man: say that .device units need udev

This was implied by the need to tag them with "systemd", but let's make this
obvious, since it's rather easy to trip over this as a user.

Inspired by https://bugzilla.redhat.com/show_bug.cgi?id=1941458.

1 files changed, 12 insertions, 13 deletions

diff --git a/man/systemd.device.xml b/man/systemd.device.xml
index 255ca3373b..596d334d5d 100644
--- a/man/systemd.device.xml
+++ b/man/systemd.device.xml
@@ -26,11 +26,10 @@
   <refsect1>
     <title>Description</title>
 
-    <para>A unit configuration file whose name ends in
-    <literal>.device</literal> encodes information about a device unit
-    as exposed in the
-    sysfs/<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
-    device tree.</para>
+    <para>A unit configuration file whose name ends in <literal>.device</literal> encodes information about a
+    device unit as exposed in the
+    sysfs/<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry> device
+    tree. This may be used to define dependencies between devices and other units.</para>
 
     <para>This unit type has no specific options. See
     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
@@ -40,14 +39,10 @@
     sections. A separate [Device] section does not
     exist, since no device-specific options may be configured.</para>
 
-    <para>systemd will dynamically create device units for all kernel
-    devices that are marked with the "systemd" udev tag (by default
-    all block and network devices, and a few others). This may be used
-    to define dependencies between devices and other units. To tag a
-    udev device, use <literal>TAG+="systemd"</literal> in the udev
-    rules file, see
-    <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
-    for details.</para>
+    <para>systemd will dynamically create device units for all kernel devices that are marked with the
+    <literal>systemd</literal> udev tag (by default all block and network devices, and a few others). Note
+    that <emphasis>if <filename>systemd-udev.service</filename> is not running, no device units will be
+    available (for example in a typical container)</emphasis>.</para>
 
     <para>Device units are named after the <filename>/sys/</filename>
     and <filename>/dev/</filename> paths they control. Example: the
@@ -57,6 +52,10 @@
     name see
     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
 
+    <para>To tag a udev device, use <literal>TAG+="systemd"</literal> in the udev rules file, see
+    <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry> for details.
+    </para>
+
     <para>Device units will be reloaded by systemd whenever the
     corresponding device generates a <literal>changed</literal> event.
     Other units can use <varname>ReloadPropagatedFrom=</varname> to react