Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml

   1 <refentry id="vidioc-g-ext-ctrls">
   2   <refmeta>
   3     <refentrytitle>ioctl VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS,
   4 VIDIOC_TRY_EXT_CTRLS</refentrytitle>
   5     &manvol;
   6   </refmeta>
   7
   8   <refnamediv>
   9     <refname>VIDIOC_G_EXT_CTRLS</refname>
  10     <refname>VIDIOC_S_EXT_CTRLS</refname>
  11     <refname>VIDIOC_TRY_EXT_CTRLS</refname>
  12     <refpurpose>Get or set the value of several controls, try control
  13 values</refpurpose>
  14   </refnamediv>
  15
  16   <refsynopsisdiv>
  17     <funcsynopsis>
  18       <funcprototype>
  19         <funcdef>int <function>ioctl</function></funcdef>
  20         <paramdef>int <parameter>fd</parameter></paramdef>
  21         <paramdef>int <parameter>request</parameter></paramdef>
  22         <paramdef>struct v4l2_ext_controls
  23 *<parameter>argp</parameter></paramdef>
  24       </funcprototype>
  25     </funcsynopsis>
  26   </refsynopsisdiv>
  27
  28   <refsect1>
  29     <title>Arguments</title>
  30
  31     <variablelist>
  32       <varlistentry>
  33         <term><parameter>fd</parameter></term>
  34         <listitem>
  35           <para>&fd;</para>
  36         </listitem>
  37       </varlistentry>
  38       <varlistentry>
  39         <term><parameter>request</parameter></term>
  40         <listitem>
  41           <para>VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS,
  42 VIDIOC_TRY_EXT_CTRLS</para>
  43         </listitem>
  44       </varlistentry>
  45       <varlistentry>
  46         <term><parameter>argp</parameter></term>
  47         <listitem>
  48           <para></para>
  49         </listitem>
  50       </varlistentry>
  51     </variablelist>
  52   </refsect1>
  53
  54   <refsect1>
  55     <title>Description</title>
  56
  57     <para>These ioctls allow the caller to get or set multiple
  58 controls atomically. Control IDs are grouped into control classes (see
  59 <xref linkend="ctrl-class" />) and all controls in the control array
  60 must belong to the same control class.</para>
  61
  62     <para>Applications must always fill in the
  63 <structfield>count</structfield>,
  64 <structfield>ctrl_class</structfield>,
  65 <structfield>controls</structfield> and
  66 <structfield>reserved</structfield> fields of &v4l2-ext-controls;, and
  67 initialize the &v4l2-ext-control; array pointed to by the
  68 <structfield>controls</structfield> fields.</para>
  69
  70     <para>To get the current value of a set of controls applications
  71 initialize the <structfield>id</structfield>,
  72 <structfield>size</structfield> and <structfield>reserved2</structfield> fields
  73 of each &v4l2-ext-control; and call the
  74 <constant>VIDIOC_G_EXT_CTRLS</constant> ioctl. String controls controls
  75 must also set the <structfield>string</structfield> field. Controls
  76 of compound types (<constant>V4L2_CTRL_FLAG_HAS_PAYLOAD</constant> is set)
  77 must set the <structfield>ptr</structfield> field.</para>
  78
  79     <para>If the <structfield>size</structfield> is too small to
  80 receive the control result (only relevant for pointer-type controls
  81 like strings), then the driver will set <structfield>size</structfield>
  82 to a valid value and return an &ENOSPC;. You should re-allocate the
  83 memory to this new size and try again. For the string type it is possible that
  84 the same issue occurs again if the string has grown in the meantime. It is
  85 recommended to call &VIDIOC-QUERYCTRL; first and use
  86 <structfield>maximum</structfield>+1 as the new <structfield>size</structfield>
  87 value. It is guaranteed that that is sufficient memory.
  88 </para>
  89
  90     <para>N-dimensional arrays are set and retrieved row-by-row. You cannot set a partial
  91 array, all elements have to be set or retrieved. The total size is calculated
  92 as <structfield>elems</structfield> * <structfield>elem_size</structfield>.
