Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml - arm/linux-arm64-legacy - Git at Google

 <refentry id="vidioc-g-ext-ctrls">
   <refmeta>
     <refentrytitle>ioctl VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS,
 VIDIOC_TRY_EXT_CTRLS</refentrytitle>
     &manvol;
   </refmeta>

   <refnamediv>
     <refname>VIDIOC_G_EXT_CTRLS</refname>
     <refname>VIDIOC_S_EXT_CTRLS</refname>
     <refname>VIDIOC_TRY_EXT_CTRLS</refname>
     <refpurpose>Get or set the value of several controls, try control
 values</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_ext_controls
 *<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_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS,
 VIDIOC_TRY_EXT_CTRLS</para>
 	</listitem>
       </varlistentry>
       <varlistentry>
 	<term><parameter>argp</parameter></term>
 	<listitem>
 	  <para></para>
 	</listitem>
       </varlistentry>
     </variablelist>
   </refsect1>

   <refsect1>
     <title>Description</title>

     <para>These ioctls allow the caller to get or set multiple
 controls atomically. Control IDs are grouped into control classes (see
 <xref linkend="ctrl-class" />) and all controls in the control array
 must belong to the same control class.</para>

     <para>Applications must always fill in the
 <structfield>count</structfield>,
 <structfield>ctrl_class</structfield>,
 <structfield>controls</structfield> and
 <structfield>reserved</structfield> fields of &v4l2-ext-controls;, and
 initialize the &v4l2-ext-control; array pointed to by the
 <structfield>controls</structfield> fields.</para>

     <para>To get the current value of a set of controls applications
 initialize the <structfield>id</structfield>,
 <structfield>size</structfield> and <structfield>reserved2</structfield> fields
 of each &v4l2-ext-control; and call the
 <constant>VIDIOC_G_EXT_CTRLS</constant> ioctl. String controls controls
 must also set the <structfield>string</structfield> field.</para>

     <para>If the <structfield>size</structfield> is too small to
 receive the control result (only relevant for pointer-type controls
 like strings), then the driver will set <structfield>size</structfield>
 to a valid value and return an &ENOSPC;. You should re-allocate the
 string memory to this new size and try again. It is possible that the
 same issue occurs again if the string has grown in the meantime. It is
 recommended to call &VIDIOC-QUERYCTRL; first and use
 <structfield>maximum</structfield>+1 as the new <structfield>size</structfield>
 value. It is guaranteed that that is sufficient memory.
 </para>

     <para>To change the value of a set of controls applications
 initialize the <structfield>id</structfield>, <structfield>size</structfield>,
 <structfield>reserved2</structfield> and
 <structfield>value/string</structfield> fields of each &v4l2-ext-control; and
 call the <constant>VIDIOC_S_EXT_CTRLS</constant> ioctl. The controls
 will only be set if <emphasis>all</emphasis> control values are
 valid.</para>

     <para>To check if a set of controls have correct values applications
 initialize the <structfield>id</structfield>, <structfield>size</structfield>,
 <structfield>reserved2</structfield> and
 <structfield>value/string</structfield> fields of each &v4l2-ext-control; and
 call the <constant>VIDIOC_TRY_EXT_CTRLS</constant> ioctl. It is up to
 the driver whether wrong values are automatically adjusted to a valid
 value or if an error is returned.</para>

     <para>When the <structfield>id</structfield> or
 <structfield>ctrl_class</structfield> is invalid drivers return an
 &EINVAL;. When the value is out of bounds drivers can choose to take
 the closest valid value or return an &ERANGE;, whatever seems more
 appropriate. In the first case the new value is set in
 &v4l2-ext-control;. If the new control value is inappropriate (e.g. the
 given menu index is not supported by the menu control), then this will
 also result in an &EINVAL; error.</para>

     <para>The driver will only set/get these controls if all control
 values are correct. This prevents the situation where only some of the
 controls were set/get. Only low-level errors (&eg; a failed i2c
 command) can still cause this situation.</para>

