.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
.. c:namespace:: V4L

.. _VIDIOC_QUERYCTRL:

*******************************************************************
ioctls VIDIOC_QUERYCTRL, VIDIOC_QUERY_EXT_CTRL and VIDIOC_QUERYMENU
*******************************************************************

Name
====

VIDIOC_QUERYCTRL - VIDIOC_QUERY_EXT_CTRL - VIDIOC_QUERYMENU - Enumerate controls and menu control items

Synopsis
========

``int ioctl(int fd, int VIDIOC_QUERYCTRL, struct v4l2_queryctrl *argp)``

.. c:macro:: VIDIOC_QUERY_EXT_CTRL

``int ioctl(int fd, VIDIOC_QUERY_EXT_CTRL, struct v4l2_query_ext_ctrl *argp)``

.. c:macro:: VIDIOC_QUERYMENU

``int ioctl(int fd, VIDIOC_QUERYMENU, struct v4l2_querymenu *argp)``

Arguments
=========

``fd``
    File descriptor returned by :c:func:`open()`.

``argp``
    Pointer to struct :c:type:`v4l2_queryctrl`, :c:type:`v4l2_query_ext_ctrl`
    or :c:type:`v4l2_querymenu` (depending on the ioctl).

Description
===========

To query the attributes of a control applications set the ``id`` field
of a struct :ref:`v4l2_queryctrl <v4l2-queryctrl>` and call the
``VIDIOC_QUERYCTRL`` ioctl with a pointer to this structure. The driver
fills the rest of the structure or returns an ``EINVAL`` error code when the
``id`` is invalid.

It is possible to enumerate controls by calling ``VIDIOC_QUERYCTRL``
with successive ``id`` values starting from ``V4L2_CID_BASE`` up to and
exclusive ``V4L2_CID_LASTP1``. Drivers may return ``EINVAL`` if a control in
this range is not supported. Further applications can enumerate private
controls, which are not defined in this specification, by starting at
``V4L2_CID_PRIVATE_BASE`` and incrementing ``id`` until the driver
returns ``EINVAL``.

In both cases, when the driver sets the ``V4L2_CTRL_FLAG_DISABLED`` flag
in the ``flags`` field this control is permanently disabled and should
be ignored by the application. [#f1]_

When the application ORs ``id`` with ``V4L2_CTRL_FLAG_NEXT_CTRL`` the
driver returns the next supported non-compound control, or ``EINVAL`` if
there is none. In addition, the ``V4L2_CTRL_FLAG_NEXT_COMPOUND`` flag
can be specified to enumerate all compound controls (i.e. controls with
type ≥ ``V4L2_CTRL_COMPOUND_TYPES`` and/or array control, in other words
controls that contain more than one value). Specify both
``V4L2_CTRL_FLAG_NEXT_CTRL`` and ``V4L2_CTRL_FLAG_NEXT_COMPOUND`` in
order to enumerate all controls, compound or not. Drivers which do not
support these flags yet always return ``EINVAL``.

The ``VIDIOC_QUERY_EXT_CTRL`` ioctl was introduced in order to better
support controls that can use compound types, and to expose additional
control information that cannot be returned in struct
:ref:`v4l2_queryctrl <v4l2-queryctrl>` since that structure is full.

``VIDIOC_QUERY_EXT_CTRL`` is used in the same way as
``VIDIOC_QUERYCTRL``, except that the ``reserved`` array must be zeroed
as well.

Additional information is required for menu controls: the names of the
menu items. To query them applications set the ``id`` and ``index``
fields of struct :ref:`v4l2_querymenu <v4l2-querymenu>` and call the
``VIDIOC_QUERYMENU`` ioctl with a pointer to this structure. The driver
fills the rest of the structure or returns an ``EINVAL`` error code when the
``id`` or ``index`` is invalid. Menu items are enumerated by calling
``VIDIOC_QUERYMENU`` with successive ``index`` values from struct
:ref:`v4l2_queryctrl <v4l2-queryctrl>` ``minimum`` to ``maximum``,
inclusive.

.. note::

   It is possible for ``VIDIOC_QUERYMENU`` to return
   an ``EINVAL`` error code for some indices between ``minimum`` and
   ``maximum``. In that case that particular menu item is not supported by
   this driver. Also note that the ``minimum`` value is not necessarily 0.

See also the examples in :ref:`control`.

