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6. vidioc-g-ext-ctrls.xml

<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>

	</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>