nixos/linux_rpi4: Broken device trees cause UART failure on Raspberry Pi 4

[hicklin]

Nixpkgs version

• Beta (25.11)

Describe the bug

Raspberry Pi 4 UARTs fails to initialise when using pkgs.linuxKernel.packages.linux_rpi4 (version 6.12.47-stable_20250916). The UARTs initialise correctly when switching to vanilla linux (version 6.12.58). See kernel logs in the relevant section below.

Additionally, this fix requires changing the console kernel parameter from ttyS0 to ttyS1.

This is likely due to a misconfigured DT overlay.

Steps to reproduce

1. Follow the instruction on the Installing NixOS on a Raspberry Pi page until and including the Installing and configuring NixOS section. Note If you want to use UART at this stage, you will have to manually modify the consol parameter as described above in boot/extlinux/extlinux.conf before the "Booting NixOS live image" section.
2. Inspect the kernel logs after reboot. This should be similar to the linux_rpi4-6.12.47 logs shared below.

To get a working UART, you can replace the kernelPackages = pkgs.linuxKernel.packages.linux_rpi4; in the configuration.nix with

    kernelPackages = pkgs.linuxPackages;
    kernelParams = lib.mkForce [
        "console=ttyS1,115200n8"
        "console=ttyAMA0,115200n8"
        "console=tty0"
        "nohibernate"
        "loglevel=7"
    	"lsm=landlock,yama,bpf"
    ];

This will use the vanilla kernel and set console=ttyS1.

Expected behaviour

The UART should be correctly initialised as shown in the linux-6.12.58 kernel logs.

Screenshots

No response

Relevant log output

## `linux-6.12.58`

Here is the kernel output, greped for "serial", when UART is working. I.e. vanilla kernel and `ttyS0` changed to `ttyS1`.


[    0.444666] fe215040.serial: ttyS1 at MMIO 0xfe215040 (irq = 25, base_baud = 62499999) is a 16550
[    1.543468] msm_serial: driver initialized
[    1.681578] usbcore: registered new interface driver usbserial_generic
[    1.688245] usbserial: USB Serial support registered for generic
[    1.874998] fe201000.serial: ttyAMA0 at MMIO 0xfe201000 (irq = 32, base_baud = 0) is a PL011 rev3
[    3.246818] serial serial0: tty port ttyAMA0 registered
[    8.628158] systemd[1]: Created slice Slice /system/serial-getty.


## `linux_rpi4-6.12.47`

Here is the kernel output when using `kernelPackages = pkgs.linuxKernel.packages.linux_rpi4;` only. Console is set to `ttyS0`.


[    0.715853] usbcore: registered new interface driver usbserial_generic
[    0.715869] usbserial: USB Serial support registered for generic
[    0.754910] uart-pl011 fe201000.serial: there is not valid maps for state default
[    0.755161] uart-pl011 fe201000.serial: cts_event_workaround enabled
[    0.755472] fe201000.serial: ttyAMA1 at MMIO 0xfe201000 (irq = 38, base_baud = 0) is a PL011 rev3
[    0.755612] serial serial0: tty port ttyAMA1 registered
[    0.756540] bcm2835-aux-uart fe215040.serial: there is not valid maps for state default
[    0.756965] bcm2835-aux-uart fe215040.serial: error -EINVAL: unable to register 8250 port
[    0.756991] bcm2835-aux-uart fe215040.serial: probe with driver bcm2835-aux-uart failed with error -22
[    8.376127] hci_uart_bcm serial0-0: supply vbat not found, using dummy regulator
[    8.376315] hci_uart_bcm serial0-0: supply vddio not found, using dummy regulator

Additional context

Hardware: Raspberry Pi 4B

System metadata

• system: "aarch64-linux"
• host os: Linux 6.12.47, NixOS, 25.11 (Xantusia), 25.11pre901419.5ae3b07d8d65
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.31.2
• channels(root): "nixos-25.11pre901419.5ae3b07d8d65"
• nixpkgs: /nix/store/75iiz86idhy5nam496h5rx9asi6mi5qf-nixos-25.11pre901419.5ae3b07d8d65/nixos

Notify maintainers

@thoughtpolice @TredwellGit @K900 @Ma27 @NeQuissimus @alyssais

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[bobrippling]

[    0.754910] uart-pl011 fe201000.serial: there is not valid maps for state default
[    0.755161] uart-pl011 fe201000.serial: cts_event_workaround enabled
[    0.755472] fe201000.serial: ttyAMA1 at MMIO 0xfe201000 (irq = 38, base_baud = 0) is a PL011 rev3
[    0.755612] serial serial0: tty port ttyAMA1 registered
[    0.756540] bcm2835-aux-uart fe215040.serial: there is not valid maps for state default
[    0.756965] bcm2835-aux-uart fe215040.serial: error -EINVAL: unable to register 8250 port
[    0.756991] bcm2835-aux-uart fe215040.serial: probe with driver bcm2835-aux-uart failed with error -22

I'm seeing this also - can confirm it started within the last few months

Sadly the workaround (generic linux) doesn't work for me as I boot from SSD and the generic kernel is missing some module config to have it detect the SSD.

[bobrippling]

A potential fix: raspberrypi/linux#7023