Display loses image after data transfer when DC pin remains high

[R0cketRacc00n]

Description:
I'm trying to use an Anycubic Kobra Neo display with Raspberry Pi OS (bookworm) on a Raspberry Pi. The display shows an image only during data transfer when the DC pin is high, but the image disappears after transfer completion when DC remains high.

Current Behavior:

Image appears only during active data transmission when DC = 1

Image disappears immediately after data transfer ends if DC pin remains high

DC pin stays high after transmission under current driver configuration

Expected Behavior:

Display should maintain the image after data transfer completes

DC pin should return to low state after data transmission

Hardware Configuration:

Display: Anycubic Kobra Neo (320x240)

Connection: SPI via MIPI DBI

DC GPIO: 24

Reset GPIO: 25

[R0cketRacc00n]

Just one additional line in the drm_mipi_dbi.c file solved the problem. Please, if possible, review and accept this change.

static int mipi_dbi_typec3_command(struct mipi_dbi *dbi, u8 *cmd,
				   u8 *par, size_t num)
{
	struct spi_device *spi = dbi->spi;
	unsigned int bpw = 8;
	u32 speed_hz;
	int ret;

	if (mipi_dbi_command_is_read(dbi, *cmd))
		return mipi_dbi_typec3_command_read(dbi, cmd, par, num);

	MIPI_DBI_DEBUG_COMMAND(*cmd, par, num);

	spi_bus_lock(spi->controller);
	gpiod_set_value_cansleep(dbi->dc, 0);
	speed_hz = mipi_dbi_spi_cmd_max_speed(spi, 1);
	ret = mipi_dbi_spi_transfer(spi, speed_hz, 8, cmd, 1);
	spi_bus_unlock(spi->controller);
	if (ret || !num)
		return ret;

	if (*cmd == MIPI_DCS_WRITE_MEMORY_START)
		bpw = dbi->write_memory_bpw;

	spi_bus_lock(spi->controller);
	gpiod_set_value_cansleep(dbi->dc, 1);
	speed_hz = mipi_dbi_spi_cmd_max_speed(spi, num);
	ret = mipi_dbi_spi_transfer(spi, speed_hz, bpw, par, num);
	spi_bus_unlock(spi->controller);

	// *** ADDED LINE: Reset DC line to low (command mode) after data transfer ***
       gpiod_set_value_cansleep(dbi->dc, 0);

	return ret;
}

[robertojguerra]

Your fix is specific for your LCD display (which is not even a product for sale), and the code is already in the kernel tree. It would be hard to merge it the way you want.

... it would be best that you prepare a script to fix your kernel module automatically when you upgrade the kernel :)

Are you going to display images/video with DRM or FB directly... or with X11 or Wayland?

[R0cketRacc00n]

Your fix is specific for your LCD display (which is not even a product for sale), and the code is already in the kernel tree. It would be hard to merge it the way you want.

To the best of my knowledge, the standards specify the state of the DC pin before and during the transmission of a command or data. However, changing the state of the DC pin after the transmission is not explicitly regulated and does not violate the standards. Similarly, sending a NOP command (0x00) after transmitting a frame of data to bring the DC pin low is also compliant with the standards.

Regarding the peculiarity of my display, I suspect the manufacturer designed it this way to combine the functions of the DC and LED pins, as keeping the DC pin at a high level causes the backlight to turn off.

... it would be best that you prepare a script to fix your kernel module automatically when you upgrade the kernel :)

I would be very grateful if you could kindly advise me on how to recompile only the panel-mipi-dbi module separately. Performing a full kernel rebuild on my RPi Zero 2W takes an impractically long time.

Are you going to display images/video with DRM or FB directly... or with X11 or Wayland?

And yes, I plan to output the image using X11 or Wayland.

[robertojguerra]

In the PC you build armbian, make sure that the panel-mipi-dbi kernel config is already set to "M" and the ".config" is saved.
Find the folder to add "userpatches", as seen in this guide:

https://github.com/robertojguerra/orangepi-zero-full-setup/blob/main/README2.md#73-add-the-tv-encoder-driver-to-the-linux-kernel-source

Make a patch file, representing the c code line you added, in the userpatch folder.
$ diff3 drm-mipi-dbi.c drm-mipi-dbi.c.new > drm-mipi-dbi.c.patch

Run armbian build again. Watch for the message that it accepted your patch file.

Hopefully, it will only compile the changed C file, or it may compile all the kernel. Find the resulting panel-mipi-dbi.ko (I can't remember the location now), and copy it to your orange pi zero /lib/modules/..., to replace the original ko file.

Install the ko normally, or force it.

https://unix.stackexchange.com/questions/499022/how-to-force-load-kernel-module-modprobe-f-on-startup

Please tell me if you can make X11 work.

Update: sorry I thought you were using armbiam. I don't know how to recompile easily in Raspberry