diff --git a/Documentation/DocBook/media/dvb/demux.xml b/Documentation/DocBook/media/dvb/demux.xml
index 86de89cfbd67..c8683d66f059 100644
--- a/Documentation/DocBook/media/dvb/demux.xml
+++ b/Documentation/DocBook/media/dvb/demux.xml
@@ -1042,7 +1042,14 @@ role="subsection"><title>DMX_ADD_PID</title>
 </para>
 <informaltable><tgroup cols="1"><tbody><row><entry
  align="char">
-<para>This ioctl is undocumented. Documentation is welcome.</para>
+<para>This ioctl call allows to add multiple PIDs to a transport stream filter
+previously set up with DMX_SET_PES_FILTER and output equal to DMX_OUT_TSDEMUX_TAP.
+</para></entry></row><row><entry align="char"><para>
+It is used by readers of /dev/dvb/adapterX/demuxY.
+</para></entry></row><row><entry align="char"><para>
+It may be called at any time, i.e. before or after the first filter on the
+shared file descriptor was started. It makes it possible to record multiple
+services without the need to de-multiplex or re-multiplex TS packets.</para>
 </entry>
  </row></tbody></tgroup></informaltable>
 <para>SYNOPSIS
@@ -1075,7 +1082,7 @@ role="subsection"><title>DMX_ADD_PID</title>
 </para>
 </entry><entry
  align="char">
-<para>Undocumented.</para>
+<para>PID number to be filtered.</para>
 </entry>
  </row></tbody></tgroup></informaltable>
 &return-value-dvb;
@@ -1087,7 +1094,15 @@ role="subsection"><title>DMX_REMOVE_PID</title>
 </para>
 <informaltable><tgroup cols="1"><tbody><row><entry
  align="char">
-<para>This ioctl is undocumented. Documentation is welcome.</para>
+<para>This ioctl call allows to remove a PID when multiple PIDs are set on a
+transport stream filter, e. g. a filter previously set up with output equal to
+DMX_OUT_TSDEMUX_TAP, created via either DMX_SET_PES_FILTER or DMX_ADD_PID.
+</para></entry></row><row><entry align="char"><para>
+It is used by readers of /dev/dvb/adapterX/demuxY.
+</para></entry></row><row><entry align="char"><para>
+It may be called at any time, i.e. before or after the first filter on the
+shared file descriptor was started. It makes it possible to record multiple
+services without the need to de-multiplex or re-multiplex TS packets.</para>
 </entry>
  </row></tbody></tgroup></informaltable>
 <para>SYNOPSIS
@@ -1120,7 +1135,7 @@ role="subsection"><title>DMX_REMOVE_PID</title>
 </para>
 </entry><entry
  align="char">
-<para>Undocumented.</para>
+<para>PID of the PES filter to be removed.</para>
 </entry>
  </row></tbody></tgroup></informaltable>
 &return-value-dvb;