     <table pgwide="1" frame="none" id="v4l2-ext-control">
       <title>struct <structname>v4l2_ext_control</structname></title>
       <tgroup cols="4">
 	&cs-ustr;
 	<tbody valign="top">
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>id</structfield></entry>
 	    <entry></entry>
 	    <entry>Identifies the control, set by the
 application.</entry>
 	  </row>
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>size</structfield></entry>
 	    <entry></entry>
 	    <entry>The total size in bytes of the payload of this
 control. This is normally 0, but for pointer controls this should be
 set to the size of the memory containing the payload, or that will
 receive the payload. If <constant>VIDIOC_G_EXT_CTRLS</constant> finds
 that this value is less than is required to store
 the payload result, then it is set to a value large enough to store the
 payload result and ENOSPC is returned. Note that for string controls
 this <structfield>size</structfield> field should not be confused with the length of the string.
 This field refers to the size of the memory that contains the string.
 The actual <emphasis>length</emphasis> of the string may well be much smaller.
 </entry>
 	  </row>
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>reserved2</structfield>[1]</entry>
 	    <entry></entry>
 	    <entry>Reserved for future extensions. Drivers and
 applications must set the array to zero.</entry>
 	  </row>
 	  <row>
 	    <entry>union</entry>
 	    <entry>(anonymous)</entry>
 	  </row>
 	  <row>
 	    <entry></entry>
 	    <entry>__s32</entry>
 	    <entry><structfield>value</structfield></entry>
 	    <entry>New value or current value.</entry>
 	  </row>
 	  <row>
 	    <entry></entry>
 	    <entry>__s64</entry>
 	    <entry><structfield>value64</structfield></entry>
 	    <entry>New value or current value.</entry>
 	  </row>
 	  <row>
 	    <entry></entry>
 	    <entry>char *</entry>
 	    <entry><structfield>string</structfield></entry>
 	    <entry>A pointer to a string.</entry>
 	  </row>
 	</tbody>
       </tgroup>
     </table>

     <table pgwide="1" frame="none" id="v4l2-ext-controls">
       <title>struct <structname>v4l2_ext_controls</structname></title>
       <tgroup cols="3">
 	&cs-str;
 	<tbody valign="top">
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>ctrl_class</structfield></entry>
 	    <entry>The control class to which all controls belong, see
 <xref linkend="ctrl-class" />. Drivers that use a kernel framework for handling
 controls will also accept a value of 0 here, meaning that the controls can
 belong to any control class. Whether drivers support this can be tested by setting
 <structfield>ctrl_class</structfield> to 0 and calling <constant>VIDIOC_TRY_EXT_CTRLS</constant>
 with a <structfield>count</structfield> of 0. If that succeeds, then the driver
 supports this feature.</entry>
 	  </row>
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>count</structfield></entry>
 	    <entry>The number of controls in the controls array. May
 also be zero.</entry>
 	  </row>
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>error_idx</structfield></entry>
 	    <entry><para>Set by the driver in case of an error. If the error is
 associated with a particular control, then <structfield>error_idx</structfield>
 is set to the index of that control. If the error is not related to a specific
 control, or the validation step failed (see below), then
 <structfield>error_idx</structfield> is set to <structfield>count</structfield>.
 The value is undefined if the ioctl returned 0 (success).</para>

 <para>Before controls are read from/written to hardware a validation step
 takes place: this checks if all controls in the list are valid controls,
 if no attempt is made to write to a read-only control or read from a write-only
 control, and any other up-front checks that can be done without accessing the
 hardware. The exact validations done during this step are driver dependent
 since some checks might require hardware access for some devices, thus making
 it impossible to do those checks up-front. However, drivers should make a
 best-effort to do as many up-front checks as possible.</para>

 <para>This check is done to avoid leaving the hardware in an inconsistent state due
 to easy-to-avoid problems. But it leads to another problem: the application needs to
 know whether an error came from the validation step (meaning that the hardware
 was not touched) or from an error during the actual reading from/writing to hardware.</para>

 <para>The, in hindsight quite poor, solution for that is to set <structfield>error_idx</structfield>
 to <structfield>count</structfield> if the validation failed. This has the
 unfortunate side-effect that it is not possible to see which control failed the
 validation. If the validation was successful and the error happened while
 accessing the hardware, then <structfield>error_idx</structfield> is less than
 <structfield>count</structfield> and only the controls up to
 <structfield>error_idx-1</structfield> were read or written correctly, and the
 state of the remaining controls is undefined.</para>