  93 These values can be obtained by calling &VIDIOC-QUERY-EXT-CTRL;.</para>
  94
  95     <para>To change the value of a set of controls applications
  96 initialize the <structfield>id</structfield>, <structfield>size</structfield>,
  97 <structfield>reserved2</structfield> and
  98 <structfield>value/value64/string/ptr</structfield> fields of each &v4l2-ext-control; and
  99 call the <constant>VIDIOC_S_EXT_CTRLS</constant> ioctl. The controls
 100 will only be set if <emphasis>all</emphasis> control values are
 101 valid.</para>
 102
 103     <para>To check if a set of controls have correct values applications
 104 initialize the <structfield>id</structfield>, <structfield>size</structfield>,
 105 <structfield>reserved2</structfield> and
 106 <structfield>value/value64/string/ptr</structfield> fields of each &v4l2-ext-control; and
 107 call the <constant>VIDIOC_TRY_EXT_CTRLS</constant> ioctl. It is up to
 108 the driver whether wrong values are automatically adjusted to a valid
 109 value or if an error is returned.</para>
 110
 111     <para>When the <structfield>id</structfield> or
 112 <structfield>ctrl_class</structfield> is invalid drivers return an
 113 &EINVAL;. When the value is out of bounds drivers can choose to take
 114 the closest valid value or return an &ERANGE;, whatever seems more
 115 appropriate. In the first case the new value is set in
 116 &v4l2-ext-control;. If the new control value is inappropriate (e.g. the
 117 given menu index is not supported by the menu control), then this will
 118 also result in an &EINVAL; error.</para>
 119
 120     <para>The driver will only set/get these controls if all control
 121 values are correct. This prevents the situation where only some of the
 122 controls were set/get. Only low-level errors (&eg; a failed i2c
 123 command) can still cause this situation.</para>
 124
 125     <table pgwide="1" frame="none" id="v4l2-ext-control">
 126       <title>struct <structname>v4l2_ext_control</structname></title>
 127       <tgroup cols="4">
 128         &cs-ustr;
 129         <tbody valign="top">
 130           <row>
 131             <entry>__u32</entry>
 132             <entry><structfield>id</structfield></entry>
 133             <entry></entry>
 134             <entry>Identifies the control, set by the
 135 application.</entry>
 136           </row>
 137           <row>
 138             <entry>__u32</entry>
 139             <entry><structfield>size</structfield></entry>
 140             <entry></entry>
 141             <entry>The total size in bytes of the payload of this
 142 control. This is normally 0, but for pointer controls this should be
 143 set to the size of the memory containing the payload, or that will
 144 receive the payload. If <constant>VIDIOC_G_EXT_CTRLS</constant> finds
 145 that this value is less than is required to store
 146 the payload result, then it is set to a value large enough to store the
 147 payload result and ENOSPC is returned. Note that for string controls
 148 this <structfield>size</structfield> field should not be confused with the length of the string.
