VIDIOC_QUERYCAP ioctl

장치 capability 조회

모든 V4L2 장치는 `VIDIOC_QUERYCAP`을 지원합니다. 이 명령은 해당 kernel device가 V4L2 명세와 호환되는지 식별하고 드라이버·하드웨어 기능을 `struct v4l2_capability`에 반환합니다.

드라이버가 이 명세와 호환되지 않으면 `EINVAL`을 반환합니다. `fd`는 열린 장치 파일이고 `argp`는 드라이버가 채울 capability 구조체 포인터입니다.

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

.. _VIDIOC_QUERYCAP:

*********************
ioctl VIDIOC_QUERYCAP
*********************

Name
====

VIDIOC_QUERYCAP - Query device capabilities

Synopsis
========

.. c:macro:: VIDIOC_QUERYCAP

``int ioctl(int fd, VIDIOC_QUERYCAP, struct v4l2_capability *argp)``

Arguments
=========

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

``argp``
    Pointer to struct :c:type:`v4l2_capability`.

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

All V4L2 devices support the ``VIDIOC_QUERYCAP`` ioctl. It is used to
identify kernel devices compatible with this specification and to obtain
information about driver and hardware capabilities. The ioctl takes a
pointer to a struct :c:type:`v4l2_capability` which is
filled by the driver. When the driver is not compatible with this
specification the ioctl returns an ``EINVAL`` error code.

struct v4l2_capability

위치 정보가 없으면 드라이버가 제어하는 장치를 `platform:vivid-000`처럼 번호로 구분합니다.

Kernel 3.1부터 `version`은 kernel numbering scheme을 따르는 V4L2 subsystem 버전입니다. Stable·배포판 kernel이 더 새 V4L2 stack을 가져오면 실행 kernel 버전과 반드시 같지는 않습니다.

예를 들어 V4L2 4.14 stack은 `KERNEL_VERSION(4, 14, 0)`이며, 각각 `(version >> 16) & 0xFF`, `(version >> 8) & 0xFF`, `version & 0xFF`로 출력할 수 있습니다.

`V4L2_CAP_DEVICE_CAPS` 자체는 `capabilities`에만 나타나며 `device_caps`에는 절대로 나타나지 않습니다.

.. c:type:: v4l2_capability

.. tabularcolumns:: |p{1.4cm}|p{2.8cm}|p{13.1cm}|

.. cssclass:: longtable

.. flat-table:: struct v4l2_capability
    :header-rows:  0
    :stub-columns: 0
    :widths:       3 4 20

    * - __u8
      - ``driver``\ [16]
      - Name of the driver, a unique NUL-terminated ASCII string. For
	example: "bttv". Driver specific applications can use this
	information to verify the driver identity. It is also useful to
	work around known bugs, or to identify drivers in error reports.

	Storing strings in fixed sized arrays is bad practice but
	unavoidable here. Drivers and applications should take precautions
	to never read or write beyond the end of the array and to make
	sure the strings are properly NUL-terminated.
    * - __u8
      - ``card``\ [32]
      - Name of the device, a NUL-terminated UTF-8 string. For example:
	"Yoyodyne TV/FM". One driver may support different brands or
	models of video hardware. This information is intended for users,
	for example in a menu of available devices. Since multiple TV
	cards of the same brand may be installed which are supported by
	the same driver, this name should be combined with the character
	device file name (e. g. ``/dev/video2``) or the ``bus_info``
	string to avoid ambiguities.
    * - __u8
      - ``bus_info``\ [32]
      - Location of the device in the system, a NUL-terminated ASCII
	string. For example: "PCI:0000:05:06.0". This information is
	intended for users, to distinguish multiple identical devices. If
	no such information is available the field must simply count the
	devices controlled by the driver ("platform:vivid-000"). The
	bus_info must start with "PCI:" for PCI boards, "PCIe:" for PCI
	Express boards, "usb-" for USB devices, "I2C:" for i2c devices,
	"ISA:" for ISA devices, "parport" for parallel port devices and
	"platform:" for platform devices.
    * - __u32
      - ``version``
      - Version number of the driver.

