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>__u32 *</entry>
 204             <entry><structfield>p_u32</structfield></entry>
 205             <entry>A pointer to a matrix control of unsigned 32-bit values.
 206 Valid if this control is of type <constant>V4L2_CTRL_TYPE_U32</constant>.</entry>
 207           </row>
 208           <row>
 209             <entry></entry>
 210             <entry>void *</entry>
 211             <entry><structfield>ptr</structfield></entry>
 212             <entry>A pointer to a compound type which can be an N-dimensional array and/or a
 213 compound type (the control's type is >= <constant>V4L2_CTRL_COMPOUND_TYPES</constant>).
 214 Valid if <constant>V4L2_CTRL_FLAG_HAS_PAYLOAD</constant> is set for this control.
 215 </entry>
 216           </row>
 217         </tbody>
 218       </tgroup>
 219     </table>
 220
 221     <table pgwide="1" frame="none" id="v4l2-ext-controls">
 222       <title>struct <structname>v4l2_ext_controls</structname></title>
 223       <tgroup cols="3">
 224         &cs-str;
 225         <tbody valign="top">
 226           <row>
 227             <entry>__u32</entry>
 228             <entry><structfield>ctrl_class</structfield></entry>
 229             <entry>The control class to which all controls belong, see
 230 <xref linkend="ctrl-class" />. Drivers that use a kernel framework for handling
 231 controls will also accept a value of 0 here, meaning that the controls can
 232 belong to any control class. Whether drivers support this can be tested by setting
 233 <structfield>ctrl_class</structfield> to 0 and calling <constant>VIDIOC_TRY_EXT_CTRLS</constant>
 234 with a <structfield>count</structfield> of 0. If that succeeds, then the driver
 235 supports this feature.</entry>
 236           </row>
 237           <row>
 238             <entry>__u32</entry>
 239             <entry><structfield>count</structfield></entry>
 240             <entry>The number of controls in the controls array. May
 241 also be zero.</entry>
 242           </row>
 243           <row>
 244             <entry>__u32</entry>
 245             <entry><structfield>error_idx</structfield></entry>
 246             <entry><para>Set by the driver in case of an error. If the error is
 247 associated with a particular control, then <structfield>error_idx</structfield>
 248 is set to the index of that control. If the error is not related to a specific
 249 control, or the validation step failed (see below), then
 250 <structfield>error_idx</structfield> is set to <structfield>count</structfield>.
 251 The value is undefined if the ioctl returned 0 (success).</para>
 252
 253 <para>Before controls are read from/written to hardware a validation step
 254 takes place: this checks if all controls in the list are valid controls,
 255 if no attempt is made to write to a read-only control or read from a write-only
 256 control, and any other up-front checks that can be done without accessing the
 257 hardware. The exact validations done during this step are driver dependent
 258 since some checks might require hardware access for some devices, thus making
 259 it impossible to do those checks up-front. However, drivers should make a
 260 best-effort to do as many up-front checks as possible.</para>
 261
 262 <para>This check is done to avoid leaving the hardware in an inconsistent state due
 263 to easy-to-avoid problems. But it leads to another problem: the application needs to
 264 know whether an error came from the validation step (meaning that the hardware
 265 was not touched) or from an error during the actual reading from/writing to hardware.</para>
 266
 267 <para>The, in hindsight quite poor, solution for that is to set <structfield>error_idx</structfield>
 268 to <structfield>count</structfield> if the validation failed. This has the
 269 unfortunate side-effect that it is not possible to see which control failed the
 270 validation. If the validation was successful and the error happened while
 271 accessing the hardware, then <structfield>error_idx</structfield> is less than
 272 <structfield>count</structfield> and only the controls up to
 273 <structfield>error_idx-1</structfield> were read or written correctly, and the
 274 state of the remaining controls is undefined.</para>
 275
 276 <para>Since <constant>VIDIOC_TRY_EXT_CTRLS</constant> does not access hardware
 277 there is also no need to handle the validation step in this special way,
 278 so <structfield>error_idx</structfield> will just be set to the control that
 279 failed the validation step instead of to <structfield>count</structfield>.
 280 This means that if <constant>VIDIOC_S_EXT_CTRLS</constant> fails with
 281 <structfield>error_idx</structfield> set to <structfield>count</structfield>,
 282 then you can call <constant>VIDIOC_TRY_EXT_CTRLS</constant> to try to discover
 283 the actual control that failed the validation step. Unfortunately, there
 284 is no <constant>TRY</constant> equivalent for <constant>VIDIOC_G_EXT_CTRLS</constant>.
 285 </para></entry>
 286           </row>
 287           <row>
 288             <entry>__u32</entry>
 289             <entry><structfield>reserved</structfield>[2]</entry>
 290             <entry>Reserved for future extensions. Drivers and
 291 applications must set the array to zero.</entry>
 292           </row>
 293           <row>
 294             <entry>&v4l2-ext-control; *</entry>
 295             <entry><structfield>controls</structfield></entry>
 296             <entry>Pointer to an array of
 297 <structfield>count</structfield> v4l2_ext_control structures. Ignored
 298 if <structfield>count</structfield> equals zero.</entry>
 299           </row>
 300         </tbody>
 301       </tgroup>
 302     </table>
 303
 304     <table pgwide="1" frame="none" id="ctrl-class">
 305       <title>Control classes</title>
 306       <tgroup cols="3">
 307         &cs-def;
 308         <tbody valign="top">
 309           <row>
 310             <entry><constant>V4L2_CTRL_CLASS_USER</constant></entry>
 311             <entry>0x980000</entry>
 312             <entry>The class containing user controls. These controls
 313 are described in <xref linkend="control" />. All controls that can be set
 314 using the &VIDIOC-S-CTRL; and &VIDIOC-G-CTRL; ioctl belong to this
 315 class.</entry>
 316           </row>
 317           <row>
 318             <entry><constant>V4L2_CTRL_CLASS_MPEG</constant></entry>
 319             <entry>0x990000</entry>
 320             <entry>The class containing MPEG compression controls.