 <para>Since <constant>VIDIOC_TRY_EXT_CTRLS</constant> does not access hardware
 there is also no need to handle the validation step in this special way,
 so <structfield>error_idx</structfield> will just be set to the control that
 failed the validation step instead of to <structfield>count</structfield>.
 This means that if <constant>VIDIOC_S_EXT_CTRLS</constant> fails with
 <structfield>error_idx</structfield> set to <structfield>count</structfield>,
 then you can call <constant>VIDIOC_TRY_EXT_CTRLS</constant> to try to discover
 the actual control that failed the validation step. Unfortunately, there
 is no <constant>TRY</constant> equivalent for <constant>VIDIOC_G_EXT_CTRLS</constant>.
 </para></entry>
 	  </row>
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>reserved</structfield>[2]</entry>
 	    <entry>Reserved for future extensions. Drivers and
 applications must set the array to zero.</entry>
 	  </row>
 	  <row>
 	    <entry>&v4l2-ext-control; *</entry>
 	    <entry><structfield>controls</structfield></entry>
 	    <entry>Pointer to an array of
 <structfield>count</structfield> v4l2_ext_control structures. Ignored
 if <structfield>count</structfield> equals zero.</entry>
 	  </row>
 	</tbody>
       </tgroup>
     </table>

     <table pgwide="1" frame="none" id="ctrl-class">
       <title>Control classes</title>
       <tgroup cols="3">
 	&cs-def;
 	<tbody valign="top">
 	  <row>
 	    <entry><constant>V4L2_CTRL_CLASS_USER</constant></entry>
 	    <entry>0x980000</entry>
 	    <entry>The class containing user controls. These controls
 are described in <xref linkend="control" />. All controls that can be set
 using the &VIDIOC-S-CTRL; and &VIDIOC-G-CTRL; ioctl belong to this
 class.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_CTRL_CLASS_MPEG</constant></entry>
 	    <entry>0x990000</entry>
 	    <entry>The class containing MPEG compression controls.
 These controls are described in <xref
 		linkend="mpeg-controls" />.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_CTRL_CLASS_CAMERA</constant></entry>
 	    <entry>0x9a0000</entry>
 	    <entry>The class containing camera controls.
 These controls are described in <xref
 		linkend="camera-controls" />.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_CTRL_CLASS_FM_TX</constant></entry>
 	    <entry>0x9b0000</entry>
 	    <entry>The class containing FM Transmitter (FM TX) controls.
 These controls are described in <xref
 		linkend="fm-tx-controls" />.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_CTRL_CLASS_FLASH</constant></entry>
 	    <entry>0x9c0000</entry>
 	    <entry>The class containing flash device controls.
 These controls are described in <xref
 		linkend="flash-controls" />.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_CTRL_CLASS_JPEG</constant></entry>
 	    <entry>0x9d0000</entry>
 	    <entry>The class containing JPEG compression controls.
 These controls are described in <xref
 		linkend="jpeg-controls" />.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_CTRL_CLASS_IMAGE_SOURCE</constant></entry>
 	    <entry>0x9e0000</entry> <entry>The class containing image
 	    source controls. These controls are described in <xref
 	    linkend="image-source-controls" />.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_CTRL_CLASS_IMAGE_PROC</constant></entry>
 	    <entry>0x9f0000</entry> <entry>The class containing image
 	    processing controls. These controls are described in <xref
 	    linkend="image-process-controls" />.</entry>
 	  </row>

 	  <row>
 	    <entry><constant>V4L2_CTRL_CLASS_FM_RX</constant></entry>
 	    <entry>0xa10000</entry>
 	    <entry>The class containing FM Receiver (FM RX) controls.
 These controls are described in <xref
 		linkend="fm-rx-controls" />.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_CTRL_CLASS_RF_TUNER</constant></entry>
 	    <entry>0xa20000</entry>
 	    <entry>The class containing RF tuner controls.
 These controls are described in <xref linkend="rf-tuner-controls" />.</entry>
 	  </row>
 	</tbody>
       </tgroup>
     </table>

