docs: howto: kernel_config

docs/howto/kernel_config.md

@@ -0,0 +1,54 @@
+# How To Config Linux Kernel in Nix
+
+Basic configuration tricks referring to [nixos.wiki: Linux Kernel](https://nixos.wiki/wiki/Linux_kernel).
+
+## The Override System in Kernel
+
+What do the `pkgs.linux.override`, `pkgs.linux.overrideAttrs` override?
+
+To answer these questions, we need first to explore how kernel derivation is generated.
+When we type `pkgs.linux`, how this derivation is generated by nix?
+
+```
+* pkgs.linux
+* pkgs/top-level/all-packages:
+  * linuxPackages.kernel;
+  * (linuxPackages = linuxKernel.packageAliases.linux_default).kernel;
+  * (linuxPackages = (linuxKernel = recurseIntoAttrs (callPackage ./linux-kernels.nix { });).packageAliases.linux_default).kernel;
+  * (recurseIntoAttrs (callPackage ./linux-kernels.nix { });).packageAliases.linux_default.kernel;
+* pkgs/top-level/linux-kernels.nix:
+  * packages.linux_6_6.kernel;
+  * (recurseIntoAttrs (vanillaPackages // rtPackages // rpiPackages // { ... })).linux_6_6.kernel
+  * (recurseIntoAttrs ({...; linux_6_6 = recurseIntoAttrs (packagesFor kernels.linux_6_6);...} // rtPackages // rpiPackages // { ... })).linux_6_6.kernel
+  * (packagesFor kernels.linux_6_6).kernel
+  * ((kernel_: {kernel = kernel_;}) kernels.linux_6_6).kernel
+  * kernels.linux_6_6
+  * ❶callPackage ../os-specific/linux/kernel/mainline.nix {
+      branch = "6.6";
+      kernelPatches = [
+        kernelPatches.bridge_stp_helper
+        kernelPatches.request_key_helper
+      ];
+    };
+* pkgs/os-specific/linux/kernel/mainline.nix
+  * buildLinux args'
+  * ❷buildLinux ((builtins.removeAttrs args [ "branch" ])
+    // {
+      inherit src version;
+
+      modDirVersion = lib.versions.pad 3 version;
+      extraMeta.branch = branch;
+    }
+    // (args.argsOverride or { }))
+  * (callPackage pkgs/os-specific/linux/kernel/generic.nix {}) args'
+  * overridableKernel args'
+  * (lib.makeOverridable ({...}: kernel.overrideAttrs (...))) args'
+  * kernel.overrideAttrs (...)
+  * ❸((callPackage ./manual-config.nix { inherit lib stdenv buildPackages; }) (basicArgs // { ... })).overrideAttrs (...)
+```
+
+因此
+
+* ❶ `override` is applied to `pkgs/os-specific/linux/kernel/mainline.nix`
+* ❷ `argsOverride` in `override (old: {argsOverride = {...} })` is applied to `pkgs/os-specific/linux/kernel/generic.nix`
+* ❸ `overrideAttrs` is applied to `pkgs/os-specific/linux/kernel/manual-config.nix`