 149 This field refers to the size of the memory that contains the string.
 150 The actual <emphasis>length</emphasis> of the string may well be much smaller.
 151 </entry>
 152           </row>
 153           <row>
 154             <entry>__u32</entry>
 155             <entry><structfield>reserved2</structfield>[1]</entry>
 156             <entry></entry>
 157             <entry>Reserved for future extensions. Drivers and
 158 applications must set the array to zero.</entry>
 159           </row>
 160           <row>
 161             <entry>union</entry>
 162             <entry>(anonymous)</entry>
 163           </row>
 164           <row>
 165             <entry></entry>
 166             <entry>__s32</entry>
 167             <entry><structfield>value</structfield></entry>
 168             <entry>New value or current value. Valid if this control is not of
 169 type <constant>V4L2_CTRL_TYPE_INTEGER64</constant> and
 170 <constant>V4L2_CTRL_FLAG_HAS_PAYLOAD</constant> is not set.</entry>
 171           </row>
 172           <row>
 173             <entry></entry>
 174             <entry>__s64</entry>
 175             <entry><structfield>value64</structfield></entry>
 176             <entry>New value or current value. Valid if this control is of
 177 type <constant>V4L2_CTRL_TYPE_INTEGER64</constant> and
 178 <constant>V4L2_CTRL_FLAG_HAS_PAYLOAD</constant> is not set.</entry>
 179           </row>
 180           <row>
 181             <entry></entry>
 182             <entry>char *</entry>
 183             <entry><structfield>string</structfield></entry>
 184             <entry>A pointer to a string. Valid if this control is of
 185 type <constant>V4L2_CTRL_TYPE_STRING</constant>.</entry>
 186           </row>
 187           <row>
 188             <entry></entry>
 189             <entry>__u8 *</entry>
 190             <entry><structfield>p_u8</structfield></entry>
 191             <entry>A pointer to a matrix control of unsigned 8-bit values.
 192 Valid if this control is of type <constant>V4L2_CTRL_TYPE_U8</constant>.</entry>
 193           </row>
 194           <row>
 195             <entry></entry>
 196             <entry>__u16 *</entry>
 197             <entry><structfield>p_u16</structfield></entry>
 198             <entry>A pointer to a matrix control of unsigned 16-bit values.
 199 Valid if this control is of type <constant>V4L2_CTRL_TYPE_U16</constant>.</entry>
 200           </row>
 201           <row>
 202             <entry></entry>
 203             <entry>void *</entry>
 204             <entry><structfield>ptr</structfield></entry>
 205             <entry>A pointer to a compound type which can be an N-dimensional array and/or a
 206 compound type (the control's type is >= <constant>V4L2_CTRL_COMPOUND_TYPES</constant>).
 207 Valid if <constant>V4L2_CTRL_FLAG_HAS_PAYLOAD</constant> is set for this control.
 208 </entry>
 209           </row>
 210         </tbody>
 211       </tgroup>
 212     </table>
 213
 214     <table pgwide="1" frame="none" id="v4l2-ext-controls">
 215       <title>struct <structname>v4l2_ext_controls</structname></title>
 216       <tgroup cols="3">
 217         &cs-str;
 218         <tbody valign="top">
 219           <row>
 220             <entry>__u32</entry>
 221             <entry><structfield>ctrl_class</structfield></entry>
 222             <entry>The control class to which all controls belong, see
 223 <xref linkend="ctrl-class" />. Drivers that use a kernel framework for handling
 224 controls will also accept a value of 0 here, meaning that the controls can
 225 belong to any control class. Whether drivers support this can be tested by setting
 226 <structfield>ctrl_class</structfield> to 0 and calling <constant>VIDIOC_TRY_EXT_CTRLS</constant>
 227 with a <structfield>count</structfield> of 0. If that succeeds, then the driver
 228 supports this feature.</entry>
 229           </row>
 230           <row>
 231             <entry>__u32</entry>
 232             <entry><structfield>count</structfield></entry>
 233             <entry>The number of controls in the controls array. May
 234 also be zero.</entry>
 235           </row>
 236           <row>
 237             <entry>__u32</entry>
 238             <entry><structfield>error_idx</structfield></entry>
 239             <entry><para>Set by the driver in case of an error. If the error is
 240 associated with a particular control, then <structfield>error_idx</structfield>
 241 is set to the index of that control. If the error is not related to a specific
 242 control, or the validation step failed (see below), then
 243 <structfield>error_idx</structfield> is set to <structfield>count</structfield>.