 321 These controls are described in <xref
 322                 linkend="mpeg-controls" />.</entry>
 323           </row>
 324           <row>
 325             <entry><constant>V4L2_CTRL_CLASS_CAMERA</constant></entry>
 326             <entry>0x9a0000</entry>
 327             <entry>The class containing camera controls.
 328 These controls are described in <xref
 329                 linkend="camera-controls" />.</entry>
 330           </row>
 331           <row>
 332             <entry><constant>V4L2_CTRL_CLASS_FM_TX</constant></entry>
 333             <entry>0x9b0000</entry>
 334             <entry>The class containing FM Transmitter (FM TX) controls.
 335 These controls are described in <xref
 336                 linkend="fm-tx-controls" />.</entry>
 337           </row>
 338           <row>
 339             <entry><constant>V4L2_CTRL_CLASS_FLASH</constant></entry>
 340             <entry>0x9c0000</entry>
 341             <entry>The class containing flash device controls.
 342 These controls are described in <xref
 343                 linkend="flash-controls" />.</entry>
 344           </row>
 345           <row>
 346             <entry><constant>V4L2_CTRL_CLASS_JPEG</constant></entry>
 347             <entry>0x9d0000</entry>
 348             <entry>The class containing JPEG compression controls.
 349 These controls are described in <xref
 350                 linkend="jpeg-controls" />.</entry>
 351           </row>
 352           <row>
 353             <entry><constant>V4L2_CTRL_CLASS_IMAGE_SOURCE</constant></entry>
 354             <entry>0x9e0000</entry> <entry>The class containing image
 355             source controls. These controls are described in <xref
 356             linkend="image-source-controls" />.</entry>
 357           </row>
 358           <row>
 359             <entry><constant>V4L2_CTRL_CLASS_IMAGE_PROC</constant></entry>
 360             <entry>0x9f0000</entry> <entry>The class containing image
 361             processing controls. These controls are described in <xref
 362             linkend="image-process-controls" />.</entry>
 363           </row>
 364
 365           <row>
 366             <entry><constant>V4L2_CTRL_CLASS_FM_RX</constant></entry>
 367             <entry>0xa10000</entry>
 368             <entry>The class containing FM Receiver (FM RX) controls.
 369 These controls are described in <xref
 370                 linkend="fm-rx-controls" />.</entry>
 371           </row>
 372           <row>
 373             <entry><constant>V4L2_CTRL_CLASS_RF_TUNER</constant></entry>
 374             <entry>0xa20000</entry>
 375             <entry>The class containing RF tuner controls.
 376 These controls are described in <xref linkend="rf-tuner-controls" />.</entry>
 377           </row>
 378         </tbody>
 379       </tgroup>
 380     </table>
 381
 382   </refsect1>
 383
 384   <refsect1>
 385     &return-value;
 386
 387     <variablelist>
 388       <varlistentry>
 389         <term><errorcode>EINVAL</errorcode></term>
 390         <listitem>
 391           <para>The &v4l2-ext-control; <structfield>id</structfield>
 392 is invalid, the &v4l2-ext-controls;
 393 <structfield>ctrl_class</structfield> is invalid, or the &v4l2-ext-control;
 394 <structfield>value</structfield> was inappropriate (e.g. the given menu
 395 index is not supported by the driver). This error code is
 396 also returned by the <constant>VIDIOC_S_EXT_CTRLS</constant> and
 397 <constant>VIDIOC_TRY_EXT_CTRLS</constant> ioctls if two or more
 398 control values are in conflict.</para>
 399         </listitem>
 400       </varlistentry>
 401       <varlistentry>
 402         <term><errorcode>ERANGE</errorcode></term>
 403         <listitem>
 404           <para>The &v4l2-ext-control; <structfield>value</structfield>
 405 is out of bounds.</para>
 406         </listitem>
 407       </varlistentry>
 408       <varlistentry>
 409         <term><errorcode>EBUSY</errorcode></term>
 410         <listitem>
 411           <para>The control is temporarily not changeable, possibly
 412 because another applications took over control of the device function
 413 this control belongs to.</para>
 414         </listitem>
 415       </varlistentry>
 416       <varlistentry>
 417         <term><errorcode>ENOSPC</errorcode></term>
 418         <listitem>
 419           <para>The space reserved for the control's payload is insufficient.
 420 The field <structfield>size</structfield> is set to a value that is enough
 421 to store the payload and this error code is returned.</para>
 422         </listitem>
 423       </varlistentry>
 424       <varlistentry>
 425         <term><errorcode>EACCES</errorcode></term>
 426         <listitem>
 427           <para>Attempt to try or set a read-only control or to get a
 428           write-only control.</para>
 429         </listitem>
 430       </varlistentry>
 431     </variablelist>
 432   </refsect1>
 433 </refentry>
 434