.. tabularcolumns:: |p{1.2cm}|p{3.6cm}|p{12.5cm}|

.. _v4l2-queryctrl:

.. cssclass:: longtable

.. flat-table:: struct v4l2_queryctrl
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2

    * - __u32
      - ``id``
      - Identifies the control, set by the application. See
	:ref:`control-id` for predefined IDs. When the ID is ORed with
	V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and
	returns the first control with a higher ID. Drivers which do not
	support this flag yet always return an ``EINVAL`` error code.
    * - __u32
      - ``type``
      - Type of control, see :c:type:`v4l2_ctrl_type`.
    * - __u8
      - ``name``\ [32]
      - Name of the control, a NUL-terminated ASCII string. This
	information is intended for the user.
    * - __s32
      - ``minimum``
      - Minimum value, inclusive. This field gives a lower bound for the
	control. See enum :c:type:`v4l2_ctrl_type` how
	the minimum value is to be used for each possible control type.
	Note that this a signed 32-bit value.
    * - __s32
      - ``maximum``
      - Maximum value, inclusive. This field gives an upper bound for the
	control. See enum :c:type:`v4l2_ctrl_type` how
	the maximum value is to be used for each possible control type.
	Note that this a signed 32-bit value.
    * - __s32
      - ``step``
      - This field gives a step size for the control. See enum
	:c:type:`v4l2_ctrl_type` how the step value is
	to be used for each possible control type. Note that this an
	unsigned 32-bit value.

	Generally drivers should not scale hardware control values. It may
	be necessary for example when the ``name`` or ``id`` imply a
	particular unit and the hardware actually accepts only multiples
	of said unit. If so, drivers must take care values are properly
	rounded when scaling, such that errors will not accumulate on
	repeated read-write cycles.

	This field gives the smallest change of an integer control
	actually affecting hardware. Often the information is needed when
	the user can change controls by keyboard or GUI buttons, rather
	than a slider. When for example a hardware register accepts values
	0-511 and the driver reports 0-65535, step should be 128.

	Note that although signed, the step value is supposed to be always
	positive.
    * - __s32
      - ``default_value``
      - The default value of a ``V4L2_CTRL_TYPE_INTEGER``, ``_BOOLEAN``,
	``_BITMASK``, ``_MENU`` or ``_INTEGER_MENU`` control. Not valid
	for other types of controls.

	.. note::

	   Drivers reset controls to their default value only when
	   the driver is first loaded, never afterwards.
    * - __u32
      - ``flags``
      - Control flags, see :ref:`control-flags`.
    * - __u32
      - ``reserved``\ [2]
      - Reserved for future extensions. Drivers must set the array to
	zero.


.. tabularcolumns:: |p{1.2cm}|p{5.5cm}|p{10.6cm}|

.. _v4l2-query-ext-ctrl:

.. cssclass:: longtable

.. flat-table:: struct v4l2_query_ext_ctrl
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2

    * - __u32
      - ``id``
      - Identifies the control, set by the application. See
	:ref:`control-id` for predefined IDs. When the ID is ORed with
	``V4L2_CTRL_FLAG_NEXT_CTRL`` the driver clears the flag and
	returns the first non-compound control with a higher ID. When the
	ID is ORed with ``V4L2_CTRL_FLAG_NEXT_COMPOUND`` the driver clears
	the flag and returns the first compound control with a higher ID.
	Set both to get the first control (compound or not) with a higher
	ID.
    * - __u32
      - ``type``
      - Type of control, see :c:type:`v4l2_ctrl_type`.
    * - char
      - ``name``\ [32]
      - Name of the control, a NUL-terminated ASCII string. This
	information is intended for the user.
    * - __s64
      - ``minimum``
      - Minimum value, inclusive. This field gives a lower bound for the
	control. See enum :c:type:`v4l2_ctrl_type` how
	the minimum value is to be used for each possible control type.
	Note that this a signed 64-bit value.
    * - __s64
      - ``maximum``
      - Maximum value, inclusive. This field gives an upper bound for the
	control. See enum :c:type:`v4l2_ctrl_type` how
	the maximum value is to be used for each possible control type.
	Note that this a signed 64-bit value.
    * - __u64
      - ``step``
      - This field gives a step size for the control. See enum
	:c:type:`v4l2_ctrl_type` how the step value is
	to be used for each possible control type. Note that this an
	unsigned 64-bit value.

	Generally drivers should not scale hardware control values. It may
	be necessary for example when the ``name`` or ``id`` imply a
	particular unit and the hardware actually accepts only multiples
	of said unit. If so, drivers must take care values are properly
	rounded when scaling, such that errors will not accumulate on
	repeated read-write cycles.

