blog: pinephone nixos: fill in some of the code TODOs

content/blog/2022-05-20-mobile-nixos-pinephone.md

@@ -5,34 +5,93 @@ description = "a flake-based quickstart and some tuning"
 extra.hidden = true
 +++
 
-there is no official, easy-to-grab NixOS image to download and flash to devices like the pinephone today. although there is [a way](https://news.ycombinator.com/item?id=30010178) to do that via the hydra build cache, it's a bit tortured and since the images built in automation don't have a user, you'll have to manually edit `/etc/fstab` (from a different machine) to add a user and then login with a USB-C keyboard.
+there is no official, easy-to-grab NixOS image to download and flash to devices like the pinephone today. although there is [a way](https://news.ycombinator.com/item?id=30010178) to do that via the hydra build cache, it's a bit tortured and since the images built in automation don't have a user, you'll have to manually edit `/etc/fstab` (from a different machine) to add a user and then login with a USB-C keyboard to setup ssh or a desktop.
 
 so first, provision an existing Nix install. the architecture doesn't matter -- i'll assume it's x86_64 and show you how to cross-compile. i recommend using a flake for this. i'm basing this around nix-22.05, so something like this:
 ```flake.nix
-TODO
+{
+  inputs = {
+    nixpkgs.url = "nixpkgs/nixos-22.05";
+  };
+
+  outputs = { self, nixpkgs }: {
+    host = nixpkgs.lib.nixosSystem {
+      system = "x86_64-linux";
+      modules = [
+        ./host.nix
+      ];
+    };
+  };
+}
 ```
 
 ```configuration.nix
-TODO
+TODO: does one need the configuration.nix wrapper on nixos-22.05?
 ```
 
 ## Bare-minimal Build
 
-for our Pinephone machine, we'll let [mobile-nixos](https://github.com/NixOS/mobile-nixos/) do the heavy lifting.
+for our Pinephone machine, we'll let [mobile-nixos](https://github.com/NixOS/mobile-nixos/) do the heavy lifting. add it as an input
 
 ```diff -u a/flake.nix b/flake.nix
-TODO: minimal NixOS install
+ inputs = {
+   nixpkgs.url = "nixpkgs/nixos-22.05";
++  mobile-nixos = {
++    url = "github:nixos/mobile-nixos";
++    flake = false;
++  };
+ };
 ```
 
-if your host machine isn't aarch64, you'll have to enable cross compiling.
+things here move fast. depending on where things are at any moment, you might be able to use the upstream `nixpkgs` in your mobile builds, or you might need to freeze the nixpkgs to a specific commit. `mobile-nixos` pins its own pkgs. if you look at [`pkgs.nix`](https://github.com/NixOS/mobile-nixos/blob/master/pkgs.nix) you can copy the rev and track it:
 
 ```diff -u a/flake.nix b/flake.nix
-TODO: enable cross compiling
+ inputs = {
+   nixpkgs.url = "nixpkgs/nixos-22.05";
+   mobile-nixos = {
+     url = "github:nixos/mobile-nixos";
+     flake = false;
+   };
++  pkgs-mobile.url = "nixpkgs/dfd82985c273aac6eced03625f454b334daae2e8";
+ };
 ```
 
+and then add a new target for building a  `pinephone-img`:
+```diff -u a/flake.nix b/flake.nix
+- outputs = { self, nixpkgs }: {
++ outputs = { self, nixpkgs, mobile-nixos, pkgs-mobile }: {
++   pinephone-img = (pkgs-mobile.lib.nixosSystem {
++     system = "aarch64-linux";
++     modules = [
++       (import "${mobile-nixos}/lib/configuration.nix" {
++         device = "pine64-pinephone";
++       })
++     ];
++   }).config.mobile.outputs.u-boot.disk-image;
+```
+
+if your host machine isn't aarch64, you'll have to enable cross compiling. the trivial way is emulation (i.e. qemu), though it could make the build take a full afternoon depending on how much of your system is available through nixcache.
+
+```diff -u a/flake.nix b/flake.nix
+ outputs = { self, nixpkgs }: {
+   host = nixpkgs.lib.nixosSystem {
+     system = "x86_64-linux";
+     modules = [
+       ./host.nix
++      { ... }: {
++        boot.binfmt.emulatedSystems = [ "aarch64-linux" ];
++      }
+     ];
+   };
+ };
+
+```
+
+if you added this emulation, rebuild your host machine to enable it (`nixos-rebuild --flake './#host' switch).
+
 then build the image with:
 ```sh
-nix build './#imgs.moby'
+nix build './#pinephone-img'
 ```
 
 the resulting image is effectively the same as what Hydra spits out in the automation: no users, and no way to login. if you're brand new to the Pinephone, i recommend flashing the image and booting as a sanity check. otherwise, skip to _Making the Image More Usable_ where we'll build a more usable image. at any time, consult the _Troubleshooting_ section if you hit something unexpected.
@@ -76,7 +135,7 @@ we don't want to re-flash the device _every_ time we change something. let's upd
 TODO: add toplevel nixosConfigurations.pinephone
 ```
 
-copy your flake over to the phone at `/etc/nixos/flake.nix` somehow. i use git for this. on the phone:
+copy your flake over to the phone at `/etc/nixos/flake.nix`. i use git for this. on the phone:
 ```sh
 ~$ git clone https://git.uninsane.org/colin/nix-files.git
 ~$ sudo rmdir /etc/nixos && sudo mv nix-files /etc/nixos
@@ -85,7 +144,9 @@ copy your flake over to the phone at `/etc/nixos/flake.nix` somehow. i use git f
 /etc/nixos$ sudo nixos-rebuild --flake "./#pinephone" switch
 ```
 
-validate this with a reboot, and you should be golden! happy nixing :-)
+validate this with a reboot, and you should be golden! i recommend mirroring your `/etc/nixos` folder to at least one other place: a github repo, `rsync` to a desktop, `duplicity` to cloud storage, etc. that way if you ever brick your phone you can restore from your latest config using the `dd` approach. managing generations and rolling back to a good one doesn't seem quite as easy to do on a phone.
+
+happy nixing :-)
 
 
 # Troubleshooting