From c8154a2db99f0d0c6b5e47c279d6b83d8e69cb8f Mon Sep 17 00:00:00 2001
From: Daniel Barlow <dan@telent.net>
Date: Sun, 4 Feb 2024 18:12:15 +0000
Subject: [PATCH] kernel: add "conditional" config

imagine: you are using a device that requires
CONFIG_MYDEVICE_FROBOZZ_DRIVER but only if CONFIG_FROBOZZ has been
specified elsewhere. Because we check that every requested config
symbol actually appears in .config then it can't be added
unconditionally or the build will fail if CONFIG_FROBOZZ wasn't asked
for.

I'm not 100% happy about this design but it's the best I've thought of
so far.
---
 devices/turris-omnia/default.nix |  9 +++++---
 modules/kernel.nix               | 37 ++++++++++++++++++++++++++++----
 2 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/devices/turris-omnia/default.nix b/devices/turris-omnia/default.nix
index 2a15a13..9c3d333 100644
--- a/devices/turris-omnia/default.nix
+++ b/devices/turris-omnia/default.nix
@@ -132,9 +132,6 @@
             PHY_MVEBU_A38X_COMPHY = "y"; # for eth2
             MARVELL_PHY = "y";
 
-            USB_XHCI_MVEBU = "y";
-            USB_XHCI_HCD = "y";
-
             MVPP2 = "y";
             MV_XOR = "y";
 
@@ -150,6 +147,12 @@
             NET_DSA = "y";
             NET_DSA_MV88E6XXX = "y"; # depends on PTP_1588_CLOCK_OPTIONAL
           };
+          conditionalConfig = {
+            USB = {
+              USB_XHCI_MVEBU = "y";
+              USB_XHCI_HCD = "y";
+            };
+          };
         };
 
         boot = {
diff --git a/modules/kernel.nix b/modules/kernel.nix
index 81e7201..f10e3d7 100644
--- a/modules/kernel.nix
+++ b/modules/kernel.nix
@@ -13,6 +13,16 @@ let
 
   type_service = pkgs.liminix.lib.types.service;
 
+  mergeConditionals = conf : conditions :
+    # for each key in conditions, if it is present in conf
+    # then merge the associated value into conf
+    lib.foldlAttrs
+      (acc: name: value:
+        if (conf ? ${name}) && (conf.${name} != "n")
+        then acc // value
+        else acc)
+      conf
+      conditions;
 in {
   options = {
     kernel = {
@@ -42,6 +52,20 @@ in {
           };
         '';
       };
+      conditionalConfig = mkOption {
+        description = ''
+          Kernel config options that should only be applied when
+          some other option is present.
+        '';
+        type = types.attrsOf (types.attrsOf types.nonEmptyStr);
+        default = {};
+        example = {
+          USB = {
+            USB_XHCI_MVEBU = "y";
+            USB_XHCI_HCD = "y";
+          };
+        };
+      };
       makeTargets = mkOption {
         type = types.listOf types.str;
       };
@@ -49,10 +73,15 @@ in {
   };
   config = {
     system.outputs =
-      let k = liminix.builders.kernel.override {
-            inherit (config.kernel) config src extraPatchPhase;
-            targets = config.kernel.makeTargets;
-          };
+      let
+        mergedConfig = mergeConditionals
+          config.kernel.config
+          config.kernel.conditionalConfig;
+        k = liminix.builders.kernel.override {
+          config = mergedConfig;
+          inherit (config.kernel) src extraPatchPhase;
+          targets = config.kernel.makeTargets;
+        };
       in {
         kernel = k.vmlinux;
         zimage = k.zImage;