 244 The value is undefined if the ioctl returned 0 (success).</para>
 245
 246 <para>Before controls are read from/written to hardware a validation step
 247 takes place: this checks if all controls in the list are valid controls,
 248 if no attempt is made to write to a read-only control or read from a write-only
 249 control, and any other up-front checks that can be done without accessing the
 250 hardware. The exact validations done during this step are driver dependent
 251 since some checks might require hardware access for some devices, thus making
 252 it impossible to do those checks up-front. However, drivers should make a
 253 best-effort to do as many up-front checks as possible.</para>
 254
 255 <para>This check is done to avoid leaving the hardware in an inconsistent state due
 256 to easy-to-avoid problems. But it leads to another problem: the application needs to
 257 know whether an error came from the validation step (meaning that the hardware
 258 was not touched) or from an error during the actual reading from/writing to hardware.</para>
 259
 260 <para>The, in hindsight quite poor, solution for that is to set <structfield>error_idx</structfield>
 261 to <structfield>count</structfield> if the validation failed. This has the
 262 unfortunate side-effect that it is not possible to see which control failed the
 263 validation. If the validation was successful and the error happened while
 264 accessing the hardware, then <structfield>error_idx</structfield> is less than
 265 <structfield>count</structfield> and only the controls up to
 266 <structfield>error_idx-1</structfield> were read or written correctly, and the
 267 state of the remaining controls is undefined.</para>
 268
 269 <para>Since <constant>VIDIOC_TRY_EXT_CTRLS</constant> does not access hardware
 270 there is also no need to handle the validation step in this special way,
 271 so <structfield>error_idx</structfield> will just be set to the control that
 272 failed the validation step instead of to <structfield>count</structfield>.
 273 This means that if <constant>VIDIOC_S_EXT_CTRLS</constant> fails with
 274 <structfield>error_idx</structfield> set to <structfield>count</structfield>,
 275 then you can call <constant>VIDIOC_TRY_EXT_CTRLS</constant> to try to discover
 276 the actual control that failed the validation step. Unfortunately, there
 277 is no <constant>TRY</constant> equivalent for <constant>VIDIOC_G_EXT_CTRLS</constant>.
 278 </para></entry>
 279           </row>
 280           <row>
 281             <entry>__u32</entry>
 282             <entry><structfield>reserved</structfield>[2]</entry>
 283             <entry>Reserved for future extensions. Drivers and
 284 applications must set the array to zero.</entry>
 285           </row>
 286           <row>
 287             <entry>&v4l2-ext-control; *</entry>
 288             <entry><structfield>controls</structfield></entry>
 289             <entry>Pointer to an array of
 290 <structfield>count</structfield> v4l2_ext_control structures. Ignored
 291 if <structfield>count</structfield> equals zero.</entry>
 292           </row>
 293         </tbody>
 294       </tgroup>
 295     </table>
 296
 297     <table pgwide="1" frame="none" id="ctrl-class">
 298       <title>Control classes</title>
 299       <tgroup cols="3">
 300         &cs-def;
 301         <tbody valign="top">
 302           <row>
 303             <entry><constant>V4L2_CTRL_CLASS_USER</constant></entry>
 304             <entry>0x980000</entry>
 305             <entry>The class containing user controls. These controls
 306 are described in <xref linkend="control" />. All controls that can be set
 307 using the &VIDIOC-S-CTRL; and &VIDIOC-G-CTRL; ioctl belong to this
 308 class.</entry>
 309           </row>
 310           <row>
 311             <entry><constant>V4L2_CTRL_CLASS_MPEG</constant></entry>
 312             <entry>0x990000</entry>
 313             <entry>The class containing MPEG compression controls.
 314 These controls are described in <xref
 315                 linkend="mpeg-controls" />.</entry>
 316           </row>
 317           <row>
 318             <entry><constant>V4L2_CTRL_CLASS_CAMERA</constant></entry>
 319             <entry>0x9a0000</entry>
 320             <entry>The class containing camera controls.
 321 These controls are described in <xref
 322                 linkend="camera-controls" />.</entry>
 323           </row>
 324           <row>
 325             <entry><constant>V4L2_CTRL_CLASS_FM_TX</constant></entry>
 326             <entry>0x9b0000</entry>
 327             <entry>The class containing FM Transmitter (FM TX) controls.
 328 These controls are described in <xref
 329                 linkend="fm-tx-controls" />.</entry>
 330           </row>
 331           <row>
 332             <entry><constant>V4L2_CTRL_CLASS_FLASH</constant></entry>
 333             <entry>0x9c0000</entry>
 334             <entry>The class containing flash device controls.
