mdev: load the tethering drivers for a phone, and bring usb0 up when it appears

general/overlay/etc/init.d/S38mdev

@@ -5,6 +5,13 @@ case "$1" in
 		echo "Starting mdev..."
 		echo /sbin/mdev > /proc/sys/kernel/hotplug
 		mdev -s
+		# Whatever was plugged in at power-on was announced before mdev became
+		# the hotplug helper, and mdev -s walks only devices with a node, not
+		# the USB interfaces a driver binds to. Announce them again, so a phone
+		# already tethering at boot gets its driver like one plugged in later.
+		for uevent in /sys/bus/usb/devices/*:*/uevent; do
+			[ -w "$uevent" ] && echo add > "$uevent"
+		done
 		;;
 
 	stop)

general/overlay/etc/mdev.conf

@@ -18,3 +18,11 @@ mmcblk[0-9]p[0-9] root:disk 660 *(/lib/mdev/automount.sh)
 
 sd[a-z] root:disk 660 *(/lib/mdev/automount.sh)
 sd[a-z][0-9] root:disk 660 *(/lib/mdev/automount.sh)
+
+# USB tethering: a phone, or a modem in RNDIS, ECM or NCM mode. The kernel
+# builds these drivers as modules and nothing else loads them, so a phone that
+# tethered gave no usb0 at all (#1112). The classes are the drivers' own; a
+# blanket MODALIAS rule would also load usbserial for a modem before
+# rndis_host, which /etc/wireless/modem exists to prevent.
+$MODALIAS=usb:.*ic(E0isc01ip03|EFisc01ip01|02isc02ipFF|02isc06ip00|02isc0Dip00)in.* root:root 660 @modprobe -q "$MODALIAS"
+$INTERFACE=usb0 root:root 660 ! *(/lib/mdev/usbnet.sh)

general/overlay/etc/network/interfaces.d/usb0

@@ -1 +1,2 @@
 iface usb0 inet dhcp
+    metric 10

general/overlay/lib/mdev/usbnet.sh

@@ -0,0 +1,47 @@
+#!/bin/sh
+#
+# usb0 comes and goes with the cable, long after S40network has run: Android
+# switches tethering off whenever the cable is pulled, so the phone is
+# re-plugged and re-enabled at any time. Bring the interface up when it appears
+# and forget it when it goes. Without the ifdown, ifupdown still records the
+# vanished interface as configured, and the next plug got "ifup: interface usb0
+# already configured" and no address.
+
+# Both halves under one lock: the cable can be pulled while udhcpc is still in
+# its foreground retries, and an ifdown that finished first would leave the
+# late ifup's state behind for the next plug to trip over. The lock orders
+# nothing, though -- a pull within a second of the plug took it first -- so
+# the add looks again once it holds it.
+lock=/var/run/usbnet.$MDEV.lock
+
+case "$ACTION" in
+	add)
+		# Only the host side of a tether. A USB gadget's usb0 -- the camera
+		# being the device -- has no DHCP server across the cable, and
+		# udhcpc's leasefail would hand it the camera's ipaddr.
+		case "$(readlink "/sys/class/net/$MDEV/device/driver")" in
+			*/rndis_host | */cdc_ether | */cdc_ncm) ;;
+			*) exit 0 ;;
+		esac
+		# In the background: mdev waits for its helper, and udhcpc tries
+		# for seconds before it daemonizes. The daemon must not take the
+		# lock with it, hence the 9>&-. And ifup must not be the last
+		# command here: ash execs that one in place of the subshell, the
+		# 9>&- then drops the lock for the whole ifup, and a pull during
+		# DHCP left usb0 recorded as configured every time.
+		(
+			flock 9
+			[ -e "/sys/class/net/$MDEV" ] || exit 0
+			# A new interface: whatever ifupdown holds for the name
+			# is left over from the last one.
+			ifdown -f "$MDEV"
+			ifup "$MDEV" 9>&- || logger -t network "$MDEV: ifup failed"
+		) 9>"$lock" </dev/null >/dev/null 2>&1 &
+		;;
+	remove)
+		(
+			flock 9
+			ifdown -f "$MDEV"
+		) 9>"$lock" >/dev/null 2>&1
+		;;
+esac