1<refentry id="vidioc-g-ext-ctrls">
2  <refmeta>
3    <refentrytitle>ioctl VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS,
4VIDIOC_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
13values</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,
42VIDIOC_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
58controls atomically. Control IDs are grouped into control classes (see
59<xref linkend="ctrl-class" />) and all controls in the control array
60must 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
67initialize 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
71initialize the <structfield>id</structfield>,
72<structfield>size</structfield> and <structfield>reserved2</structfield> fields
73of each &v4l2-ext-control; and call the
74<constant>VIDIOC_G_EXT_CTRLS</constant> ioctl. String controls controls
75must also set the <structfield>string</structfield> field. Controls
76of compound types (<constant>V4L2_CTRL_FLAG_HAS_PAYLOAD</constant> is set)
77must set the <structfield>ptr</structfield> field.</para>
78
79    <para>If the <structfield>size</structfield> is too small to
80receive the control result (only relevant for pointer-type controls
81like strings), then the driver will set <structfield>size</structfield>
82to a valid value and return an &ENOSPC;. You should re-allocate the
83memory to this new size and try again. For the string type it is possible that
84the same issue occurs again if the string has grown in the meantime. It is
85recommended to call &VIDIOC-QUERYCTRL; first and use
86<structfield>maximum</structfield>+1 as the new <structfield>size</structfield>
87value. 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
91array, all elements have to be set or retrieved. The total size is calculated
92as <structfield>elems</structfield> * <structfield>elem_size</structfield>.
93These values can be obtained by calling &VIDIOC-QUERY-EXT-CTRL;.</para>
94
95    <para>To change the value of a set of controls applications
96initialize 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
99call the <constant>VIDIOC_S_EXT_CTRLS</constant> ioctl. The controls
100will only be set if <emphasis>all</emphasis> control values are
101valid.</para>
102
103    <para>To check if a set of controls have correct values applications
104initialize 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
107call the <constant>VIDIOC_TRY_EXT_CTRLS</constant> ioctl. It is up to
108the driver whether wrong values are automatically adjusted to a valid
109value 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
114the closest valid value or return an &ERANGE;, whatever seems more
115appropriate. In the first case the new value is set in
116&v4l2-ext-control;. If the new control value is inappropriate (e.g. the
117given menu index is not supported by the menu control), then this will
118also result in an &EINVAL; error.</para>
119
120    <para>The driver will only set/get these controls if all control
121values are correct. This prevents the situation where only some of the
122controls were set/get. Only low-level errors (&eg; a failed i2c
123command) 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
135application.</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
142control. This is normally 0, but for pointer controls this should be
143set to the size of the memory containing the payload, or that will
144receive the payload. If <constant>VIDIOC_G_EXT_CTRLS</constant> finds
145that this value is less than is required to store
146the payload result, then it is set to a value large enough to store the
147payload result and ENOSPC is returned. Note that for string controls
148this <structfield>size</structfield> field should not be confused with the length of the string.
149This field refers to the size of the memory that contains the string.
150The 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
158applications 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
169type <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
177type <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
185type <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.
192Valid 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.
199Valid 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
206compound type (the control's type is >= <constant>V4L2_CTRL_COMPOUND_TYPES</constant>).
207Valid 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
224controls will also accept a value of 0 here, meaning that the controls can
225belong 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>
227with a <structfield>count</structfield> of 0. If that succeeds, then the driver
228supports 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
234also 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
240associated with a particular control, then <structfield>error_idx</structfield>
241is set to the index of that control. If the error is not related to a specific
242control, or the validation step failed (see below), then
243<structfield>error_idx</structfield> is set to <structfield>count</structfield>.
244The value is undefined if the ioctl returned 0 (success).</para>
245
246<para>Before controls are read from/written to hardware a validation step
247takes place: this checks if all controls in the list are valid controls,
248if no attempt is made to write to a read-only control or read from a write-only
249control, and any other up-front checks that can be done without accessing the
250hardware. The exact validations done during this step are driver dependent
251since some checks might require hardware access for some devices, thus making
252it impossible to do those checks up-front. However, drivers should make a
253best-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
256to easy-to-avoid problems. But it leads to another problem: the application needs to
257know whether an error came from the validation step (meaning that the hardware
258was 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>
261to <structfield>count</structfield> if the validation failed. This has the
262unfortunate side-effect that it is not possible to see which control failed the
263validation. If the validation was successful and the error happened while
264accessing 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
267state of the remaining controls is undefined.</para>
268
269<para>Since <constant>VIDIOC_TRY_EXT_CTRLS</constant> does not access hardware
270there is also no need to handle the validation step in this special way,
271so <structfield>error_idx</structfield> will just be set to the control that
272failed the validation step instead of to <structfield>count</structfield>.
273This means that if <constant>VIDIOC_S_EXT_CTRLS</constant> fails with
274<structfield>error_idx</structfield> set to <structfield>count</structfield>,
275then you can call <constant>VIDIOC_TRY_EXT_CTRLS</constant> to try to discover
276the actual control that failed the validation step. Unfortunately, there
277is 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
284applications 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
291if <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
306are described in <xref linkend="control" />. All controls that can be set
307using the &VIDIOC-S-CTRL; and &VIDIOC-G-CTRL; ioctl belong to this
308class.</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.
314These 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.
321These 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.
328These 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.
335These 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.
342These 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.
362These 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.
369These 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>
385is 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
388index is not supported by the driver). This error code is
389also returned by the <constant>VIDIOC_S_EXT_CTRLS</constant> and
390<constant>VIDIOC_TRY_EXT_CTRLS</constant> ioctls if two or more
391control 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>
398is 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
405because another applications took over control of the device function
406this 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.
413The field <structfield>size</structfield> is set to a value that is enough
414to 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
428