   </refsect1>

   <refsect1>
     &return-value;

     <variablelist>
       <varlistentry>
 	<term><errorcode>EINVAL</errorcode></term>
 	<listitem>
 	  <para>The &v4l2-ext-control; <structfield>id</structfield>
 is invalid, the &v4l2-ext-controls;
 <structfield>ctrl_class</structfield> is invalid, or the &v4l2-ext-control;
 <structfield>value</structfield> was inappropriate (e.g. the given menu
 index is not supported by the driver). This error code is
 also returned by the <constant>VIDIOC_S_EXT_CTRLS</constant> and
 <constant>VIDIOC_TRY_EXT_CTRLS</constant> ioctls if two or more
 control values are in conflict.</para>
 	</listitem>
       </varlistentry>
       <varlistentry>
 	<term><errorcode>ERANGE</errorcode></term>
 	<listitem>
 	  <para>The &v4l2-ext-control; <structfield>value</structfield>
 is out of bounds.</para>
 	</listitem>
       </varlistentry>
       <varlistentry>
 	<term><errorcode>EBUSY</errorcode></term>
 	<listitem>
 	  <para>The control is temporarily not changeable, possibly
 because another applications took over control of the device function
 this control belongs to.</para>
 	</listitem>
       </varlistentry>
       <varlistentry>
 	<term><errorcode>ENOSPC</errorcode></term>
 	<listitem>
 	  <para>The space reserved for the control's payload is insufficient.
 The field <structfield>size</structfield> is set to a value that is enough
 to store the payload and this error code is returned.</para>
 	</listitem>
       </varlistentry>
       <varlistentry>
 	<term><errorcode>EACCES</errorcode></term>
 	<listitem>
 	  <para>Attempt to try or set a read-only control or to get a
 	  write-only control.</para>
 	</listitem>
       </varlistentry>
     </variablelist>
   </refsect1>
 </refentry>
	<refentry id="vidioc-g-ext-ctrls">
	<refmeta>
	<refentrytitle>ioctl VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS,
	VIDIOC_TRY_EXT_CTRLS</refentrytitle>
	&manvol;
	</refmeta>

	<refnamediv>
	<refname>VIDIOC_G_EXT_CTRLS</refname>
	<refname>VIDIOC_S_EXT_CTRLS</refname>
	<refname>VIDIOC_TRY_EXT_CTRLS</refname>
	<refpurpose>Get or set the value of several controls, try control
	values</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_ext_controls
	*<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_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS,
	VIDIOC_TRY_EXT_CTRLS</para>
	</listitem>
	</varlistentry>
	<varlistentry>
	<term><parameter>argp</parameter></term>
	<listitem>
	<para></para>
	</listitem>
	</varlistentry>
	</variablelist>
	</refsect1>

	<refsect1>
	<title>Description</title>

	<para>These ioctls allow the caller to get or set multiple
	controls atomically. Control IDs are grouped into control classes (see
	<xref linkend="ctrl-class" />) and all controls in the control array
	must belong to the same control class.</para>

	<para>Applications must always fill in the
	<structfield>count</structfield>,
	<structfield>ctrl_class</structfield>,
	<structfield>controls</structfield> and
	<structfield>reserved</structfield> fields of &v4l2-ext-controls;, and
	initialize the &v4l2-ext-control; array pointed to by the
	<structfield>controls</structfield> fields.</para>

	<para>To get the current value of a set of controls applications
	initialize the <structfield>id</structfield>,
	<structfield>size</structfield> and <structfield>reserved2</structfield> fields
	of each &v4l2-ext-control; and call the
	<constant>VIDIOC_G_EXT_CTRLS</constant> ioctl. String controls controls
	must also set the <structfield>string</structfield> field.</para>

	<para>If the <structfield>size</structfield> is too small to
	receive the control result (only relevant for pointer-type controls
	like strings), then the driver will set <structfield>size</structfield>
	to a valid value and return an &ENOSPC;. You should re-allocate the
	string memory to this new size and try again. It is possible that the
	same issue occurs again if the string has grown in the meantime. It is
	recommended to call &VIDIOC-QUERYCTRL; first and use
	<structfield>maximum</structfield>+1 as the new <structfield>size</structfield>
	value. It is guaranteed that that is sufficient memory.
	</para>

