VIDIOC_ENUM_FREQ_BANDS ioctl

목적, 호출 형식과 인자

`VIDIOC_ENUM_FREQ_BANDS`는 tuner 또는 modulator가 지원하는 주파수 대역을 열거하는 ioctl입니다. 호출 형식은 `int ioctl(int fd, VIDIOC_ENUM_FREQ_BANDS, struct v4l2_frequency_band *argp)`입니다.

`fd`는 `open()`이 반환한 파일 디스크립터이고, `argp`는 조회할 tuner·유형·대역 순번과 반환 정보를 담는 `struct v4l2_frequency_band`를 가리킵니다.

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

.. _VIDIOC_ENUM_FREQ_BANDS:

****************************
ioctl VIDIOC_ENUM_FREQ_BANDS
****************************

Name
====

VIDIOC_ENUM_FREQ_BANDS - Enumerate supported frequency bands

Synopsis
========

.. c:macro:: VIDIOC_ENUM_FREQ_BANDS

``int ioctl(int fd, VIDIOC_ENUM_FREQ_BANDS, struct v4l2_frequency_band *argp)``

Arguments
=========

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

``argp``
    Pointer to struct :c:type:`v4l2_frequency_band`.

지원 조건과 열거 절차

응용 프로그램은 `v4l2_frequency_band`의 `tuner`, `type`, `index`를 초기화하고 `reserved` 배열을 0으로 만든 뒤 ioctl을 호출합니다. 대응하는 tuner 또는 modulator에 `V4L2_TUNER_CAP_FREQ_BANDS` capability가 설정된 경우에만 이 ioctl이 지원됩니다.

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

Enumerates the frequency bands that a tuner or modulator supports. To do
this applications initialize the ``tuner``, ``type`` and ``index``
fields, and zero out the ``reserved`` array of a struct
:c:type:`v4l2_frequency_band` and call the
:ref:`VIDIOC_ENUM_FREQ_BANDS` ioctl with a pointer to this structure.

This ioctl is supported if the ``V4L2_TUNER_CAP_FREQ_BANDS`` capability
of the corresponding tuner/modulator is set.

.. tabularcolumns:: |p{2.9cm}|p{2.9cm}|p{5.8cm}|p{2.9cm}|p{2.4cm}|

v4l2_frequency_band 구조체

`type`은 `v4l2_tuner.type`과 같은 값입니다. `/dev/radioX` 장치 노드는 `V4L2_TUNER_RADIO`, 그 밖의 노드는 `V4L2_TUNER_ANALOG_TV`를 사용합니다. 현재는 radio modulator만 지원하므로 modulator에도 `V4L2_TUNER_RADIO`를 설정합니다.

`capability`의 `V4L2_TUNER_CAP_LOW` 또는 `V4L2_TUNER_CAP_1HZ`는 선택한 tuner/modulator의 모든 주파수 band에서 같아야 합니다. 즉 모든 band에 해당 capability가 있거나 어느 band에도 없어야 합니다.

`rangelow`와 `rangehigh`의 기본 단위는 62.5 kHz입니다. `V4L2_TUNER_CAP_LOW`가 설정되면 62.5 Hz, `V4L2_TUNER_CAP_1HZ`가 설정되면 1 Hz 단위를 사용합니다.

현재는 주파수 band 하나당 변조 방식 하나만 지원합니다. 여러 변조 방식을 한 band에 표현해야 한다면 API 작업이 더 필요하며 원문이 안내한 linux-media 메일링 리스트에 논의해야 합니다.

.. c:type:: v4l2_frequency_band

.. flat-table:: struct v4l2_frequency_band
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2 1 1

    * - __u32
      - ``tuner``
      - The tuner or modulator index number. This is the same value as in
	the struct :c:type:`v4l2_input` ``tuner`` field and
	the struct :c:type:`v4l2_tuner` ``index`` field, or
	the struct :c:type:`v4l2_output` ``modulator`` field
	and the struct :c:type:`v4l2_modulator` ``index``
	field.
    * - __u32
      - ``type``
      - The tuner type. This is the same value as in the struct
	:c:type:`v4l2_tuner` ``type`` field. The type must be
	set to ``V4L2_TUNER_RADIO`` for ``/dev/radioX`` device nodes, and
	to ``V4L2_TUNER_ANALOG_TV`` for all others. Set this field to
	``V4L2_TUNER_RADIO`` for modulators (currently only radio
	modulators are supported). See :c:type:`v4l2_tuner_type`
    * - __u32
      - ``index``
      - Identifies the frequency band, set by the application.
    * - __u32
      - ``capability``
      - :cspan:`2` The tuner/modulator capability flags for this
	frequency band, see :ref:`tuner-capability`. The
	``V4L2_TUNER_CAP_LOW`` or ``V4L2_TUNER_CAP_1HZ`` capability must
	be the same for all frequency bands of the selected
	tuner/modulator. So either all bands have that capability set, or
	none of them have that capability.
    * - __u32
      - ``rangelow``
      - :cspan:`2` The lowest tunable frequency in units of 62.5 kHz, or
	if the ``capability`` flag ``V4L2_TUNER_CAP_LOW`` is set, in units
	of 62.5 Hz, for this frequency band. A 1 Hz unit is used when the
	``capability`` flag ``V4L2_TUNER_CAP_1HZ`` is set.
    * - __u32
      - ``rangehigh``
      - :cspan:`2` The highest tunable frequency in units of 62.5 kHz,
	or if the ``capability`` flag ``V4L2_TUNER_CAP_LOW`` is set, in
	units of 62.5 Hz, for this frequency band. A 1 Hz unit is used
	when the ``capability`` flag ``V4L2_TUNER_CAP_1HZ`` is set.
    * - __u32
      - ``modulation``
      - :cspan:`2` The supported modulation systems of this frequency
	band. See :ref:`band-modulation`.

	.. note::

	   Currently only one modulation system per frequency band
	   is supported. More work will need to be done if multiple
	   modulation systems are possible. Contact the linux-media
	   mailing list
	   (`https://linuxtv.org/lists.php <https://linuxtv.org/lists.php>`__)
	   if you need such functionality.
    * - __u32
      - ``reserved``\ [9]
      - Reserved for future extensions.

	Applications and drivers must set the array to zero.


.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.5cm}|

Band 변조 방식

.. _band-modulation:

.. flat-table:: Band Modulation Systems
    :header-rows:  0
    :stub-columns: 0
    :widths:       3 1 4

    * - ``V4L2_BAND_MODULATION_VSB``
      - 0x02
      - Vestigial Sideband modulation, used for analog TV.
    * - ``V4L2_BAND_MODULATION_FM``
      - 0x04
      - Frequency Modulation, commonly used for analog radio.
    * - ``V4L2_BAND_MODULATION_AM``
      - 0x08
      - Amplitude Modulation, commonly used for analog radio.

반환값과 EINVAL

성공하면 0을 반환합니다. 오류가 발생하면 -1을 반환하고 `errno`를 적절한 값으로 설정하며, 공통 오류 코드는 Generic Error Codes 장을 따릅니다.

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 ``tuner`` or ``index`` is out of bounds or the ``type`` field is
    wrong.