since no device-specific options may be
configured.</para>
- <para>systemd will automatically create dynamic 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.</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>Device units are named after the
<filename>/sys</filename> and
<para>The settings of device units may either be
configured via unit files, or directly from the udev
- database (which is recommended). The following udev
+ database (which is recommended). The following udev device
properties are understood by systemd:</para>
<variablelist class='udev-directives'>
<varlistentry>
<term><varname>SYSTEMD_WANTS=</varname></term>
+ <term><varname>SYSTEMD_USER_WANTS=</varname></term>
<listitem><para>Adds dependencies of
- type <varname>Wants</varname> from
- this unit to all listed units. This
- may be used to activate arbitrary
- units, when a specific device becomes
- available. Note that this and the
+ type <varname>Wants</varname> from the
+ device unit to all listed units. The
+ first form is used by the system
+ systemd instance, the second by user
+ systemd instances. Those settings may
+ be used to activate arbitrary units
+ when a specific device becomes
+ available.</para>
+
+ <para>Note that this and the
other tags are not taken into account
unless the device is tagged with the
<literal>systemd</literal> string in
the udev database, because otherwise
- the device is not exposed as systemd
- unit.</para></listitem>
+ the device is not exposed as a systemd
+ unit (see above).</para>
+
+ <para>Note that systemd will only act
+ on <varname>Wants</varname>
+ dependencies when a device first
+ becomes active. It will not act on
+ them if they are added to devices that
+ are already active. Use
+ <varname>SYSTEMD_READY=</varname> (see
+ below) to influence on which udev
+ event to trigger the dependencies.
+ </para></listitem>
</varlistentry>
<varlistentry>
<varlistentry>
<term><varname>SYSTEMD_READY=</varname></term>
- <listitem><para>If set to 0 systemd
+ <listitem><para>If set to 0, systemd
will consider this device unplugged
even if it shows up in the udev
tree. If this property is unset or set
- to 1 the device will be considered
- plugged the moment it shows up in the
+ to 1, the device will be considered
+ plugged if it is visible in the
udev tree. This property has no
influence on the behavior when a
device disappears from the udev
- tree. This option is useful to support
+ tree.</para>
+
+ <para>This option is useful to support
devices that initially show up in an
- uninitialized state in the tree, and for
- which a changed event is generated the
- moment they are fully set
- up.</para></listitem>
+ uninitialized state in the tree, and
+ for which a <literal>changed</literal>
+ event is generated the moment they are
+ fully set up. Note that
+ <varname>SYSTEMD_WANTS=</varname> (see
+ above) is not acted on as long as
+ <varname>SYSTEMD_READY=0</varname> is
+ set for a device.</para></listitem>
</varlistentry>
<varlistentry>
~ianmdlvl / elogind.git / blobdiff