units: add usb-gadget target

Linux can be run on a device meant to act as a USB peripheral. In order
for a machine to act as such a USB device it has to be equipped with
a UDC - USB Device Controller.

This patch adds a target reached when UDC becomes available. It can be used
for activating e.g. a service unit which composes a USB gadget with
configfs and activates it.

man/systemd.special.xml

@@ -83,6 +83,7 @@
     <filename>time-sync.target</filename>,
     <filename>timers.target</filename>,
     <filename>umount.target</filename>,
+    <filename>usb-gadget.target</filename>,
     <!-- slices --><filename>-.slice</filename>,
     <filename>system.slice</filename>,
     <filename>user.slice</filename>,
@@ -799,6 +800,16 @@
             dynamically when audio hardware is found.</para>
           </listitem>
         </varlistentry>
+        <varlistentry>
+          <term><filename>usb-gadget.target</filename></term>
+          <listitem>
+            <para>This target is started automatically as soon as a
+            USB Device Controller becomes available at boot.</para>
+
+            <para>This may be used to pull in usb gadget
+            dynamically when UDC hardware is found.</para>
+          </listitem>
+        </varlistentry>
       </variablelist>
     </refsect2>
 

rules/99-systemd.rules.in

@@ -56,6 +56,8 @@ SUBSYSTEM=="printer", TAG+="systemd", ENV{SYSTEMD_WANTS}+="printer.target"
 SUBSYSTEM=="usb", KERNEL=="lp*", TAG+="systemd", ENV{SYSTEMD_WANTS}+="printer.target"
 SUBSYSTEM=="usb", ENV{DEVTYPE}=="usb_device", ENV{ID_USB_INTERFACES}=="*:0701??:*", TAG+="systemd", ENV{SYSTEMD_WANTS}+="printer.target"
 
+SUBSYSTEM=="udc", ACTION=="add", TAG+="systemd", ENV{SYSTEMD_WANTS}+="usb-gadget.target"
+
 # Apply sysctl variables to network devices (and only to those) as they appear.
 ACTION=="add", SUBSYSTEM=="net", KERNEL!="lo", RUN+="@rootlibexecdir@/systemd-sysctl --prefix=/net/ipv4/conf/$name --prefix=/net/ipv4/neigh/$name --prefix=/net/ipv6/conf/$name --prefix=/net/ipv6/neigh/$name"
 

units/usb-gadget.target

@@ -0,0 +1,12 @@
+#  SPDX-License-Identifier: LGPL-2.1+
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Hardware activated USB gadget
+Documentation=man:systemd.special(7)