<refentry id="vidioc-dbg-g-register">
  <refmeta>
    <refentrytitle>ioctl VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER</refentrytitle>
    &manvol;
  </refmeta>

  <refnamediv>
    <refname>VIDIOC_DBG_G_REGISTER</refname>
    <refname>VIDIOC_DBG_S_REGISTER</refname>
    <refpurpose>Read or write hardware registers</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcprototype>
	<funcdef>int <function>ioctl</function></funcdef>
	<paramdef>int <parameter>fd</parameter></paramdef>
	<paramdef>int <parameter>request</parameter></paramdef>
	<paramdef>struct v4l2_dbg_register *<parameter>argp</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
    <funcsynopsis>
      <funcprototype>
	<funcdef>int <function>ioctl</function></funcdef>
	<paramdef>int <parameter>fd</parameter></paramdef>
	<paramdef>int <parameter>request</parameter></paramdef>
	<paramdef>const struct v4l2_dbg_register
*<parameter>argp</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Arguments</title>

    <variablelist>
      <varlistentry>
	<term><parameter>fd</parameter></term>
	<listitem>
	  <para>&fd;</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><parameter>request</parameter></term>
	<listitem>
	  <para>VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><parameter>argp</parameter></term>
	<listitem>
	  <para></para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Description</title>

    <note>
      <title>Experimental</title>

      <para>This is an <link linkend="experimental">experimental</link>
interface and may change in the future.</para>
    </note>

    <para>For driver debugging purposes these ioctls allow test
applications to access hardware registers directly. Regular
applications must not use them.</para>

    <para>Since writing or even reading registers can jeopardize the
system security, its stability and damage the hardware, both ioctls
require superuser privileges. Additionally the Linux kernel must be
compiled with the <constant>CONFIG_VIDEO_ADV_DEBUG</constant> option
to enable these ioctls.</para>

    <para>To write a register applications must initialize all fields
of a &v4l2-dbg-register; and call
<constant>VIDIOC_DBG_S_REGISTER</constant> with a pointer to this
structure. The <structfield>match.type</structfield> and
<structfield>match.addr</structfield> or <structfield>match.name</structfield>
fields select a chip on the TV
card, the <structfield>reg</structfield> field specifies a register
number and the <structfield>val</structfield> field the value to be
written into the register.</para>

    <para>To read a register applications must initialize the
<structfield>match.type</structfield>,
<structfield>match.addr</structfield> or <structfield>match.name</structfield> and
<structfield>reg</structfield> fields, and call
<constant>VIDIOC_DBG_G_REGISTER</constant> with a pointer to this
structure. On success the driver stores the register value in the
<structfield>val</structfield> field. On failure the structure remains
unchanged.</para>

    <para>When <structfield>match.type</structfield> is
<constant>V4L2_CHIP_MATCH_BRIDGE</constant>,
<structfield>match.addr</structfield> selects the nth non-sub-device chip
on the TV card.  The number zero always selects the host chip, &eg; the
chip connected to the PCI or USB bus. You can find out which chips are
present with the &VIDIOC-DBG-G-CHIP-INFO; ioctl.</para>

    <para>When <structfield>match.type</structfield> is
<constant>V4L2_CHIP_MATCH_I2C_DRIVER</constant>,
<structfield>match.name</structfield> contains the I2C driver name.
For instance
<constant>"saa7127"</constant> will match any chip
supported by the saa7127 driver, regardless of its &i2c; bus address.
When multiple chips supported by the same driver are present, the
effect of these ioctls is undefined. Again with the
&VIDIOC-DBG-G-CHIP-INFO; ioctl you can find out which &i2c; chips are
present.</para>

    <para>When <structfield>match.type</structfield> is
<constant>V4L2_CHIP_MATCH_I2C_ADDR</constant>,
<structfield>match.addr</structfield> selects a chip by its 7 bit &i2c;
bus address.</para>

    <para>When <structfield>match.type</structfield> is
<constant>V4L2_CHIP_MATCH_AC97</constant>,
<structfield>match.addr</structfield> selects the nth AC97 chip
on the TV card.</para>

    <para>When <structfield>match.type</structfield> is
<constant>V4L2_CHIP_MATCH_SUBDEV</constant>,
<structfield>match.addr</structfield> selects the nth sub-device.</para>

