man: import "Multi-Seat on Linux" into sd-login(3)

The document on the wiki is partially outdated and not very visible.  Let's
import the gist of it here. The original text is retained, with only grammar
and stylistic and formatting changes.

1 files changed, 145 insertions, 5 deletions

diff --git a/man/sd-login.xml b/man/sd-login.xml
index 6861fbe257..5022ff6be3 100644
--- a/man/sd-login.xml
+++ b/man/sd-login.xml
@@ -66,11 +66,6 @@
     and monitor seat, login session and user status information on the
     local system. </para>
 
-    <para>See <ulink
-    url="https://www.freedesktop.org/wiki/Software/systemd/multiseat">Multi-Seat
-    on Linux</ulink> for an introduction into multi-seat support on
-    Linux, the background for this set of APIs.</para>
-
     <para>Note that these APIs only allow purely passive access and
     monitoring of seats, sessions and users. To actively make changes
     to the seat configuration, terminate login sessions, or switch
@@ -115,6 +110,146 @@
     implemented.</para>
   </refsect1>
 
+  <refsect1>
+    <title>Definition of Terms</title>
+
+    <variablelist>
+      <varlistentry>
+        <term>seat</term>
+
+        <listitem><para>A seat consists of all hardware devices assigned to a specific
+        workplace. It consists of at least one graphics device, and usually also includes
+        keyboard, mouse. It can also include video cameras, sound cards and more. Seats
+        are identified by seat names, which are short strings (&lt;= 64 chars), that start
+        with the four characters <literal>seat</literal> followed by at least one more
+        character from the range [a-zA-Z0-9], <literal>_</literal> and
+        <literal>-</literal>. They are suitable for inclusion in file names. Seat names
+        may or may not be stable and may be reused if a seat becomes available again.
+        </para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term>session</term>
+
+        <listitem><para>A session is defined by the time a user is logged in until they
+        log out. A session is bound to one or no seats (the latter for 'virtual' ssh
+        logins). Multiple sessions can be attached to the same seat, but only one of them
+        can be active, the others are in the background. A session is identified by a
+        short string.</para>
+
+        <para>
+        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+        ensures that audit sessions are identical to systemd sessions, and uses the audit
+        session ID as session ID in systemd (if auditing is enabled). The session
+        identifier too shall be considered a short string (&lt;= 64 chars) consisting only
+        of [a-zA-Z0-9], <literal>_</literal> and <literal>-</literal>, suitable for
+        inclusion in a file name. Session IDs are unique on the local machine and are
+        never reused as long as the machine is online. A user (the way we know it on UNIX)
+        corresponds to the person using a computer. A single user can have multiple
+        sessions open at the same time. A user is identified by a numeric user id (UID) or
+        a user name (a string). A multi-session system allows multiple user sessions on
+        the same seat at the same time. A multi-seat system allows multiple independent
+        seats that can be individually and simultaneously used by different users.</para>
+        </listitem>
+      </varlistentry>
+    </variablelist>
+
+    <para>All hardware devices that are eligible to being assigned to a seat, are assigned
+    to one. A device can be assigned to only one seat at a time. If a device is not
+    assigned to any particular other seat it is implicitly assigned to the special default
+    seat called <literal>seat0</literal>.</para>
+
+    <para>Note that hardware like printers, hard disks or network cards is generally not
+    assigned to a specific seat. They are available to all seats equally. (Well, with one
+    exception: USB sticks can be assigned to a seat.)</para>
+
+    <para><literal>seat0</literal> always exists.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>udev Rules</title>
+
+    <para>Assignment of hardware devices to seats is managed inside the udev database, via
+    settings on the devices:</para>
+
+    <variablelist>
+      <varlistentry>
+        <term>Tag <literal>seat</literal></term>
+
+        <listitem><para>When set, a device is eligible to be assigned to a seat. This tag
+        is set for graphics devices, mice, keyboards, video cards, sound cards and
+        more. Note that some devices like sound cards consist of multiple subdevices
+        (i.e. a PCM for input and another one for output). This tag will be set only for
+        the originating device, not for the individual subdevices. A UI for configuring
+        assignment of devices to seats should enumerate and subscribe to all devices with
+        this tag set and show them in the UI. Note that USB hubs can be assigned to a seat
+        as well, in which case all (current and future) devices plugged into it will also
+        be assigned to the same seat (unless they are explicitly assigned to another
+        seat).
+        </para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term>Tag <literal>master-of-seat</literal></term>
+
+        <listitem><para>When set, this device is enough for a seat to be considered
+        existent. This tag is usually set for the framebuffer device of graphics cards. A
+        seat hence consists of an arbitrary number of devices marked with the
+        <literal>seat</literal> tag, but (at least) one of these devices needs to be
+        tagged with <literal>master-of-seat</literal> before the seat is actually
+        considered to be around.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term>Property <varname>ID_SEAT</varname></term>
+
+        <listitem><para>This property specifies the name of the seat a specific device is
+        assigned to. If not set the device is assigned to <literal>seat0</literal>. Also,
+        to speed up enumeration of hardware belonging to a specific seat, the seat is also
+        set as tag on the device. I.e. if the property
+        <varname>ID_SEAT=seat-waldo</varname> is set for a device, the tag
+        <literal>seat-waldo</literal> will be set as well. Note that if a device is
+        assigned to <literal>seat0</literal>, it will usually not carry such a tag and you
+        need to enumerate all devices and check the <varname>ID_SEAT</varname> property
+        manually. Again, if a device is assigned to seat0 this is visible on the device in
+        two ways: with a property <varname>ID_SEAT=seat0</varname> and with no property
+        <varname>ID_SEAT</varname> set for it at all.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term>Property <varname>ID_AUTOSEAT</varname></term>
+
+        <listitem><para>When set to <literal>1</literal>, this device automatically
+        generates a new and independent seat, which is named after the path of the
+        device. This is set for specialized USB hubs like the Plugable devices, which when
+        plugged in should create a hotplug seat without further configuration.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term>Property <varname>ID_FOR_SEAT</varname></term>
+
+        <listitem><para>When creating additional (manual) seats starting from a graphics
+        device this is a good choice to name the seat after. It is created from the path
+        of the device. This is useful in UIs for configuring seats: as soon as you create
+        a new seat from a graphics device, read this property and prefix it with
+        <literal>seat-</literal> and use it as name for the seat.</para></listitem>
+      </varlistentry>
+    </variablelist>
+
+    <para>A seat exists only and exclusively because a properly tagged device with the
+    right <varname>ID_SEAT</varname> property exists. Besides udev rules there is no
+    persistent data about seats stored on disk.</para>
+
+    <para>Note that
+    <citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+    manages ACLs on a number of device classes, to allow user code to access the device
+    nodes attached to a seat as long as the user has an active session on it. This is
+    mostly transparent to applications. As mentioned above, for certain user software it
+    might be a good idea to watch whether they can access device nodes instead of thinking
+    about seats.</para>
+  </refsect1>
+
   <xi:include href="libsystemd-pkgconfig.xml" />
 
   <refsect1>
@@ -130,6 +265,11 @@
       <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
       <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
     </para>
+
+    <para>
+      <ulink url="https://www.freedesktop.org/wiki/Software/systemd/multiseat">Multi-Seat on Linux</ulink>
+      for an introduction to multi-seat support on Linux and the background for this set of APIs.
+    </para>
   </refsect1>
 
 </refentry>