Documentation/DocBook/v4l/vidioc-g-priority.xml

   1 <refentry id="vidioc-g-priority">
   2   <refmeta>
   3     <refentrytitle>ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY</refentrytitle>
   4     &manvol;
   5   </refmeta>
   6
   7   <refnamediv>
   8     <refname>VIDIOC_G_PRIORITY</refname>
   9     <refname>VIDIOC_S_PRIORITY</refname>
  10     <refpurpose>Query or request the access priority associated with a
  11 file descriptor</refpurpose>
  12   </refnamediv>
  13
  14   <refsynopsisdiv>
  15     <funcsynopsis>
  16       <funcprototype>
  17         <funcdef>int <function>ioctl</function></funcdef>
  18         <paramdef>int <parameter>fd</parameter></paramdef>
  19         <paramdef>int <parameter>request</parameter></paramdef>
  20         <paramdef>enum v4l2_priority *<parameter>argp</parameter></paramdef>
  21       </funcprototype>
  22     </funcsynopsis>
  23     <funcsynopsis>
  24       <funcprototype>
  25         <funcdef>int <function>ioctl</function></funcdef>
  26         <paramdef>int <parameter>fd</parameter></paramdef>
  27         <paramdef>int <parameter>request</parameter></paramdef>
  28         <paramdef>const enum v4l2_priority *<parameter>argp</parameter></paramdef>
  29       </funcprototype>
  30     </funcsynopsis>
  31   </refsynopsisdiv>
  32
  33   <refsect1>
  34     <title>Arguments</title>
  35
  36     <variablelist>
  37       <varlistentry>
  38         <term><parameter>fd</parameter></term>
  39         <listitem>
  40           <para>&fd;</para>
  41         </listitem>
  42       </varlistentry>
  43       <varlistentry>
  44         <term><parameter>request</parameter></term>
  45         <listitem>
  46           <para>VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY</para>
  47         </listitem>
  48       </varlistentry>
  49       <varlistentry>
  50         <term><parameter>argp</parameter></term>
  51         <listitem>
  52           <para>Pointer to an enum v4l2_priority type.</para>
  53         </listitem>
  54       </varlistentry>
  55     </variablelist>
  56   </refsect1>
  57
  58   <refsect1>
  59     <title>Description</title>
  60
  61     <para>To query the current access priority
  62 applications call the <constant>VIDIOC_G_PRIORITY</constant> ioctl
  63 with a pointer to an enum v4l2_priority variable where the driver stores
  64 the current priority.</para>
  65
  66     <para>To request an access priority applications store the
  67 desired priority in an enum v4l2_priority variable and call
  68 <constant>VIDIOC_S_PRIORITY</constant> ioctl with a pointer to this
  69 variable.</para>
  70
  71     <table frame="none" pgwide="1" id="v4l2-priority">
  72       <title>enum v4l2_priority</title>
  73       <tgroup cols="3">
  74         &cs-def;
  75         <tbody valign="top">
  76           <row>
  77             <entry><constant>V4L2_PRIORITY_UNSET</constant></entry>
  78             <entry>0</entry>
  79             <entry></entry>
  80           </row>
  81           <row>
  82             <entry><constant>V4L2_PRIORITY_BACKGROUND</constant></entry>
  83             <entry>1</entry>
  84             <entry>Lowest priority, usually applications running in
  85 background, for example monitoring VBI transmissions. A proxy
  86 application running in user space will be necessary if multiple
  87 applications want to read from a device at this priority.</entry>
  88           </row>
  89           <row>
  90             <entry><constant>V4L2_PRIORITY_INTERACTIVE</constant></entry>
  91             <entry>2</entry>
  92             <entry></entry>
  93           </row>
  94           <row>
  95             <entry><constant>V4L2_PRIORITY_DEFAULT</constant></entry>
  96             <entry>2</entry>
  97             <entry>Medium priority, usually applications started and
  98 interactively controlled by the user. For example TV viewers, Teletext
  99 browsers, or just "panel" applications to change the channel or video
 100 controls. This is the default priority unless an application requests
 101 another.</entry>
 102           </row>
 103           <row>
 104             <entry><constant>V4L2_PRIORITY_RECORD</constant></entry>
 105             <entry>3</entry>
 106             <entry>Highest priority. Only one file descriptor can have
 107 this priority, it blocks any other fd from changing device properties.
 108 Usually applications which must not be interrupted, like video
 109 recording.</entry>
 110           </row>
 111         </tbody>
 112       </tgroup>
 113     </table>
 114   </refsect1>
 115
 116   <refsect1>
 117     &return-value;
 118
 119     <variablelist>
 120       <varlistentry>
 121         <term><errorcode>EINVAL</errorcode></term>
 122         <listitem>
 123           <para>The requested priority value is invalid, or the
 124 driver does not support access priorities.</para>
 125         </listitem>
 126       </varlistentry>
 127       <varlistentry>
 128         <term><errorcode>EBUSY</errorcode></term>
 129         <listitem>
 130           <para>Another application already requested higher
 131 priority.</para>
 132         </listitem>
 133       </varlistentry>
 134     </variablelist>
 135   </refsect1>
 136 </refentry>
 137
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