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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+
+.. _extended-controls:
+
+*********************
+Extended Controls API
+*********************
+
+
+Introduction
+============
+
+The control mechanism as originally designed was meant to be used for
+user settings (brightness, saturation, etc). However, it turned out to
+be a very useful model for implementing more complicated driver APIs
+where each driver implements only a subset of a larger API.
+
+The MPEG encoding API was the driving force behind designing and
+implementing this extended control mechanism: the MPEG standard is quite
+large and the currently supported hardware MPEG encoders each only
+implement a subset of this standard. Further more, many parameters
+relating to how the video is encoded into an MPEG stream are specific to
+the MPEG encoding chip since the MPEG standard only defines the format
+of the resulting MPEG stream, not how the video is actually encoded into
+that format.
+
+Unfortunately, the original control API lacked some features needed for
+these new uses and so it was extended into the (not terribly originally
+named) extended control API.
+
+Even though the MPEG encoding API was the first effort to use the
+Extended Control API, nowadays there are also other classes of Extended
+Controls, such as Camera Controls and FM Transmitter Controls. The
+Extended Controls API as well as all Extended Controls classes are
+described in the following text.
+
+
+The Extended Control API
+========================
+
+Three new ioctls are available:
+:ref:`VIDIOC_G_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`,
+:ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` and
+:ref:`VIDIOC_TRY_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`. These ioctls act
+on arrays of controls (as opposed to the
+:ref:`VIDIOC_G_CTRL <VIDIOC_G_CTRL>` and
+:ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` ioctls that act on a single
+control). This is needed since it is often required to atomically change
+several controls at once.
+
+Each of the new ioctls expects a pointer to a struct
+:c:type:`v4l2_ext_controls`. This structure
+contains a pointer to the control array, a count of the number of
+controls in that array and a control class. Control classes are used to
+group similar controls into a single class. For example, control class
+``V4L2_CTRL_CLASS_USER`` contains all user controls (i. e. all controls
+that can also be set using the old :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>`
+ioctl). Control class ``V4L2_CTRL_CLASS_CODEC`` contains controls
+relating to codecs.
+
+All controls in the control array must belong to the specified control
+class. An error is returned if this is not the case.
+
+It is also possible to use an empty control array (``count`` == 0) to check
+whether the specified control class is supported.
+
+The control array is a struct
+:c:type:`v4l2_ext_control` array. The
+struct :c:type:`v4l2_ext_control` is very similar to
+struct :c:type:`v4l2_control`, except for the fact that
+it also allows for 64-bit values and pointers to be passed.
+
+Since the struct :c:type:`v4l2_ext_control` supports
+pointers it is now also possible to have controls with compound types
+such as N-dimensional arrays and/or structures. You need to specify the
+``V4L2_CTRL_FLAG_NEXT_COMPOUND`` when enumerating controls to actually
+be able to see such compound controls. In other words, these controls
+with compound types should only be used programmatically.
+
+Since such compound controls need to expose more information about
+themselves than is possible with :ref:`VIDIOC_QUERYCTRL <VIDIOC_QUERYCTRL>`
+the :ref:`VIDIOC_QUERY_EXT_CTRL <VIDIOC_QUERYCTRL>` ioctl was added. In
+particular, this ioctl gives the dimensions of the N-dimensional array if
+this control consists of more than one element.
+
+.. note::
+
+   #. It is important to realize that due to the flexibility of controls it is
+      necessary to check whether the control you want to set actually is
+      supported in the driver and what the valid range of values is. So use
+      :ref:`VIDIOC_QUERYCTRL` to check this.
+
+   #. It is possible that some of the menu indices in a control of
+      type ``V4L2_CTRL_TYPE_MENU`` may not be supported (``VIDIOC_QUERYMENU``
+      will return an error). A good example is the list of supported MPEG
+      audio bitrates. Some drivers only support one or two bitrates, others
+      support a wider range.
+
+All controls use machine endianness.
+
+
+Enumerating Extended Controls
+=============================
+
+The recommended way to enumerate over the extended controls is by using
+:ref:`VIDIOC_QUERYCTRL` in combination with the
+``V4L2_CTRL_FLAG_NEXT_CTRL`` flag:
+
+
+.. code-block:: c
+
+    struct v4l2_queryctrl qctrl;
+
+    qctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL;
+    while (0 == ioctl (fd, VIDIOC_QUERYCTRL, &qctrl)) {
+	/* ... */
+	qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
+    }
+
+The initial control ID is set to 0 ORed with the
+``V4L2_CTRL_FLAG_NEXT_CTRL`` flag. The ``VIDIOC_QUERYCTRL`` ioctl will
+return the first control with a higher ID than the specified one. When
+no such controls are found an error is returned.
+
+If you want to get all controls within a specific control class, then
+you can set the initial ``qctrl.id`` value to the control class and add
+an extra check to break out of the loop when a control of another
+control class is found:
+
+
+.. code-block:: c
+
+    qctrl.id = V4L2_CTRL_CLASS_CODEC | V4L2_CTRL_FLAG_NEXT_CTRL;
+    while (0 == ioctl(fd, VIDIOC_QUERYCTRL, &qctrl)) {
+	if (V4L2_CTRL_ID2CLASS(qctrl.id) != V4L2_CTRL_CLASS_CODEC)
+	    break;
+	/* ... */
+	qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
+    }
+
+The 32-bit ``qctrl.id`` value is subdivided into three bit ranges: the
+top 4 bits are reserved for flags (e. g. ``V4L2_CTRL_FLAG_NEXT_CTRL``)
+and are not actually part of the ID. The remaining 28 bits form the
+control ID, of which the most significant 12 bits define the control
+class and the least significant 16 bits identify the control within the
+control class. It is guaranteed that these last 16 bits are always
+non-zero for controls. The range of 0x1000 and up are reserved for
+driver-specific controls. The macro ``V4L2_CTRL_ID2CLASS(id)`` returns
+the control class ID based on a control ID.
+
+If the driver does not support extended controls, then
+``VIDIOC_QUERYCTRL`` will fail when used in combination with
+``V4L2_CTRL_FLAG_NEXT_CTRL``. In that case the old method of enumerating
+control should be used (see :ref:`enum_all_controls`). But if it is
+supported, then it is guaranteed to enumerate over all controls,
+including driver-private controls.
+
+
+Creating Control Panels
+=======================
+
+It is possible to create control panels for a graphical user interface
+where the user can select the various controls. Basically you will have
+to iterate over all controls using the method described above. Each
+control class starts with a control of type
+``V4L2_CTRL_TYPE_CTRL_CLASS``. ``VIDIOC_QUERYCTRL`` will return the name
+of this control class which can be used as the title of a tab page
+within a control panel.
+
+The flags field of struct :ref:`v4l2_queryctrl <v4l2-queryctrl>` also
+contains hints on the behavior of the control. See the
+:ref:`VIDIOC_QUERYCTRL` documentation for more
+details.