	This field gives the smallest change of an integer control
	actually affecting hardware. Often the information is needed when
	the user can change controls by keyboard or GUI buttons, rather
	than a slider. When for example a hardware register accepts values
	0-511 and the driver reports 0-65535, step should be 128.
    * - __s64
      - ``default_value``
      - The default value of a ``V4L2_CTRL_TYPE_INTEGER``, ``_INTEGER64``,
	``_BOOLEAN``, ``_BITMASK``, ``_MENU``, ``_INTEGER_MENU``, ``_U8``
	or ``_U16`` control. Not valid for other types of controls.

	.. note::

	   Drivers reset controls to their default value only when
	   the driver is first loaded, never afterwards.
    * - __u32
      - ``flags``
      - Control flags, see :ref:`control-flags`.
    * - __u32
      - ``elem_size``
      - The size in bytes of a single element of the array. Given a char
	pointer ``p`` to a 3-dimensional array you can find the position
	of cell ``(z, y, x)`` as follows:
	``p + ((z * dims[1] + y) * dims[0] + x) * elem_size``.
	``elem_size`` is always valid, also when the control isn't an
	array. For string controls ``elem_size`` is equal to
	``maximum + 1``.
    * - __u32
      - ``elems``
      - The number of elements in the N-dimensional array. If this control
	is not an array, then ``elems`` is 1. The ``elems`` field can
	never be 0.
    * - __u32
      - ``nr_of_dims``
      - The number of dimension in the N-dimensional array. If this
	control is not an array, then this field is 0.
    * - __u32
      - ``dims[V4L2_CTRL_MAX_DIMS]``
      - The size of each dimension. The first ``nr_of_dims`` elements of
	this array must be non-zero, all remaining elements must be zero.
    * - __u32
      - ``reserved``\ [32]
      - Reserved for future extensions. Applications and drivers must set
	the array to zero.


.. tabularcolumns:: |p{1.2cm}|p{3.0cm}|p{13.1cm}|

.. _v4l2-querymenu:

.. flat-table:: struct v4l2_querymenu
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2

    * - __u32
      - ``id``
      - Identifies the control, set by the application from the respective
	struct :ref:`v4l2_queryctrl <v4l2-queryctrl>` ``id``.
    * - __u32
      - ``index``
      - Index of the menu item, starting at zero, set by the application.
    * - union {
      - (anonymous)
    * - __u8
      - ``name``\ [32]
      - Name of the menu item, a NUL-terminated ASCII string. This
	information is intended for the user. This field is valid for
	``V4L2_CTRL_TYPE_MENU`` type controls.
    * - __s64
      - ``value``
      - Value of the integer menu item. This field is valid for
	``V4L2_CTRL_TYPE_INTEGER_MENU`` type controls.
    * - }
      -
    * - __u32
      - ``reserved``
      - Reserved for future extensions. Drivers must set the array to
	zero.

.. c:type:: v4l2_ctrl_type

.. raw:: latex

   \footnotesize

.. tabularcolumns:: |p{6.5cm}|p{1.5cm}|p{1.1cm}|p{1.5cm}|p{6.8cm}|

.. cssclass:: longtable

.. flat-table:: enum v4l2_ctrl_type
    :header-rows:  1
    :stub-columns: 0
    :widths:       30 5 5 5 55

