How to Modify U-Boot Environment Variables for Raspberry Pi 4 Boot Settings

Customizing U-Boot Environment Variables on Raspberry Pi

U-Boot gives you fine-grained control over the boot process that the standard Raspberry Pi bootloader doesn't. By tweaking U-Boot's environment variables, you can change boot delays, pass custom kernel arguments, set up multi-boot configurations, and script complex boot logic. If you're building embedded Linux systems on a Pi 4 (or Pi 5), understanding U-Boot's environment is a fundamental skill.

The Raspberry Pi doesn't ship with U-Boot by default—it uses Broadcom's proprietary bootloader. So the first assumption here is that you've already replaced or chained U-Boot into your boot flow, which is common when using Yocto, Buildroot, or a custom embedded Linux distro.

Prerequisites

Raspberry Pi 4 (or Pi 5—U-Boot supports both now) with U-Boot already installed
MicroSD card with your Linux image and U-Boot configured
Serial console access (USB-to-UART adapter connected to GPIO 14/15) or HDMI + keyboard
Comfort with command-line operations

Parts and Tools

Raspberry Pi 4 or 5
Appropriate power supply (USB-C, 5V/3A minimum)
MicroSD card with U-Boot-based image
USB-to-UART serial adapter (strongly recommended over HDMI for U-Boot work)
Terminal emulator (minicom, picocom, or PuTTY)

Steps

Connect to the U-Boot Console
Hook up your serial adapter to the Pi's UART pins (GPIO 14 = TX, GPIO 15 = RX, plus a ground connection). Open your terminal emulator at 115200 baud, 8N1. Power on the Pi and immediately start hitting a key—U-Boot's autoboot countdown is usually just a few seconds. If you see the U-Boot> prompt, you're in.

Watch out: if you're using HDMI instead of serial, you might miss the autoboot window because HDMI initialization adds delay. Serial is much more reliable for U-Boot interaction.
View Current Environment Variables
At the U-Boot prompt, dump all current variables:
```
printenv
```
This will list everything—bootcmd, bootargs, bootdelay, load addresses, and more. To check a specific variable:
```
printenv bootdelay
printenv bootargs
```
Take note of the current values before changing anything. If something breaks, you'll want to know what to revert to.
Change the Boot Delay
The bootdelay variable controls how many seconds U-Boot waits before automatically running bootcmd. Set it to give yourself enough time to interrupt:
```
setenv bootdelay 5
```
Set it to -1 to disable autoboot entirely (U-Boot will wait at the prompt forever)—useful during development. Set it to 0 for instant boot in production, though you'll lose the ability to interrupt without a serial break signal.
Modify Boot Arguments
The bootargs variable gets passed to the Linux kernel as its command line. This is where you configure console output, root filesystem, and other kernel parameters:
```
setenv bootargs 'console=serial0,115200 root=/dev/mmcblk0p2 rootwait'
```
Some common additions you might want:
- quiet — suppress most kernel boot messages
- loglevel=4 — control kernel log verbosity
- rootfstype=ext4 — explicitly set filesystem type
- init=/sbin/init — specify the init system path
You can also build bootargs dynamically using other variables. For example:
```
setenv console_args 'console=serial0,115200'
setenv root_args 'root=/dev/mmcblk0p2 rootwait'
setenv bootargs '${console_args} ${root_args}'
```
This makes it easier to swap individual pieces without retyping the whole string.
Save the Environment
Changes made with setenv only exist in RAM until you explicitly save them:
```
saveenv
```
You should see output like Saving Environment to FAT... OK (or to MMC, or to SPI flash, depending on your U-Boot build). If saveenv fails with an error, your U-Boot might not be configured with a writable environment storage backend—check your U-Boot build configuration.
Reboot and Verify
Reset the board:
```
reset
```
Watch the boot output. You should see the new bootdelay countdown and the kernel receiving your updated bootargs. On Linux, you can verify the kernel command line with:
```
cat /proc/cmdline
```

Troubleshooting

Can't interrupt autoboot: Try holding down a key before powering on. If bootdelay is 0, you may need to send a serial break signal (most terminal emulators have a menu option for this). Worst case, you can mount the SD card on another computer and edit the U-Boot environment file directly.
saveenv fails: This usually means U-Boot was compiled without support for persistent environment storage on your media. You'll need to rebuild U-Boot with the right CONFIG_ENV_IS_IN_* option enabled (typically CONFIG_ENV_IS_IN_FAT for Pi SD card setups).
Kernel panics after changing bootargs: Double-check your root= parameter points to the correct partition. Use mmc part at the U-Boot prompt to list partitions and verify. Also make sure rootwait is included—without it, the kernel may try to mount the root filesystem before the SD card driver is ready.
Want to reset to defaults: If you've saved bad environment variables and can't boot, remove the environment file from the boot partition (it's typically uboot.env on a FAT partition) using another computer. U-Boot will fall back to its compiled-in defaults on the next boot.

Going Further

U-Boot's environment is much more than just static variables. You can write boot scripts with conditionals, implement A/B partition switching for OTA updates, and use bootcmd to chain complex boot sequences. If you're building a product, look into U-Boot's distro boot mechanism (boot_targets and boot_scripts) which provides a standardized way to find and boot a kernel across different storage media. For the Pi 5 specifically, make sure you're using a U-Boot version from 2024 or later—earlier versions had incomplete BCM2712 support.