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I'm trying to use a Raspberry Pi with a UART peripheral and a non-Raspbian Linux distribution. It seems like most of these distributions use U-Boot. I'm running into two problems:

  1. U-Boot outputs on the serial console by default, which confuses the peripheral (it mistakes the logging messages as input)
  2. My peripheral tries to respond to these messages, which interrupts the U-Boot loader and prevents the kernel from booting

It seems like the possible solutions to this problem are:

  1. Disable the serial console/UART in U-Boot
  2. Boot the kernel directly (i.e. remove U-Boot from the boot process)

I've done a lot of searching, and it doesn't seem like there's a way to disable the U-Boot serial console output at runtime (i.e. without patching and recompiling U-Boot). I'm sure I could figure out how to have the Pi bootloader load the Linux kernel directly (like Raspbian does), but that seems like it might break with kernel updates.

I'm trying to investigate a third option: is it possible to have the UART disabled at boot time and enable it from the operating system instead? Failing that, are there any other workarounds to prevent U-Boot from using the serial console, or should I give up and use Raspbian?

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Here is the video where it is explained step by step how to prevent U-boot console from interrupting autoboot and sending debug messages on UART. I know links only answers are frowned upon, so here' s a quick breakdown of a solution:

Install the dependencies

sudo apt install git make gcc gcc-aarch64-linux-gnu

Git clone the official u-boot repository. Alternatively you can git clone my fork of repository, where I already have the necessary changes for silent autoboot - but if you need the latest version, then you need to clone the official repository and make changes yourself.

git clone --depth 1 git://git.denx.de/u-boot.git

cd u-boot

Find the raspberry pi config files - they depend on the model, configs/rpi_3_defconfig for Raspberry Pi 3, configs/rpi_4_defconfig for Raspberry Pi 4, and so on. Add the following lines to the end of the file

CONFIG_BOOTDELAY=-2
CONFIG_SILENT_CONSOLE=y
CONFIG_SYS_DEVICE_NULLDEV=y
CONFIG_SILENT_CONSOLE_UPDATE_ON_SET=y
CONFIG_SILENT_U_BOOT_ONLY=y

The first line removes the boot delay, so autoboot will not be interrupted by messages sent on UART interface. Next four lines enable silent boot, so U-boot will not send any messages on UART itself, because the messages might in turn confuse your device. One more little thing left, set silent boot environmental variable. Change include/configs/rpi.h file

#define CONFIG_EXTRA_ENV_SETTINGS \
    "dhcpuboot=usb start; dhcp u-boot.uimg; bootm\0" \
    "silent=1\0" \
    ENV_DEVICE_SETTINGS \
    ENV_DFU_SETTINGS \
    ENV_MEM_LAYOUT_SETTINGS \
    BOOTENV

Now configure with

make rpi_3_defconfig
# or 
make rpi_4_defconfig

from repository main folder and build with

make CROSS_COMPILE=aarch64-linux-gnu-

When build process finishes you will have a u-boot.bin file, which you need to rename and copy to Raspberry Pi SD card (e.g. to /boot/firmware/uboot_rpi_4.bin for a Raspberry Pi 4). Now your Raspberry Pi will not be disturbed by any messages on UART during boot. The UART functionality after boot will not be affected.

Relevant docs:

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