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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:02 EST.

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