 335 These controls are described in <xref
 336                 linkend="flash-controls" />.</entry>
 337           </row>
 338           <row>
 339             <entry><constant>V4L2_CTRL_CLASS_JPEG</constant></entry>
 340             <entry>0x9d0000</entry>
 341             <entry>The class containing JPEG compression controls.
 342 These controls are described in <xref
 343                 linkend="jpeg-controls" />.</entry>
 344           </row>
 345           <row>
 346             <entry><constant>V4L2_CTRL_CLASS_IMAGE_SOURCE</constant></entry>
 347             <entry>0x9e0000</entry> <entry>The class containing image
 348             source controls. These controls are described in <xref
 349             linkend="image-source-controls" />.</entry>
 350           </row>
 351           <row>
 352             <entry><constant>V4L2_CTRL_CLASS_IMAGE_PROC</constant></entry>
 353             <entry>0x9f0000</entry> <entry>The class containing image
 354             processing controls. These controls are described in <xref
 355             linkend="image-process-controls" />.</entry>
 356           </row>
 357
 358           <row>
 359             <entry><constant>V4L2_CTRL_CLASS_FM_RX</constant></entry>
 360             <entry>0xa10000</entry>
 361             <entry>The class containing FM Receiver (FM RX) controls.
 362 These controls are described in <xref
 363                 linkend="fm-rx-controls" />.</entry>
 364           </row>
 365           <row>
 366             <entry><constant>V4L2_CTRL_CLASS_RF_TUNER</constant></entry>
 367             <entry>0xa20000</entry>
 368             <entry>The class containing RF tuner controls.
 369 These controls are described in <xref linkend="rf-tuner-controls" />.</entry>
 370           </row>
 371         </tbody>
 372       </tgroup>
 373     </table>
 374
 375   </refsect1>
 376
 377   <refsect1>
 378     &return-value;
 379
 380     <variablelist>
 381       <varlistentry>
 382         <term><errorcode>EINVAL</errorcode></term>
 383         <listitem>
 384           <para>The &v4l2-ext-control; <structfield>id</structfield>
 385 is invalid, the &v4l2-ext-controls;
 386 <structfield>ctrl_class</structfield> is invalid, or the &v4l2-ext-control;
 387 <structfield>value</structfield> was inappropriate (e.g. the given menu
 388 index is not supported by the driver). This error code is
 389 also returned by the <constant>VIDIOC_S_EXT_CTRLS</constant> and
 390 <constant>VIDIOC_TRY_EXT_CTRLS</constant> ioctls if two or more
 391 control values are in conflict.</para>
 392         </listitem>
 393       </varlistentry>
 394       <varlistentry>
 395         <term><errorcode>ERANGE</errorcode></term>
 396         <listitem>
 397           <para>The &v4l2-ext-control; <structfield>value</structfield>
 398 is out of bounds.</para>
 399         </listitem>
 400       </varlistentry>
 401       <varlistentry>
 402         <term><errorcode>EBUSY</errorcode></term>
 403         <listitem>
 404           <para>The control is temporarily not changeable, possibly
 405 because another applications took over control of the device function
 406 this control belongs to.</para>
 407         </listitem>
 408       </varlistentry>
 409       <varlistentry>
 410         <term><errorcode>ENOSPC</errorcode></term>
 411         <listitem>
 412           <para>The space reserved for the control's payload is insufficient.
 413 The field <structfield>size</structfield> is set to a value that is enough
 414 to store the payload and this error code is returned.</para>
 415         </listitem>
 416       </varlistentry>
 417       <varlistentry>
 418         <term><errorcode>EACCES</errorcode></term>
 419         <listitem>
 420           <para>Attempt to try or set a read-only control or to get a
 421           write-only control.</para>
 422         </listitem>
 423       </varlistentry>
 424     </variablelist>
 425   </refsect1>
 426 </refentry>
 427