man/udev.xml: add doc about hwdb database

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>

1 files changed, 35 insertions, 0 deletions

diff --git a/man/udev.xml b/man/udev.xml
index 2719c867e4..0a2a383a7d 100644
--- a/man/udev.xml
+++ b/man/udev.xml
@@ -697,6 +697,41 @@
       </variablelist>
   </refsect1>
 
+  <refsect1><title>Hardware Database Files</title>
+      <para>The hwdb files are read from the files located in the
+      system hwdb directory <filename>/lib/udev/hwdb.d</filename>,
+      the volatile runtime directory <filename>/run/udev/hwdb.d</filename>
+      and the local administration directory <filename>/etc/udev/hwdb.d</filename>.
+      All hwdb files are collectively sorted and processed in lexical order,
+      regardless of the directories in which they live. However, files with
+      identical filenames replace each other. Files in <filename>/etc</filename>
+      have the highest priority, files in <filename>/run</filename> take precedence
+      over files with the same name in <filename>/lib</filename>. This can be
+      used to override an hwdb file with a local file if needed;
+      a symlink in <filename>/etc</filename> with the same name as a hwdb file in
+      <filename>/lib</filename>, pointing to <filename>/dev/null</filename>,
+      disables the hwdb file entirely. hwdb files must have the extension
+      <filename>.hwdb</filename>; other extensions are ignored.</para>
+
+      <para>The hwdb file contains data records consisting of matches and
+      associated key-value pairs. Every record in the hwdb starts with one or
+      more match string, specifying a shell glob to compare the database
+      lookup string against. Multiple match lines are specified in additional
+      consecutive lines. Every match line is compared indivdually, they are
+      combined by OR. Every match line must start at the first character of
+      the line.</para>
+
+      <para>The match lines are followed by one or more key-value pair lines, which
+      are recognized by a leading space character. The key name and value are separated
+      by <literal>=</literal>. An empty line signifies the end
+      of a record. Lines beginning with <literal>#</literal> are ignored.</para>
+
+      <para>The content of all hwdb files is read by
+      <citerefentry><refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+      and compiled to a binary database located at <filename>/etc/udev/hwdb.bin</filename>.
+      During runtime only the binary database is used.</para>
+  </refsect1>
+
   <refsect1>
     <title>See Also</title>
     <para><citerefentry>