serial: tty: Add a driver for the RPi firmware UART

On Raspberry Pi 4 and earlier models the firmware provides
a low speed (up to 115200 baud) bit-bashed UART on arbitrary
GPIOs using the second VPU core.

This has only been properly tested at 9600. Higher rates
appear to work but YMMV.

This can be useful for debug or managing additional low
speed peripherals if the hardware PL011 and 8250 hardware
UARTs are already used for console / bluetooth.

The firmware driver requires a fixed core clock frequency
and also requires the VPU PWM audio driver to be disabled
(dtparam=audio=off)

Runtime configuration is handled via the vc-mailbox APIs
with the FIFO buffers being allocated in uncached VPU
addressable memory. The FIFO pointers are stored in spare
VideoCore multi-core sync registers in order to reduce the number
of uncached SDRAM accesses thereby reducing jitter.

Signed-off-by: Tim Gover <tim.gover@raspberrypi.com>

Author: timg236

drivers/tty/serial/Kconfig

@@ -1578,6 +1578,17 @@ config SERIAL_NUVOTON_MA35D1_CONSOLE
 	  but you can alter that using a kernel command line option such as
 	  "console=ttyNVTx".
 
+config SERIAL_RPI_FW
+	tristate "Raspberry Pi Firmware software UART support"
+	depends on ARM_AMBA || COMPILE_TEST
+	select SERIAL_CORE
+	help
+	  This selects the Raspberry Pi firmware UART. This is a bit-bashed
+	  implementation running on the Raspbery Pi VPU core.
+	  This is not supported on Raspberry Pi 5 or newer platforms.
+
+	  If unsure, say N.
+
 endmenu
 
 config SERIAL_MCTRL_GPIO

drivers/tty/serial/Makefile

@@ -88,6 +88,7 @@ obj-$(CONFIG_SERIAL_MILBEAUT_USIO) += milbeaut_usio.o
 obj-$(CONFIG_SERIAL_SIFIVE)	+= sifive.o
 obj-$(CONFIG_SERIAL_LITEUART) += liteuart.o
 obj-$(CONFIG_SERIAL_SUNPLUS)	+= sunplus-uart.o
+obj-$(CONFIG_SERIAL_RPI_FW) += rpi-fw-uart.o
 
 # GPIOLIB helpers for modem control lines
 obj-$(CONFIG_SERIAL_MCTRL_GPIO)	+= serial_mctrl_gpio.o