    * - Type
      - ``minimum``
      - ``step``
      - ``maximum``
      - Description
    * - ``V4L2_CTRL_TYPE_INTEGER``
      - any
      - any
      - any
      - An integer-valued control ranging from minimum to maximum
	inclusive. The step value indicates the increment between values.
    * - ``V4L2_CTRL_TYPE_BOOLEAN``
      - 0
      - 1
      - 1
      - A boolean-valued control. Zero corresponds to "disabled", and one
	means "enabled".
    * - ``V4L2_CTRL_TYPE_MENU``
      - ≥ 0
      - 1
      - N-1
      - The control has a menu of N choices. The names of the menu items
	can be enumerated with the ``VIDIOC_QUERYMENU`` ioctl.
    * - ``V4L2_CTRL_TYPE_INTEGER_MENU``
      - ≥ 0
      - 1
      - N-1
      - The control has a menu of N choices. The values of the menu items
	can be enumerated with the ``VIDIOC_QUERYMENU`` ioctl. This is
	similar to ``V4L2_CTRL_TYPE_MENU`` except that instead of strings,
	the menu items are signed 64-bit integers.
    * - ``V4L2_CTRL_TYPE_BITMASK``
      - 0
      - n/a
      - any
      - A bitmask field. The maximum value is the set of bits that can be
	used, all other bits are to be 0. The maximum value is interpreted
	as a __u32, allowing the use of bit 31 in the bitmask.
    * - ``V4L2_CTRL_TYPE_BUTTON``
      - 0
      - 0
      - 0
      - A control which performs an action when set. Drivers must ignore
	the value passed with ``VIDIOC_S_CTRL`` and return an ``EACCES`` error
	code on a ``VIDIOC_G_CTRL`` attempt.
    * - ``V4L2_CTRL_TYPE_INTEGER64``
      - any
      - any
      - any
      - A 64-bit integer valued control. Minimum, maximum and step size
	cannot be queried using ``VIDIOC_QUERYCTRL``. Only
	``VIDIOC_QUERY_EXT_CTRL`` can retrieve the 64-bit min/max/step
	values, they should be interpreted as n/a when using
	``VIDIOC_QUERYCTRL``.
    * - ``V4L2_CTRL_TYPE_STRING``
      - ≥ 0
      - ≥ 1
      - ≥ 0
      - The minimum and maximum string lengths. The step size means that
	the string must be (minimum + N * step) characters long for N ≥ 0.
	These lengths do not include the terminating zero, so in order to
	pass a string of length 8 to
	:ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` you need to
	set the ``size`` field of struct
	:c:type:`v4l2_ext_control` to 9. For
	:ref:`VIDIOC_G_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` you can set
	the ``size`` field to ``maximum`` + 1. Which character encoding is
	used will depend on the string control itself and should be part
	of the control documentation.
    * - ``V4L2_CTRL_TYPE_CTRL_CLASS``
      - n/a
      - n/a
      - n/a
      - This is not a control. When ``VIDIOC_QUERYCTRL`` is called with a
	control ID equal to a control class code (see :ref:`ctrl-class`)
	+ 1, the ioctl returns the name of the control class and this
	control type. Older drivers which do not support this feature
	return an ``EINVAL`` error code.
    * - ``V4L2_CTRL_TYPE_U8``
      - any
      - any
      - any
      - An unsigned 8-bit valued control ranging from minimum to maximum
	inclusive. The step value indicates the increment between values.
    * - ``V4L2_CTRL_TYPE_U16``
      - any
      - any
      - any
      - An unsigned 16-bit valued control ranging from minimum to maximum
	inclusive. The step value indicates the increment between values.
    * - ``V4L2_CTRL_TYPE_U32``
      - any
      - any
      - any
      - An unsigned 32-bit valued control ranging from minimum to maximum
	inclusive. The step value indicates the increment between values.
    * - ``V4L2_CTRL_TYPE_MPEG2_QUANTISATION``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_mpeg2_quantisation`, containing MPEG-2
	quantisation matrices for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_MPEG2_SEQUENCE``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_mpeg2_sequence`, containing MPEG-2
	sequence parameters for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_MPEG2_PICTURE``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_mpeg2_picture`, containing MPEG-2
	picture parameters for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_AREA``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_area`, containing the width and the height
        of a rectangular area. Units depend on the use case.
    * - ``V4L2_CTRL_TYPE_H264_SPS``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_h264_sps`, containing H264
	sequence parameters for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_H264_PPS``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_h264_pps`, containing H264
	picture parameters for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_H264_SCALING_MATRIX``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_h264_scaling_matrix`, containing H264
	scaling matrices for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_H264_SLICE_PARAMS``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_h264_slice_params`, containing H264
	slice parameters for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_H264_DECODE_PARAMS``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_h264_decode_params`, containing H264
	decode parameters for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_FWHT_PARAMS``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_fwht_params`, containing FWHT
	parameters for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_HEVC_SPS``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_hevc_sps`, containing HEVC Sequence
	Parameter Set for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_HEVC_PPS``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_hevc_pps`, containing HEVC Picture
	Parameter Set for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_HEVC_SLICE_PARAMS``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_hevc_slice_params`, containing HEVC
	slice parameters for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_HEVC_SCALING_MATRIX``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_hevc_scaling_matrix`, containing HEVC
	scaling matrix for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_VP8_FRAME``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_vp8_frame`, containing VP8
	frame parameters for stateless video decoders.
    * - ``V4L2_CTRL_TYPE_HEVC_DECODE_PARAMS``
      - n/a
      - n/a
      - n/a
      - A struct :c:type:`v4l2_ctrl_hevc_decode_params`, containing HEVC
	decoding parameters for stateless video decoders.

