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update default rpi config
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117
README.md
117
README.md
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@ -1,27 +1,22 @@
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# raspberry-pi-nix
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NixOS modules that make building images for raspberry-pi products
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easier. Most of the work in this repository is based on work in
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[nixos-hardware](https://github.com/NixOS/nixos-hardware) and
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[nixpkgs](https://github.com/NixOS/nixpkgs). Additionally, be aware
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that I am no expert and this repo is the product of me fooling around
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with some pis.
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The primary goal of this software is to make it easy to create
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working NixOS configurations for Raspberry Pi products. Specifically,
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this repository aims to deliver the following benefits:
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This flake provides nixos modules that correspond to different
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raspberry-pi products. These modules can be included in nixos
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configurations and aim to deliver the following benefits:
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1. Configure the kernel, device tree, and u-boot in a way that is
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compatible with the hardware.
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2. Provide a nix interface to device tree configuration that will be
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familiar to those who have used raspberry-pi's config.txt based
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configuration.
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1. Configure the kernel, device tree, and boot loader in a way that is
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compatible with the hardware and proprietary firmware.
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2. Provide a nix interface to Raspberry Pi/device tree configuration
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that will be familiar to those who have used Raspberry Pi's
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[config.txt based
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configuration](https://www.raspberrypi.com/documentation/computers/config_txt.html).
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3. Make it easy to build an image suitable for flashing to an sd-card,
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without a need to first go through an installation media.
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The important modules are `overlay/default.nix`, `rpi/default.nix`,
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and `rpi/device-tree.nix`. The other modules for i2c, i2s, etc are
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mostly wrappers that set common device tree settings for you.
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and `rpi/config.nix`. The other modules for i2c, i2s, etc are mostly
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wrappers that set `config.txt` settings and enable required kernel
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modules.
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## Example
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@ -46,27 +41,12 @@ mostly wrappers that set common device tree settings for you.
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useDHCP = false;
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interfaces = { wlan0.useDHCP = true; };
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};
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hardware.raspberry-pi = {
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i2c.enable = true;
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audio.enable = true;
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fkms-3d.enable = true;
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deviceTree = {
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dt-overlays = [{
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overlay = "imx477"; # add the overlay for the HQ camera
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args = [ ];
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}];
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};
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};
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};
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in {
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nixosConfigurations = {
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rpi-zero-2-w-example = nixosSystem {
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rpi-example = nixosSystem {
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system = "aarch64-linux";
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modules = [ raspberry-pi-nix.rpi-zero-2-w basic-config ];
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};
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rpi-4b-example = nixosSystem {
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system = "aarch64-linux";
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modules = [ raspberry-pi-nix.rpi-4b basic-config ];
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modules = [ raspberry-pi-nix.rpi basic-config ];
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};
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};
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};
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@ -77,25 +57,64 @@ mostly wrappers that set common device tree settings for you.
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An image suitable for flashing to an sd-card can be found at the
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attribute `config.system.build.sdImage`. For example, if you wanted to
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build an image for `rpi-zero-2-w-example` in the above configuration
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build an image for `rpi-example` in the above configuration
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example you could run:
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```
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nix build '.#nixosConfigurations.rpi-zero-2-w-example.config.system.build.sdImage'
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nix build '.#nixosConfigurations.rpi-example.config.system.build.sdImage'
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```
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## Other notes
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## The firmware partition
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The sd-image built is partitioned in the same way as the aarch64
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installation media from nixpkgs: There is a firmware partition that
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contains necessary firmware, u-boot, and config.txt. Then there is
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another partition that contains everything else. After the sd-image is
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built, nixos system updates will not change anything in the firmware
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partition ever again. New kernels and device tree configurations will
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remain on the nixos partition and be booted by u-boot in the firmware
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The image produced by this package is partitioned in the same way as
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the aarch64 installation media from nixpkgs: There is a firmware
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partition that contains necessary firmware, u-boot, and
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config.txt. Then there is another partition (labeled `NIXOS_SD`) that
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contains everything else. The firmware and `config.txt` file are
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managed by NixOS modules defined in this package. Additionally, NixOS
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system activation will update the firmware and `config.txt` in the
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firmware partition __in place__. Linux kernels are stored in the
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`NIXOS_SD` partition and will be booted by u-boot in the firmware
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partition.
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So, while you can control device tree params and overlays through your
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nixos system configuration, if you want to modify other config.txt
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variables this must be done manually by mounting the partition and
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modifying the config.txt file.
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## `config.txt` generation
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As noted, the `config.txt` file is generated by the NixOS
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configuration and automatically updated on system activation.
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## Firmware partition implementation notes
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In Raspberry Pi devices the proprietary firmware manipulates the
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device tree in a number of ways before handing it off to the kernel
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(or in our case, to u-boot). The transformations that are performed
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aren't documented so well (although I have found [this
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list](https://forums.raspberrypi.com/viewtopic.php?t=329799#p1974233)
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).
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This manipulation makes it difficult to use the device tree configured
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directly by NixOS as you must know and reproduce the manipulation
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performed by the proprietary firmware.
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Even if the manipulation was reproduced, some convenience would be
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missed out. For example, the firmware can detect hardware during boot
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and automatically configure the device tree accordingly, rather than
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requiring a NixOS system rebuild with a different device tree for
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different hardware. Examples of what I mean by hardware include: the
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specific Raspberry Pi device booting the image, connected cameras, and
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connected displays.
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So, in order to avoid the headaches associated with failing to
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reproduce some firmware device tree manipulation, and to reap the
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benefits afforded by the firmware device tree configuration, u-boot is
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configured to use the device tree that it is given (i.e. the one that
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the raspberry pi firmware loads and manipulates). As a consequence,
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device tree configuration is controlled via the [config.txt
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file](https://www.raspberrypi.com/documentation/computers/config_txt.html).
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Additionally, the firmware, device trees, and overlays from the
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`raspberrypifw` package populates the firmware partition on system
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activation. This package is kept up to date by the overlay applied by
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this package, so you don't need configure this. However, if you want
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to use different firmware you can override that package to do so.
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