system-target: reviews implementation

Since the initial implementation I learned enough to know where it was
slighly wrong. I think this fixes most of the slight wrongness.

devices/motorola-addison/kernel/default.nix

@@ -13,7 +13,7 @@
 # This is because motorola ships an armv7l userspace from stock ROM.
 #
 # in local.nix:
-#  mobile.system.platform = lib.mkForce "armv7a-linux";
+#  mobile.system.system = lib.mkForce "armv7l-linux";
 #
 
 let

modules/hardware-generic.nix

@@ -15,6 +15,6 @@ in
   };
 
   config = {
-    mobile.system.platform = lib.mkIf cfg.generic-x86_64.enable "x86_64-linux";
+    mobile.system.system = lib.mkIf cfg.generic-x86_64.enable "x86_64-linux";
   };
 }

modules/hardware-qualcomm.nix

@@ -27,19 +27,19 @@ in
   config = mkMerge [
     {
       mobile = mkIf cfg.qualcomm-msm8939.enable {
-        system.platform = "aarch64-linux";
+        system.system = "aarch64-linux";
         quirks.qualcomm.msm-fb-handle.enable = true;
       };
     }
     {
       mobile = mkIf cfg.qualcomm-msm8953.enable {
-        system.platform = "aarch64-linux";
+        system.system = "aarch64-linux";
         quirks.qualcomm.msm-fb-handle.enable = true;
       };
     }
     {
       mobile = mkIf cfg.qualcomm-apq8064-1aa.enable {
-        system.platform = "armv7a-linux";
+        system.system = "armv7l-linux";
         quirks.qualcomm.msm-fb-refresher.enable = true;
       };
     }

modules/hardware-rockchip.nix

@@ -17,7 +17,7 @@ in
   config = mkMerge [
     {
       mobile = mkIf cfg.rockchip-op1.enable {
-        system.platform = "aarch64-linux";
+        system.system = "aarch64-linux";
       };
     }
   ];

modules/system-target.nix

@@ -1,24 +1,35 @@
 { config, lib, pkgs, ... }:
 
-# FIXME : current implementation only works for native x86_64 built hosts.
-
 with lib;
 let
   cfg = config.mobile.system;
 
-  target_types = {
-    aarch64-linux = lib.systems.examples.aarch64-multiplatform;
-    armv7a-linux = lib.systems.examples.armv7l-hf-multiplatform;
-    x86_64-linux = { config = "x86_64-unknown-linux-gnu"; };
+  # Mapping from system types to config types
+  # A simplified view of <nixpkgs/lib/systems/examples.nix>
+  config_types = {
+    aarch64-linux = "aarch64-unknown-linux-gnu";
+    armv7l-linux = "armv7l-unknown-linux-gnueabihf";
+    x86_64-linux = "x86_64-unknown-linux-gnu";
   };
 
-  # Hmmm, this doesn't feel right, but it does work.
-  host_platform = (import <nixpkgs> {}).buildPackages.hostPlatform;
+  # Derived from config_types
+  target_types = lib.attrNames config_types;
+
+  # Builds the expected "platform" set for cross-compilation from the given
+  # system name.
+  selectPlatform = system: {
+    inherit system;
+    platform = lib.systems.platforms.selectBySystem system;
+    config = config_types.${system};
+  };
+
+  # The platform selected by the configuration
+  selectedPlatform = selectPlatform cfg.system;
 in
 {
   options.mobile = {
-    system.platform = mkOption {
-      type = types.enum (lib.attrNames target_types);
+    system.system = mkOption {
+      type = types.enum target_types;
       description = ''
         Defines the kind of target architecture system the device is.
 
@@ -30,14 +41,25 @@ in
   config = {
     assertions = [
       {
-        assertion = pkgs.targetPlatform.system == cfg.platform;
-        message = "pkgs.targetPlatform.system expected to be `${cfg.platform}`, is `${pkgs.targetPlatform.system}`";
+        assertion = pkgs.targetPlatform.system == cfg.system;
+        message = "pkgs.targetPlatform.system expected to be `${cfg.system}`, is `${pkgs.targetPlatform.system}`";
       }
     ];
 
     nixpkgs.crossSystem = lib.mkIf
-      ( target_types.${cfg.platform}.config != host_platform.config )
-      (target_types.${cfg.platform} // { system = cfg.platform; }) # FIXME : WHY? I didn't need to add system before :/
+      (
+        let
+          result = selectedPlatform.system != builtins.currentSystem;
+        in
+        builtins.trace
+        "Building with crossSystem?: ${selectedPlatform.system} != ${builtins.currentSystem} → ${if result then "true" else "false"}"
+        result
+      )
+      (
+        builtins.trace
+        "    crossSystem: config: ${selectedPlatform.config}"
+        selectedPlatform
+      )
     ;
   };
 }