	<para>To change the value of a set of controls applications
	initialize the <structfield>id</structfield>, <structfield>size</structfield>,
	<structfield>reserved2</structfield> and
	<structfield>value/string</structfield> fields of each &v4l2-ext-control; and
	call the <constant>VIDIOC_S_EXT_CTRLS</constant> ioctl. The controls
	will only be set if <emphasis>all</emphasis> control values are
	valid.</para>

	<para>To check if a set of controls have correct values applications
	initialize the <structfield>id</structfield>, <structfield>size</structfield>,
	<structfield>reserved2</structfield> and
	<structfield>value/string</structfield> fields of each &v4l2-ext-control; and
	call the <constant>VIDIOC_TRY_EXT_CTRLS</constant> ioctl. It is up to
	the driver whether wrong values are automatically adjusted to a valid
	value or if an error is returned.</para>

	<para>When the <structfield>id</structfield> or
	<structfield>ctrl_class</structfield> is invalid drivers return an
	&EINVAL;. When the value is out of bounds drivers can choose to take
	the closest valid value or return an &ERANGE;, whatever seems more
	appropriate. In the first case the new value is set in
	&v4l2-ext-control;. If the new control value is inappropriate (e.g. the
	given menu index is not supported by the menu control), then this will
	also result in an &EINVAL; error.</para>

	<para>The driver will only set/get these controls if all control
	values are correct. This prevents the situation where only some of the
	controls were set/get. Only low-level errors (&eg; a failed i2c
	command) can still cause this situation.</para>

	<table pgwide="1" frame="none" id="v4l2-ext-control">
	<title>struct <structname>v4l2_ext_control</structname></title>
	<tgroup cols="4">
	&cs-ustr;
	<tbody valign="top">
	<row>
	<entry>__u32</entry>
	<entry><structfield>id</structfield></entry>
	<entry></entry>
	<entry>Identifies the control, set by the
	application.</entry>
	</row>
	<row>
	<entry>__u32</entry>
	<entry><structfield>size</structfield></entry>
	<entry></entry>
	<entry>The total size in bytes of the payload of this
	control. This is normally 0, but for pointer controls this should be
	set to the size of the memory containing the payload, or that will
	receive the payload. If <constant>VIDIOC_G_EXT_CTRLS</constant> finds
	that this value is less than is required to store
	the payload result, then it is set to a value large enough to store the
	payload result and ENOSPC is returned. Note that for string controls
	this <structfield>size</structfield> field should not be confused with the length of the string.
	This field refers to the size of the memory that contains the string.
	The actual <emphasis>length</emphasis> of the string may well be much smaller.
	</entry>
	</row>
	<row>
	<entry>__u32</entry>
	<entry><structfield>reserved2</structfield>[1]</entry>
	<entry></entry>
	<entry>Reserved for future extensions. Drivers and
	applications must set the array to zero.</entry>
	</row>
	<row>
	<entry>union</entry>
	<entry>(anonymous)</entry>
	</row>
	<row>
	<entry></entry>
	<entry>__s32</entry>
	<entry><structfield>value</structfield></entry>
	<entry>New value or current value.</entry>
	</row>
	<row>
	<entry></entry>
	<entry>__s64</entry>
	<entry><structfield>value64</structfield></entry>
	<entry>New value or current value.</entry>
	</row>
	<row>
	<entry></entry>
	<entry>char *</entry>
	<entry><structfield>string</structfield></entry>
	<entry>A pointer to a string.</entry>
	</row>
	</tbody>
	</tgroup>
	</table>

