1 .. Permission is granted to copy, distribute and/or modify this
2 .. document under the terms of the GNU Free Documentation License,
3 .. Version 1.1 or any later version published by the Free Software
4 .. Foundation, with no Invariant Sections, no Front-Cover Texts
5 .. and no Back-Cover Texts. A copy of the license is included at
6 .. Documentation/media/uapi/fdl-appendix.rst.
8 .. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
10 .. _extended-controls:
20 The control mechanism as originally designed was meant to be used for
21 user settings (brightness, saturation, etc). However, it turned out to
22 be a very useful model for implementing more complicated driver APIs
23 where each driver implements only a subset of a larger API.
25 The MPEG encoding API was the driving force behind designing and
26 implementing this extended control mechanism: the MPEG standard is quite
27 large and the currently supported hardware MPEG encoders each only
28 implement a subset of this standard. Further more, many parameters
29 relating to how the video is encoded into an MPEG stream are specific to
30 the MPEG encoding chip since the MPEG standard only defines the format
31 of the resulting MPEG stream, not how the video is actually encoded into
34 Unfortunately, the original control API lacked some features needed for
35 these new uses and so it was extended into the (not terribly originally
36 named) extended control API.
38 Even though the MPEG encoding API was the first effort to use the
39 Extended Control API, nowadays there are also other classes of Extended
40 Controls, such as Camera Controls and FM Transmitter Controls. The
41 Extended Controls API as well as all Extended Controls classes are
42 described in the following text.
45 The Extended Control API
46 ========================
48 Three new ioctls are available:
49 :ref:`VIDIOC_G_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`,
50 :ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` and
51 :ref:`VIDIOC_TRY_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`. These ioctls act
52 on arrays of controls (as opposed to the
53 :ref:`VIDIOC_G_CTRL <VIDIOC_G_CTRL>` and
54 :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` ioctls that act on a single
55 control). This is needed since it is often required to atomically change
56 several controls at once.
58 Each of the new ioctls expects a pointer to a struct
59 :c:type:`v4l2_ext_controls`. This structure
60 contains a pointer to the control array, a count of the number of
61 controls in that array and a control class. Control classes are used to
62 group similar controls into a single class. For example, control class
63 ``V4L2_CTRL_CLASS_USER`` contains all user controls (i. e. all controls
64 that can also be set using the old :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>`
65 ioctl). Control class ``V4L2_CTRL_CLASS_MPEG`` contains all controls
66 relating to MPEG encoding, etc.
68 All controls in the control array must belong to the specified control
69 class. An error is returned if this is not the case.
71 It is also possible to use an empty control array (``count`` == 0) to check
72 whether the specified control class is supported.
74 The control array is a struct
75 :c:type:`v4l2_ext_control` array. The
76 struct :c:type:`v4l2_ext_control` is very similar to
77 struct :c:type:`v4l2_control`, except for the fact that
78 it also allows for 64-bit values and pointers to be passed.
80 Since the struct :c:type:`v4l2_ext_control` supports
81 pointers it is now also possible to have controls with compound types
82 such as N-dimensional arrays and/or structures. You need to specify the
83 ``V4L2_CTRL_FLAG_NEXT_COMPOUND`` when enumerating controls to actually
84 be able to see such compound controls. In other words, these controls
85 with compound types should only be used programmatically.
87 Since such compound controls need to expose more information about
88 themselves than is possible with
89 :ref:`VIDIOC_QUERYCTRL` the
90 :ref:`VIDIOC_QUERY_EXT_CTRL <VIDIOC_QUERYCTRL>` ioctl was added. In
91 particular, this ioctl gives the dimensions of the N-dimensional array
92 if this control consists of more than one element.
96 #. It is important to realize that due to the flexibility of controls it is
97 necessary to check whether the control you want to set actually is
98 supported in the driver and what the valid range of values is. So use
99 the :ref:`VIDIOC_QUERYCTRL` (or :ref:`VIDIOC_QUERY_EXT_CTRL
100 <VIDIOC_QUERYCTRL>`) and :ref:`VIDIOC_QUERYMENU <VIDIOC_QUERYCTRL>`
101 ioctls to check this.
103 #. It is possible that some of the menu indices in a control of
104 type ``V4L2_CTRL_TYPE_MENU`` may not be supported (``VIDIOC_QUERYMENU``
105 will return an error). A good example is the list of supported MPEG
106 audio bitrates. Some drivers only support one or two bitrates, others
107 support a wider range.
109 All controls use machine endianness.
112 Enumerating Extended Controls
113 =============================
115 The recommended way to enumerate over the extended controls is by using
116 :ref:`VIDIOC_QUERYCTRL` in combination with the
117 ``V4L2_CTRL_FLAG_NEXT_CTRL`` flag:
122 struct v4l2_queryctrl qctrl;
124 qctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL;
125 while (0 == ioctl (fd, VIDIOC_QUERYCTRL, &qctrl)) {
127 qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
130 The initial control ID is set to 0 ORed with the
131 ``V4L2_CTRL_FLAG_NEXT_CTRL`` flag. The ``VIDIOC_QUERYCTRL`` ioctl will
132 return the first control with a higher ID than the specified one. When
133 no such controls are found an error is returned.
135 If you want to get all controls within a specific control class, then
136 you can set the initial ``qctrl.id`` value to the control class and add
137 an extra check to break out of the loop when a control of another
138 control class is found:
143 qctrl.id = V4L2_CTRL_CLASS_MPEG | V4L2_CTRL_FLAG_NEXT_CTRL;
144 while (0 == ioctl(fd, VIDIOC_QUERYCTRL, &qctrl)) {
145 if (V4L2_CTRL_ID2CLASS(qctrl.id) != V4L2_CTRL_CLASS_MPEG)
148 qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
151 The 32-bit ``qctrl.id`` value is subdivided into three bit ranges: the
152 top 4 bits are reserved for flags (e. g. ``V4L2_CTRL_FLAG_NEXT_CTRL``)
153 and are not actually part of the ID. The remaining 28 bits form the
154 control ID, of which the most significant 12 bits define the control
155 class and the least significant 16 bits identify the control within the
156 control class. It is guaranteed that these last 16 bits are always
157 non-zero for controls. The range of 0x1000 and up are reserved for
158 driver-specific controls. The macro ``V4L2_CTRL_ID2CLASS(id)`` returns
159 the control class ID based on a control ID.
161 If the driver does not support extended controls, then
162 ``VIDIOC_QUERYCTRL`` will fail when used in combination with
163 ``V4L2_CTRL_FLAG_NEXT_CTRL``. In that case the old method of enumerating
164 control should be used (see :ref:`enum_all_controls`). But if it is
165 supported, then it is guaranteed to enumerate over all controls,
166 including driver-private controls.
169 Creating Control Panels
170 =======================
172 It is possible to create control panels for a graphical user interface
173 where the user can select the various controls. Basically you will have
174 to iterate over all controls using the method described above. Each
175 control class starts with a control of type
176 ``V4L2_CTRL_TYPE_CTRL_CLASS``. ``VIDIOC_QUERYCTRL`` will return the name
177 of this control class which can be used as the title of a tab page
178 within a control panel.
180 The flags field of struct :ref:`v4l2_queryctrl <v4l2-queryctrl>` also
181 contains hints on the behavior of the control. See the
182 :ref:`VIDIOC_QUERYCTRL` documentation for more