[PATCH 0/2] drm/tiny: Add RAiO RA8875 display controller driver support

[Adam Azuddin <azuddinadam@gmail.com>]

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. More critically, the GRAM write path
is stateful: an MRWC register-address transaction followed by a streaming
pixel pump under a single CS assertion. I thought this may be too
complicated to cater towards, but please let me know if this is the
correct approach.

My ER-TFTM050-3 board has RST tied NC, and VDD is a fixed supply
set via an on-board solder jumper (3.3V/5V) rather than software-controllable,
so reset-gpios/vcc-supply are not implemented in either the driver or the binding.
I can add both as optional properties in v2 if reviewers know of RA8875 boards
that do need this.

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

Ran fbtest (https://git.kernel.org/pub/scm/linux/kernel/git/geert/fbtest.git) against the
resulting framebuffer:
  ./fbtest -f /dev/fb1
  Using drawops cfb16 (16 bpp packed pixels)
  Available visuals:
    Monochrome
    Grayscale 32
    Truecolor 5:6:5:0
  Using visops truecolor
  Running all tests
  test001: PASSED
  test002: PASSED
  test003: PASSED
  test004: PASSED
  test005: PASSED
  test006: PASSED
  test008: PASSED
  test009: PASSED
  test010: PASSED
  Benchmarking... 10x10 squares: 74.24 Mpixels/s
  Benchmarking... 20x20 squares: 152.49 Mpixels/s
  Benchmarking... 50x50 squares: 334.09 Mpixels/s
  Benchmarking... 100x100 squares: 591.46 Mpixels/s
  Benchmarking... 200x200 squares: 999.49 Mpixels/s
  test012: PASSED
  Benchmarking... R5 circles: 45.90 Mpixels/s
  Benchmarking... R10 circles: 98.84 Mpixels/s
  Benchmarking... R25 circles: 236.37 Mpixels/s
  Benchmarking... R50 circles: 459.98 Mpixels/s
  Benchmarking... R100 circles: 851.43 Mpixels/s
  test013: PASSED

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   |  66 ++
 .../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                 | 739 ++++++++++++++++++
 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