller driver

 .../bindings/display/panel/raio,ra8875.yaml   |  86 +++
 .../devicetree/bindings/vendor-prefixes.yaml  |   2 +
 MAINTAINERS                                   |   6 +
 drivers/gpu/drm/tiny/Kconfig                  |  13 +
 drivers/gpu/drm/tiny/Makefile                 |   1 +
 drivers/gpu/drm/tiny/ra8875.c                 | 719 ++++++++++++++++++
 6 files changed, 827 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/display/panel/raio,ra8875.yaml
 create mode 100644 drivers/gpu/drm/tiny/ra8875.c

2.55.0