Fixing low display brightness on LUKS password prompt

Given:

• Framework Laptop 13 AMD 7040 edition;
• Arch Linux with busybox-based initrd;
• Encrypted drive: LVM partitions on LUKS

The problem: The laptop display brightness fluctuates after booting when running on battery power.

1. At first, the brightness is rather high, and systemd-boot entries are clearly visible.
2. However, after selecting the kernel to boot, the brightness drops to minimum. You hardly see anything when the boot reaches LUKS password prompt.
3. The boot process continues after successfully entering the password. When systemd kicks in, the systemd-backlight service restores the brightness.

The solution: set a brightness on startup via a udev rule.

1. Add a new udev rule which sets brightness through ACPI:

   1	SUBSYSTEM=="backlight", ACTION=="add", KERNEL=="amdgpu_bl1", ATTR{brightness}="20000"

   save it to /etc/udev/rules.d/10-backlight.rules.

2. Include the rule path to mkinitcpio's configuration in /etc/mkinitcpio.conf:

   1	FILES = ("/etc/udev/rules.d/10-backlight.rules")

3. Generate a new initrd by running mkinitcpio -P.

4. Take out the power cable and reboot the machine. Enjoy the visible decryption password prompt :).