A10M Display with 12-Pin Header

The printer was supplied with Marlin firmware, and the display worked correctly.
However, after switching to Klipper, the display stopped working. I even found that I’m not the first:
How to diagnose why the Geeetech A10X display shows nothing

I initially thought it was just a matter of pin definitions, so I decided to map the pins from Marlin firmware to Klipper firmware.
Using the board configuration for Geeetech A10 V4.1 and the mapping from Arduino pin numbers (e.g., 1) to AVR ports (e.g., E1)

We get the following for YHCB2004:

#define in Marlin	Arduino #	AVR port	Schematic NET	H2 pin	Reality
YHCB2004_CLK	5	PE3	LCM_D6	4	CS
YHCB2004_MOSI	21	PD0	LCM_D5	5	CLK
YHCB2004_MISO	36	PC1	LCM_D7	3	MOSI
BTN_EN1	16	PH1	LCM_D4	7	BTN_EN1
BTN_EN2	17	PH0	LCM_EN	8	BTN_EN2
BTN_ENC	19	PD2	EC_PRESS	9	BTN_ENC
			RESET	11
			BEEP	12
			V5	1
			GND	2,6,10

According to the source code, the display is based on the
AiP31068
and uses the following library for communication:
LiquidCrystal_AIP31068

In summary, the pin definitions are incorrect in Marlin firmware (they were reordered to make it work under the wrong names). See the issue in MarlinFirmware.

To make it work in Klipper firmware, it was necessary to send commands in a 9-bit format as required by the AIP31068, but the existing driver only supported 8 bits.
I solved the issue by creating a flexible SPI driver that supports any number of bits. However, it wasn’t accepted due to the risks of changing a core driver, especially since this display is rare. Kevin (the author of Klipper) proposed grouping eight 9-bit commands into nine bytes of data and sending them at once.
This solution worked, and a new driver was created and merged into Klipper.