The primary goal of this flake is to make it easy to create working NixOS configurations for Raspberry Pi products. Specifically, this repository aims to deliver the following benefits:
- Configure the kernel, device tree, and boot loader in a way that is compatible with the hardware and proprietary firmware.
- Provide a nix interface to Raspberry Pi/device tree configuration that will be familiar to those who have used Raspberry Pi's config.txt based configuration.
- Make it easy to build an image suitable for flashing to an sd-card, without the need to first go through an installation media.
The important modules are overlay/default.nix, rpi/default.nix,
and rpi/config.nix. The other modules are mostly wrappers that set
config.txt settings and enable required kernel modules.
See the rpi-example config in this flake for an example config built by CI.
This repo uses the raspberry pi linux kernel fork, and compiling linux takes a while. CI pushes kernel builds to the nix-community cachix cache that you may use to avoid compiling linux yourself. The cache can be found at https://nix-community.cachix.org, and you can follow the instructions there to use this cache.
Include the provided sd-image nixos module this flake provides, then an image
suitable for flashing to an sd-card can be found at the attribute
config.system.build.sdImage. For example, if you wanted to build an image for
rpi-example in the above configuration example you could run:
nix build '.#nixosConfigurations.rpi-example.config.system.build.sdImage'
The image produced by this package is partitioned in the same way as the aarch64
installation media from nixpkgs: There is a firmware partition that contains
necessary firmware, the kernel or u-boot, and config.txt. Then there is another
partition (labeled NIXOS_SD) that contains everything else. The firmware and
config.txt file are managed by NixOS modules defined in this
package. Additionally, a systemd service will update the firmware and
config.txt in the firmware partition in place. If uboot is enabled then
linux kernels are stored in the NIXOS_SD partition and will be booted by
u-boot in the firmware partition.
As noted, the config.txt file is generated by the NixOS
configuration and automatically updated on when the nix configuration
is modified.
The relevant nixos option is
hardware.raspberry-pi.config. Configuration is partitioned into
three sections:
- Base device tree parameters
base-dt-params - Device tree overlays
dt-overlays - Firmware options
options
Other than that, the format follows pretty closely to the config.txt format. For example:
hardware.raspberry-pi.config = {
cm4 = {
options = {
otg_mode = {
enable = true;
value = true;
};
};
};
pi4 = {
options = {
arm_boost = {
enable = true;
value = true;
};
};
dt-overlays = {
vc4-kms-v3d = {
enable = true;
params = { cma-512 = { enable = true; }; };
};
};
};
all = {
options = {
# The firmware will start our u-boot binary rather than a
# linux kernel.
kernel = {
enable = true;
value = "u-boot-rpi-arm64.bin";
};
arm_64bit = {
enable = true;
value = true;
};
enable_uart = {
enable = true;
value = true;
};
avoid_warnings = {
enable = true;
value = true;
};
camera_auto_detect = {
enable = true;
value = true;
};
display_auto_detect = {
enable = true;
value = true;
};
disable_overscan = {
enable = true;
value = true;
};
};
dt-overlays = {
vc4-kms-v3d = {
enable = true;
params = { };
};
};
base-dt-params = {
krnbt = {
enable = true;
value = "on";
};
spi = {
enable = true;
value = "on";
};
};
};
};generates the following config.txt:
# This is a generated file. Do not edit!
[all]
arm_64bit=1
avoid_warnings=1
camera_auto_detect=1
disable_overscan=1
display_auto_detect=1
enable_uart=1
kernel=u-boot-rpi-arm64.bin
dtparam=krnbt=on
dtparam=spi=on
dtoverlay=vc4-kms-v3d
dtoverlay=
[cm4]
otg_mode=1
[pi4]
arm_boost=1
dtoverlay=vc4-kms-v3d
dtparam=cma-512
dtoverlay=
If you want to preview the generated config.txt, you can find
it at the path config.hardware.raspberry-pi.config-output. For
example, if you had the above configuration then you could build the
config.txt file with:
nix build '.#nixosConfigurations.rpi-example.config.hardware.raspberry-pi.config-output'
In Raspberry Pi devices the proprietary firmware manipulates the device tree in a number of ways before handing it off to the kernel (or in our case, to u-boot). The transformations that are performed aren't documented so well (although I have found this list ).
This manipulation makes it difficult to use the device tree configured directly by NixOS as the proprietary firmware's manipulation must be known and reproduced.
Even if the manipulation were successfully reproduced, some benefits would be lost. For example, the firmware can detect connected hardware during boot and automatically configure the device tree accordingly before passing it onto the kernel. If this firmware device tree is ignored then a NixOS system rebuild with a different device tree would be required when swapping connected hardware. Examples of what I mean by hardware include: the specific Raspberry Pi device booting the image, connected cameras, and connected displays.
So, in order to avoid the headaches associated with failing to reproduce some firmware device tree manipulation, and to reap the benefits afforded by the firmware device tree configuration, the bootloader is configured to use the device tree that it is given (i.e. the one that the raspberry pi firmware loads and manipulates). As a consequence, device tree configuration is controlled via the config.txt file.
Additionally, the firmware, device trees, and overlays from the raspberrypifw
package populate the firmware partition. This package is kept up to date by the
overlay applied by this package, so you don't need configure this. However, if
you want to use different firmware you can override that package to do so.
- Pi 5 u-boot devices other than sd-cards (i.e. usb, nvme).