	<table pgwide="1" frame="none" id="v4l2-ext-controls">
	<title>struct <structname>v4l2_ext_controls</structname></title>
	<tgroup cols="3">
	&cs-str;
	<tbody valign="top">
	<row>
	<entry>__u32</entry>
	<entry><structfield>ctrl_class</structfield></entry>
	<entry>The control class to which all controls belong, see
	<xref linkend="ctrl-class" />. Drivers that use a kernel framework for handling
	controls will also accept a value of 0 here, meaning that the controls can
	belong to any control class. Whether drivers support this can be tested by setting
	<structfield>ctrl_class</structfield> to 0 and calling <constant>VIDIOC_TRY_EXT_CTRLS</constant>
	with a <structfield>count</structfield> of 0. If that succeeds, then the driver
	supports this feature.</entry>
	</row>
	<row>
	<entry>__u32</entry>
	<entry><structfield>count</structfield></entry>
	<entry>The number of controls in the controls array. May
	also be zero.</entry>
	</row>
	<row>
	<entry>__u32</entry>
	<entry><structfield>error_idx</structfield></entry>
	<entry><para>Set by the driver in case of an error. If the error is
	associated with a particular control, then <structfield>error_idx</structfield>
	is set to the index of that control. If the error is not related to a specific
	control, or the validation step failed (see below), then
	<structfield>error_idx</structfield> is set to <structfield>count</structfield>.
	The value is undefined if the ioctl returned 0 (success).</para>

	<para>Before controls are read from/written to hardware a validation step
	takes place: this checks if all controls in the list are valid controls,
	if no attempt is made to write to a read-only control or read from a write-only
	control, and any other up-front checks that can be done without accessing the
	hardware. The exact validations done during this step are driver dependent
	since some checks might require hardware access for some devices, thus making
	it impossible to do those checks up-front. However, drivers should make a
	best-effort to do as many up-front checks as possible.</para>

	<para>This check is done to avoid leaving the hardware in an inconsistent state due
	to easy-to-avoid problems. But it leads to another problem: the application needs to
	know whether an error came from the validation step (meaning that the hardware
	was not touched) or from an error during the actual reading from/writing to hardware.</para>

	<para>The, in hindsight quite poor, solution for that is to set <structfield>error_idx</structfield>
	to <structfield>count</structfield> if the validation failed. This has the
	unfortunate side-effect that it is not possible to see which control failed the
	validation. If the validation was successful and the error happened while
	accessing the hardware, then <structfield>error_idx</structfield> is less than
	<structfield>count</structfield> and only the controls up to
	<structfield>error_idx-1</structfield> were read or written correctly, and the
	state of the remaining controls is undefined.</para>

	<para>Since <constant>VIDIOC_TRY_EXT_CTRLS</constant> does not access hardware
	there is also no need to handle the validation step in this special way,
	so <structfield>error_idx</structfield> will just be set to the control that
	failed the validation step instead of to <structfield>count</structfield>.
	This means that if <constant>VIDIOC_S_EXT_CTRLS</constant> fails with
	<structfield>error_idx</structfield> set to <structfield>count</structfield>,
	then you can call <constant>VIDIOC_TRY_EXT_CTRLS</constant> to try to discover
	the actual control that failed the validation step. Unfortunately, there
	is no <constant>TRY</constant> equivalent for <constant>VIDIOC_G_EXT_CTRLS</constant>.
	</para></entry>
	</row>
	<row>
	<entry>__u32</entry>
	<entry><structfield>reserved</structfield>[2]</entry>
	<entry>Reserved for future extensions. Drivers and
	applications must set the array to zero.</entry>
	</row>
	<row>
	<entry>&v4l2-ext-control; *</entry>
	<entry><structfield>controls</structfield></entry>
	<entry>Pointer to an array of
	<structfield>count</structfield> v4l2_ext_control structures. Ignored
	if <structfield>count</structfield> equals zero.</entry>
	</row>
	</tbody>
	</tgroup>
	</table>