.. raw:: latex

   \normalsize

.. tabularcolumns:: |p{7.3cm}|p{1.8cm}|p{8.2cm}|

.. cssclass:: longtable

.. _control-flags:

.. flat-table:: Control Flags
    :header-rows:  0
    :stub-columns: 0
    :widths:       3 1 4

    * - ``V4L2_CTRL_FLAG_DISABLED``
      - 0x0001
      - This control is permanently disabled and should be ignored by the
	application. Any attempt to change the control will result in an
	``EINVAL`` error code.
    * - ``V4L2_CTRL_FLAG_GRABBED``
      - 0x0002
      - This control is temporarily unchangeable, for example because
	another application took over control of the respective resource.
	Such controls may be displayed specially in a user interface.
	Attempts to change the control may result in an ``EBUSY`` error code.
    * - ``V4L2_CTRL_FLAG_READ_ONLY``
      - 0x0004
      - This control is permanently readable only. Any attempt to change
	the control will result in an ``EINVAL`` error code.
    * - ``V4L2_CTRL_FLAG_UPDATE``
      - 0x0008
      - A hint that changing this control may affect the value of other
	controls within the same control class. Applications should update
	their user interface accordingly.
    * - ``V4L2_CTRL_FLAG_INACTIVE``
      - 0x0010
      - This control is not applicable to the current configuration and
	should be displayed accordingly in a user interface. For example
	the flag may be set on a MPEG audio level 2 bitrate control when
	MPEG audio encoding level 1 was selected with another control.
    * - ``V4L2_CTRL_FLAG_SLIDER``
      - 0x0020
      - A hint that this control is best represented as a slider-like
	element in a user interface.
    * - ``V4L2_CTRL_FLAG_WRITE_ONLY``
      - 0x0040
      - This control is permanently writable only. Any attempt to read the
	control will result in an ``EACCES`` error code error code. This flag
	is typically present for relative controls or action controls
	where writing a value will cause the device to carry out a given
	action (e. g. motor control) but no meaningful value can be
	returned.
    * - ``V4L2_CTRL_FLAG_VOLATILE``
      - 0x0080
      - This control is volatile, which means that the value of the
	control changes continuously. A typical example would be the
	current gain value if the device is in auto-gain mode. In such a
	case the hardware calculates the gain value based on the lighting
	conditions which can change over time.

	.. note::

	   Setting a new value for a volatile control will be ignored
	   unless
	   :ref:`V4L2_CTRL_FLAG_EXECUTE_ON_WRITE <FLAG_EXECUTE_ON_WRITE>`
	   is also set.
	   Setting a new value for a volatile control will *never* trigger a
	   :ref:`V4L2_EVENT_CTRL_CH_VALUE <ctrl-changes-flags>` event.
    * - ``V4L2_CTRL_FLAG_HAS_PAYLOAD``
      - 0x0100
      - This control has a pointer type, so its value has to be accessed
	using one of the pointer fields of struct
	:c:type:`v4l2_ext_control`. This flag is set
	for controls that are an array, string, or have a compound type.
	In all cases you have to set a pointer to memory containing the
	payload of the control.
    * .. _FLAG_EXECUTE_ON_WRITE:

      - ``V4L2_CTRL_FLAG_EXECUTE_ON_WRITE``
      - 0x0200
      - The value provided to the control will be propagated to the driver
	even if it remains constant. This is required when the control
	represents an action on the hardware. For example: clearing an
	error flag or triggering the flash. All the controls of the type
	``V4L2_CTRL_TYPE_BUTTON`` have this flag set.
    * .. _FLAG_MODIFY_LAYOUT:

      - ``V4L2_CTRL_FLAG_MODIFY_LAYOUT``
      - 0x0400
      - Changing this control value may modify the layout of the
        buffer (for video devices) or the media bus format (for sub-devices).

	A typical example would be the ``V4L2_CID_ROTATE`` control.

	Note that typically controls with this flag will also set the
	``V4L2_CTRL_FLAG_GRABBED`` flag when buffers are allocated or
	streaming is in progress since most drivers do not support changing
	the format in that case.

Return Value
============

On success 0 is returned, on error -1 and the ``errno`` variable is set
appropriately. The generic error codes are described at the
:ref:`Generic Error Codes <gen-errors>` chapter.

EINVAL
    The struct :ref:`v4l2_queryctrl <v4l2-queryctrl>` ``id`` is
    invalid. The struct :ref:`v4l2_querymenu <v4l2-querymenu>` ``id``
    is invalid or ``index`` is out of range (less than ``minimum`` or
    greater than ``maximum``) or this particular menu item is not
    supported by the driver.

EACCES
    An attempt was made to read a write-only control.

.. [#f1]
   ``V4L2_CTRL_FLAG_DISABLED`` was intended for two purposes: Drivers
   can skip predefined controls not supported by the hardware (although
   returning ``EINVAL`` would do as well), or disable predefined and private
   controls after hardware detection without the trouble of reordering
   control arrays and indices (``EINVAL`` cannot be used to skip private
   controls because it would prematurely end the enumeration).