pine64-pinephone: Add service to control the modem

devices/pine64-pinephone/default.nix

@@ -1,6 +1,10 @@
 { config, lib, pkgs, ... }:
 
 {
+  imports = [
+    ./modem.nix
+  ];
+
   mobile.device.name = "pine64-pinephone";
   mobile.device.identity = {
     name = "PinePhone";
@@ -38,10 +42,5 @@
     gpio set 115   # R
     gpio set 114   # G
     gpio clear 116 # B
-
-    # Properly shut off EG25 by pulling up PWRKEY.
-    gpio set 35
-    sleep 1
-    gpio clear 35
   '';
 }

devices/pine64-pinephone/modem.nix

@@ -0,0 +1,53 @@
+{ config, lib, pkgs, ... }:
+
+# This, by default, boots the phone with the modem powered down.
+# In addition, the systemd service does not, by default, start the modem.
+{
+
+  # Shut down the modem in early boot
+  mobile.quirks.u-boot.additionalCommands = ''
+    # Properly shut off EG25 by pulling up PWRKEY.
+    gpio set 35
+    sleep 1
+    gpio clear 35
+  '';
+
+  # Ensure we have systemd tagging on the modem.
+  # Probably is not needed.
+  services.udev.extraRules =
+    let
+      path = "/devices/platform/soc/1c1b000.usb/*/net/wwan0";
+    in ''
+      ACTION=="add", DEVPATH=="${path}", TAG+="systemd"
+      ACTION=="remove", DEVPATH=="${path}", TAG+="systemd"
+    ''
+  ;
+
+  # This service, which by default is not "wantedBy" multi-user.target, allows
+  # the user to start the modem using a systemd service.
+  # In addition, the service status always reflects the current status of the
+  # modem; whether it has been turned off or on using systemd or not.
+  systemd.services =
+    let
+      dotDeviceName = "sys-devices-platform-soc-1c1b000.usb-usb3-3\\x2d1-3\\x2d1:1.4-net-wwan0.device";
+    in {
+    "modem-control" = {
+      bindsTo = [ dotDeviceName ];
+      wantedBy = lib.mkForce [ dotDeviceName ];
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = true;
+
+        ExecStart = pkgs.writeShellScript "start-modem" ''
+          echo 'Powering modem on...'
+          echo 1 > /sys/class/modem-power/modem-power/device/powered
+        '';
+
+        ExecStop  = pkgs.writeShellScript "stop-modem" ''
+          echo 'Powering modem off...'
+          echo 0 > /sys/class/modem-power/modem-power/device/powered
+        '';
+      };
+    };
+  };
+}