	<table pgwide="1" frame="none" id="ctrl-class">
	<title>Control classes</title>
	<tgroup cols="3">
	&cs-def;
	<tbody valign="top">
	<row>
	<entry><constant>V4L2_CTRL_CLASS_USER</constant></entry>
	<entry>0x980000</entry>
	<entry>The class containing user controls. These controls
	are described in <xref linkend="control" />. All controls that can be set
	using the &VIDIOC-S-CTRL; and &VIDIOC-G-CTRL; ioctl belong to this
	class.</entry>
	</row>
	<row>
	<entry><constant>V4L2_CTRL_CLASS_MPEG</constant></entry>
	<entry>0x990000</entry>
	<entry>The class containing MPEG compression controls.
	These controls are described in <xref
	linkend="mpeg-controls" />.</entry>
	</row>
	<row>
	<entry><constant>V4L2_CTRL_CLASS_CAMERA</constant></entry>
	<entry>0x9a0000</entry>
	<entry>The class containing camera controls.
	These controls are described in <xref
	linkend="camera-controls" />.</entry>
	</row>
	<row>
	<entry><constant>V4L2_CTRL_CLASS_FM_TX</constant></entry>
	<entry>0x9b0000</entry>
	<entry>The class containing FM Transmitter (FM TX) controls.
	These controls are described in <xref
	linkend="fm-tx-controls" />.</entry>
	</row>
	<row>
	<entry><constant>V4L2_CTRL_CLASS_FLASH</constant></entry>
	<entry>0x9c0000</entry>
	<entry>The class containing flash device controls.
	These controls are described in <xref
	linkend="flash-controls" />.</entry>
	</row>
	<row>
	<entry><constant>V4L2_CTRL_CLASS_JPEG</constant></entry>
	<entry>0x9d0000</entry>
	<entry>The class containing JPEG compression controls.
	These controls are described in <xref
	linkend="jpeg-controls" />.</entry>
	</row>
	<row>
	<entry><constant>V4L2_CTRL_CLASS_IMAGE_SOURCE</constant></entry>
	<entry>0x9e0000</entry> <entry>The class containing image
	source controls. These controls are described in <xref
	linkend="image-source-controls" />.</entry>
	</row>
	<row>
	<entry><constant>V4L2_CTRL_CLASS_IMAGE_PROC</constant></entry>
	<entry>0x9f0000</entry> <entry>The class containing image
	processing controls. These controls are described in <xref
	linkend="image-process-controls" />.</entry>
	</row>

	<row>
	<entry><constant>V4L2_CTRL_CLASS_FM_RX</constant></entry>
	<entry>0xa10000</entry>
	<entry>The class containing FM Receiver (FM RX) controls.
	These controls are described in <xref
	linkend="fm-rx-controls" />.</entry>
	</row>
	<row>
	<entry><constant>V4L2_CTRL_CLASS_RF_TUNER</constant></entry>
	<entry>0xa20000</entry>
	<entry>The class containing RF tuner controls.
	These controls are described in <xref linkend="rf-tuner-controls" />.</entry>
	</row>
	</tbody>
	</tgroup>
	</table>

	</refsect1>

	<refsect1>
	&return-value;

	<variablelist>
	<varlistentry>
	<term><errorcode>EINVAL</errorcode></term>
	<listitem>
	<para>The &v4l2-ext-control; <structfield>id</structfield>
	is invalid, the &v4l2-ext-controls;
	<structfield>ctrl_class</structfield> is invalid, or the &v4l2-ext-control;
	<structfield>value</structfield> was inappropriate (e.g. the given menu
	index is not supported by the driver). This error code is
	also returned by the <constant>VIDIOC_S_EXT_CTRLS</constant> and
	<constant>VIDIOC_TRY_EXT_CTRLS</constant> ioctls if two or more
	control values are in conflict.</para>
	</listitem>
	</varlistentry>
	<varlistentry>
	<term><errorcode>ERANGE</errorcode></term>
	<listitem>
	<para>The &v4l2-ext-control; <structfield>value</structfield>
	is out of bounds.</para>
	</listitem>
	</varlistentry>
	<varlistentry>
	<term><errorcode>EBUSY</errorcode></term>
	<listitem>
	<para>The control is temporarily not changeable, possibly
	because another applications took over control of the device function
	this control belongs to.</para>
	</listitem>
	</varlistentry>
	<varlistentry>
	<term><errorcode>ENOSPC</errorcode></term>
	<listitem>
	<para>The space reserved for the control's payload is insufficient.
	The field <structfield>size</structfield> is set to a value that is enough
	to store the payload and this error code is returned.</para>
	</listitem>
	</varlistentry>
	<varlistentry>
	<term><errorcode>EACCES</errorcode></term>
	<listitem>
	<para>Attempt to try or set a read-only control or to get a
	write-only control.</para>
	</listitem>
	</varlistentry>
	</variablelist>
	</refsect1>
	</refentry>