ivbakula / uart2-serdev

UART2

• Simple slave character device on uart2 for raspberry pi 4B

• The device is defined in the device tree. For some odd reason it doesn't work when defined in device tree overlay

• The device driver is built upon serdev bus and currently supports just writing to the device.

• UART2 pins (raspberry pi 4b):

• tx - GPIO 0
• rx - GPIO 1
• cts - GPIO 2
• rts - GPIO 3
• gnd - pin 6, 9, 14, 20, 25, 30, 34, 39

To test this device and module, simply hook up logic analyzer (or arduino) to GPIO pin 0 and GND and run:

$ sudo sh -c 'echo test > /dev/serdev'

Recommended readings:

• <linux_source_root>/Documentation/devicetree/usage-model.rst
• <linux_source_root>/arch/arm/boot/dts/bcm2711.dtsi
• <linux_source_root>/arch/arm/boot/dts/bcm2711-rpi-4-b.dts
• https://elinux.org/Device_Tree_Reference
• https://www.raspberrypi.org/documentation/configuration/device-tree.md

For more on serdev bus, check out these files:

• <linux_source_root>/drivers/tty/serdev/core.c
• <linux_source_root>/include/linux/serdev.h
• <linux_source_root>/drivers/nfc/s3fwrn5/uart.c
• <linux_source_root>/drivers/bluetooth/btmtkuart.c
• <linux_source_root>/drivers/bluetooth/hci_nokia.c
• <linux_source_root>/drivers/nfc/pn533/uart.c

Building on raspberry pi (raspbian buster)

Install dependencies:

$ sudo apt-get install raspberrypi-kernel-headers build-essential bc git wget bison flex libssl-dev make libncurses-dev
$ sudo apt install device-tree-compiler

Obtain raspberri pi kernel:

$ git clone https://github.com/raspberrypi/linux.git

Patch and build device tree

$ cd linux			# cd into raspberry pi linux kernel source tree
$ git branch uart
$ git checkout uart
$ git am uart2_device.diff	# apply patch ./uart2_device.diff
$ make bcm2711_defconfig
$ make dtbs			# build device tree

Install newly built device tree:

$ sudo cp arch/arm/boot/dts/bcm2711-rpi-4-b.dtb /boot/bcm2711-uart2-slave-rpi-4-b.dtb

Add this line to /boot/config.txt

device_tree = bcm2711-uart2-slave-rpi-4-b.dtb

TODO

• reading from the device
• set baud at runtime
• hook it up to some real hardware