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.

The firmware driver is designed to support 19200 baud. Higher
rates up to 115200 seem to work but there may be more jitter.

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

@@ -1600,6 +1600,17 @@ config SERIAL_ESP32_ACM
 	  snippet may be used:
 	    earlycon=esp32s3acm,mmio32,0x60038000
 
+config SERIAL_RPI_FW
+	tristate "Raspberry Pi Firmware software UART support"
+	depends on RASPBERRYPI_FIRMWARE || 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

@@ -71,6 +71,7 @@ obj-$(CONFIG_SERIAL_QCOM_GENI)		+= qcom_geni_serial.o
 obj-$(CONFIG_SERIAL_QE)			+= ucc_uart.o
 obj-$(CONFIG_SERIAL_RDA)		+= rda-uart.o
 obj-$(CONFIG_SERIAL_RP2)		+= rp2.o
+obj-$(CONFIG_SERIAL_RPI_FW) 		+= rpi-fw-uart.o
 obj-$(CONFIG_SERIAL_SA1100)		+= sa1100.o
 obj-$(CONFIG_SERIAL_SAMSUNG)		+= samsung_tty.o
 obj-$(CONFIG_SERIAL_SB1250_DUART)	+= sb1250-duart.o