This repository includes a FreeRTOS UART sample application which can run on Raspberry Pi 4B.
This FreeRTOS porting uses UART2(PL011). The sample application is designed to be launched by u-boot and to operate together with 64-bit Linux.
Implementation is based on another FreeRTOS porting for Raspberry Pi 3 by eggman [1] (many thanks to him!).
The sample application runs on the CPU core #3 on your Raspberry Pi 4B board. A specified memory region (0x20000000 - 0x207FFFFF) is dedicated to the sample application. Modify FreeRTOS/Demo/CORTEX_A72_64-bit_Raspberrypi4/uart/src/raspberrypi4.ld if you want to change the memory usage.
[1] https://github.com/eggman/FreeRTOS-raspi3
UART1(mini UART) must be enabled to manipulate u-boot. Add enable_uart=1 to config.txt. See https://www.raspberrypi.org/documentation/configuration/uart.md for the detail.
You need to install a GCC toolset for aarch64, and can get it from [2]. Don't forget to add its binary path to $PATH.
I used AArch64 ELF bare-metal target (aarch64-none-elf) version 9.2.1 for this repository.
A pre-built u-boot image provided by Ubuntu or Debian may not have the dcache command on the u-boot prompt. You need compile and install u-boot having cache commands if u-boot provided by your Linux distribution does not have it.
(1) Source code download
$ git clone https://github.com/u-boot/u-boot
(2) Compilation
$ cd u-boot
$ export CROSS_COMPILE=aarch64-none-elf-
$ echo 'CONFIG_CMD_CACHE=y' >> ./configs/rpi_4_defconfig
$ make rpi_4_defconfig
$ make -j4 (if your PC has 4 processor cores)
(CROSS_COMPILE must be changed depending on a compiler you installed)
(3) Copy the binary to your SD card
$ sudo ./u-boot.bin /path/to/sd_boot_partition/kernel8.img
(The new file name must be kernel8.img)
Very simple! Just execute the following commands.
$ cd Demo/CORTEX_A72_64-bit_Raspberrypi4/uart
$ make CROSS=aarch64-none-elf-
(CROSS must be changed depending on a compiler you installed)
(1) Copy the obtained binary to your SD card
$ sudo ./uart.elf /path/to/sd_boot_partition/
(2) Get the u-boot command on your Raspberry Pi 4B board
Insert your SD card into your board, then power it on.
(3) Launch the FreeRTOS sample program on the u-boot prompt
ext4load mmc 0:2 0x28000000 /path/to/uart.elf
mw 0xf0 0x20000000
dcache flush
bootelf 0x28000000
mmc 0:2 in the ext4load command execution will vary depending on your SD card configuration. Don't forget to use the fatload command if you copied the sample program binary to a FAT partition.
You will see output by the UART sample program.
****************************
FreeRTOS UART Sample
(This sample uses UART2)
****************************
00000FF8
...
This is little bit complicated. Follow the procedure below.
You must remove PL011 related nodes indicating UART[0,2-5] from a device tree file your board uses. You must also add a memory region 0x20000000 - 0x207FFFFF to the reserved-memory scope in the device tree file.
I put a modified device tree file derived from ubuntu 20.04 LTS at ./dts/bcm2711-rpi-4-b-rtos.dts for testing. You can compile it by executing the dtc command below.
$ dtc -O dtb -I dts ./bcm2711-rpi-4-b-rtos.dts -o ./bcm2711-rpi-4-b-rtos.dtb
You will obtain bcm2711-rpi-4-b-rtos.dtb. Copy it to an SD card partition where Linux kernel will load it. You may need to rename the binary dtb file or to modify a board configuration file such as config.txt to make it possible for Linux kernel to load it properly.
You need to add a macro -D__LINUX__ to CFLAGS in Makefile. This macro adds a special function to avoid GIC configuration change by Linux.
$ cd Demo/CORTEX_A72_64-bit_Raspberrypi4/uart/
$ grep ^CFLAGS Makefile
CFLAGS = -mcpu=cortex-a72 -fpic -ffreestanding -std=gnu99 -O2 -Wall -Wextra -I$(INCLUDEPATH1) -I$(INCLUDEPATH2) -I$(INCLUDEPATH3) -DGUEST -D__LINUX__
$ make CROSS=aarch64-none-elf-
(CROSS must be changed depending on a compiler you installed)
Same as 4-(1) above.
Add maxcpus=3 to cmdline.txt. This enables Linux to use only CPU cores #0-2. The CPU core #3 can be used by FreeRTOS safely.
Same as 4-(3). Execute the following commands on the u-boot prompt.
ext4load mmc 0:2 0x28000000 /path/to/uart.elf
mw 0xf0 0x20000000
dcache flush
bootelf 0x28000000
But you will see only a message
Waiting until Linux starts booting up ...
on UART2(PL011) until you launch Linux.
Quite simple. Just execute
run bootcmd
on the u-boot prompt. You will see Linux boot process output on UART1(mini UART) and FreeRTOS UART output on UART2(PL011).
MIT License (see LICENSE.md for the detail).