    <note>
      <title>Success not guaranteed</title>

      <para>Due to a flaw in the Linux &i2c; bus driver these ioctls may
return successfully without actually reading or writing a register. To
catch the most likely failure we recommend a &VIDIOC-DBG-G-CHIP-INFO;
call confirming the presence of the selected &i2c; chip.</para>
    </note>

    <para>These ioctls are optional, not all drivers may support them.
However when a driver supports these ioctls it must also support
&VIDIOC-DBG-G-CHIP-INFO;. Conversely it may support
<constant>VIDIOC_DBG_G_CHIP_INFO</constant> but not these ioctls.</para>

    <para><constant>VIDIOC_DBG_G_REGISTER</constant> and
<constant>VIDIOC_DBG_S_REGISTER</constant> were introduced in Linux
2.6.21, but their API was changed to the one described here in kernel 2.6.29.</para>

    <para>We recommended the <application>v4l2-dbg</application>
utility over calling these ioctls directly. It is available from the
LinuxTV v4l-dvb repository; see <ulink
url="http://linuxtv.org/repo/">http://linuxtv.org/repo/</ulink> for
access instructions.</para>

    <!-- Note for convenience vidioc-dbg-g-chip-ident.sgml
	 contains a duplicate of this table. -->
    <table pgwide="1" frame="none" id="v4l2-dbg-match">
      <title>struct <structname>v4l2_dbg_match</structname></title>
      <tgroup cols="4">
	&cs-ustr;
	<tbody valign="top">
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>type</structfield></entry>
	    <entry>See <xref linkend="ident-chip-match-types" /> for a list of
possible types.</entry>
	  </row>
	  <row>
	    <entry>union</entry>
	    <entry>(anonymous)</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>__u32</entry>
	    <entry><structfield>addr</structfield></entry>
	    <entry>Match a chip by this number, interpreted according
to the <structfield>type</structfield> field.</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>char</entry>
	    <entry><structfield>name[32]</structfield></entry>
	    <entry>Match a chip by this name, interpreted according
to the <structfield>type</structfield> field.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>


    <table pgwide="1" frame="none" id="v4l2-dbg-register">
      <title>struct <structname>v4l2_dbg_register</structname></title>
      <tgroup cols="4">
	<colspec colname="c1" />
	<colspec colname="c2" />
	<colspec colname="c4" />
	<tbody valign="top">
	  <row>
	    <entry>struct v4l2_dbg_match</entry>
	    <entry><structfield>match</structfield></entry>
	    <entry>How to match the chip, see <xref linkend="v4l2-dbg-match" />.</entry>
	  </row>
	  <row>
	    <entry>__u64</entry>
	    <entry><structfield>reg</structfield></entry>
	    <entry>A register number.</entry>
	  </row>
	  <row>
	    <entry>__u64</entry>
	    <entry><structfield>val</structfield></entry>
	    <entry>The value read from, or to be written into the
register.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

    <!-- Note for convenience vidioc-dbg-g-chip-ident.sgml
	 contains a duplicate of this table. -->
    <table pgwide="1" frame="none" id="chip-match-types">
      <title>Chip Match Types</title>
      <tgroup cols="3">
	&cs-def;
	<tbody valign="top">
	  <row>
	    <entry><constant>V4L2_CHIP_MATCH_BRIDGE</constant></entry>
	    <entry>0</entry>
	    <entry>Match the nth chip on the card, zero for the
	    bridge chip. Does not match sub-devices.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_CHIP_MATCH_I2C_DRIVER</constant></entry>
	    <entry>1</entry>
	    <entry>Match an &i2c; chip by its driver name.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_CHIP_MATCH_I2C_ADDR</constant></entry>
	    <entry>2</entry>
	    <entry>Match a chip by its 7 bit &i2c; bus address.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_CHIP_MATCH_AC97</constant></entry>
	    <entry>3</entry>
	    <entry>Match the nth anciliary AC97 chip.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_CHIP_MATCH_SUBDEV</constant></entry>
	    <entry>4</entry>
	    <entry>Match the nth sub-device.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>
  </refsect1>

  <refsect1>
    &return-value;

    <variablelist>
      <varlistentry>
	<term><errorcode>EPERM</errorcode></term>
	<listitem>
	  <para>Insufficient permissions. Root privileges are required
to execute these ioctls.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>
</refentry>