	Starting with kernel 3.1, the version reported is provided by the
	V4L2 subsystem following the kernel numbering scheme. However, it
	may not always return the same version as the kernel if, for
	example, a stable or distribution-modified kernel uses the V4L2
	stack from a newer kernel.

	The version number is formatted using the ``KERNEL_VERSION()``
	macro. For example if the media stack corresponds to the V4L2
	version shipped with Kernel 4.14, it would be equivalent to:
    * - :cspan:`2`

	``#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))``

	``__u32 version = KERNEL_VERSION(4, 14, 0);``

	``printf ("Version: %u.%u.%u\\n",``

	``(version >> 16) & 0xFF, (version >> 8) & 0xFF, version & 0xFF);``
    * - __u32
      - ``capabilities``
      - Available capabilities of the physical device as a whole, see
	:ref:`device-capabilities`. The same physical device can export
	multiple devices in /dev (e.g. /dev/videoX, /dev/vbiY and
	/dev/radioZ). The ``capabilities`` field should contain a union of
	all capabilities available around the several V4L2 devices
	exported to userspace. For all those devices the ``capabilities``
	field returns the same set of capabilities. This allows
	applications to open just one of the devices (typically the video
	device) and discover whether video, vbi and/or radio are also
	supported.
    * - __u32
      - ``device_caps``
      - Device capabilities of the opened device, see
	:ref:`device-capabilities`. Should contain the available
	capabilities of that specific device node. So, for example,
	``device_caps`` of a radio device will only contain radio related
	capabilities and no video or vbi capabilities. This field is only
	set if the ``capabilities`` field contains the
	``V4L2_CAP_DEVICE_CAPS`` capability. Only the ``capabilities``
	field can have the ``V4L2_CAP_DEVICE_CAPS`` capability,
	``device_caps`` will never set ``V4L2_CAP_DEVICE_CAPS``.
    * - __u32
      - ``reserved``\ [3]
      - Reserved for future extensions. Drivers must set this array to
	zero.


.. tabularcolumns:: |p{7.0cm}|p{2.6cm}|p{7.7cm}|

Device capability 플래그

`v4l2_framebuffer`에는 `v4l2_buf_type` 필드가 없으므로 overlay 종류는 드라이버 capability로 암시됩니다. 따라서 드라이버는 두 overlay flag 가운데 하나를 설정하면 다른 하나를 지워야 합니다.

EDID input 장치는 보통 VESA Enhanced Display Data Channel을 지원하는 EEPROM이지만 microcontroller일 수도 있습니다. Output EDID는 serial port로 접근하는 HDMI splitter 같은 외부 장치에서 읽을 수 있습니다.

.. _device-capabilities:

.. cssclass:: longtable

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

    * - ``V4L2_CAP_VIDEO_CAPTURE``
      - 0x00000001
      - The device supports the single-planar API through the
	:ref:`Video Capture <capture>` interface.
    * - ``V4L2_CAP_VIDEO_CAPTURE_MPLANE``
      - 0x00001000
      - The device supports the :ref:`multi-planar API <planar-apis>`
	through the :ref:`Video Capture <capture>` interface.
    * - ``V4L2_CAP_VIDEO_OUTPUT``
      - 0x00000002
      - The device supports the single-planar API through the
	:ref:`Video Output <output>` interface.
    * - ``V4L2_CAP_VIDEO_OUTPUT_MPLANE``
      - 0x00002000
      - The device supports the :ref:`multi-planar API <planar-apis>`
	through the :ref:`Video Output <output>` interface.
    * - ``V4L2_CAP_VIDEO_M2M``
      - 0x00008000
      - The device supports the single-planar API through the Video
	Memory-To-Memory interface.
    * - ``V4L2_CAP_VIDEO_M2M_MPLANE``
      - 0x00004000
      - The device supports the :ref:`multi-planar API <planar-apis>`
	through the Video Memory-To-Memory interface.
    * - ``V4L2_CAP_VIDEO_OVERLAY``
      - 0x00000004
      - The device supports the :ref:`Video Overlay <overlay>`
	interface. A video overlay device typically stores captured images
	directly in the video memory of a graphics card, with hardware
	clipping and scaling.
    * - ``V4L2_CAP_VBI_CAPTURE``
      - 0x00000010
      - The device supports the :ref:`Raw VBI Capture <raw-vbi>`
	interface, providing Teletext and Closed Caption data.
    * - ``V4L2_CAP_VBI_OUTPUT``
      - 0x00000020
      - The device supports the :ref:`Raw VBI Output <raw-vbi>`
	interface.
    * - ``V4L2_CAP_SLICED_VBI_CAPTURE``
      - 0x00000040
      - The device supports the :ref:`Sliced VBI Capture <sliced>`
	interface.
    * - ``V4L2_CAP_SLICED_VBI_OUTPUT``
      - 0x00000080
      - The device supports the :ref:`Sliced VBI Output <sliced>`
	interface.
    * - ``V4L2_CAP_RDS_CAPTURE``
      - 0x00000100
      - The device supports the :ref:`RDS <rds>` capture interface.
    * - ``V4L2_CAP_VIDEO_OUTPUT_OVERLAY``
      - 0x00000200
      - The device supports the :ref:`Video Output Overlay <osd>` (OSD)
	interface. Unlike the *Video Overlay* interface, this is a
	secondary function of video output devices and overlays an image
	onto an outgoing video signal. When the driver sets this flag, it
	must clear the ``V4L2_CAP_VIDEO_OVERLAY`` flag and vice
	versa. [#f1]_
    * - ``V4L2_CAP_HW_FREQ_SEEK``
      - 0x00000400
      - The device supports the
	:ref:`VIDIOC_S_HW_FREQ_SEEK` ioctl
	for hardware frequency seeking.
    * - ``V4L2_CAP_RDS_OUTPUT``
      - 0x00000800
      - The device supports the :ref:`RDS <rds>` output interface.
    * - ``V4L2_CAP_TUNER``
      - 0x00010000
      - The device has some sort of tuner to receive RF-modulated video
	signals. For more information about tuner programming see
	:ref:`tuner`.
    * - ``V4L2_CAP_AUDIO``
      - 0x00020000
      - The device has audio inputs or outputs. It may or may not support
	audio recording or playback, in PCM or compressed formats. PCM
	audio support must be implemented as ALSA or OSS interface. For
	more information on audio inputs and outputs see :ref:`audio`.
    * - ``V4L2_CAP_RADIO``
      - 0x00040000
      - This is a radio receiver.
    * - ``V4L2_CAP_MODULATOR``
      - 0x00080000
      - The device has some sort of modulator to emit RF-modulated
	video/audio signals. For more information about modulator
	programming see :ref:`tuner`.
    * - ``V4L2_CAP_SDR_CAPTURE``
      - 0x00100000
      - The device supports the :ref:`SDR Capture <sdr>` interface.
    * - ``V4L2_CAP_EXT_PIX_FORMAT``
      - 0x00200000
      - The device supports the struct
	:c:type:`v4l2_pix_format` extended fields.
    * - ``V4L2_CAP_SDR_OUTPUT``
      - 0x00400000
      - The device supports the :ref:`SDR Output <sdr>` interface.
    * - ``V4L2_CAP_META_CAPTURE``
      - 0x00800000
      - The device supports the :ref:`metadata` capture interface.
    * - ``V4L2_CAP_READWRITE``
      - 0x01000000
      - The device supports the :c:func:`read()` and/or
	:c:func:`write()` I/O methods.
    * - ``V4L2_CAP_EDID``
      - 0x02000000
      - The device stores the EDID for a video input, or retrieves the EDID for a video
        output. It is a standalone EDID device, so no video streaming etc. will take place.

        For a video input this is typically an eeprom that supports the
        :ref:`VESA Enhanced Display Data Channel Standard <vesaeddc>`. It can be something
        else as well, for example a micro controller.

        For a video output this is typically read from an external device such as an
        HDMI splitter accessed by a serial port.
    * - ``V4L2_CAP_STREAMING``
      - 0x04000000
      - The device supports the :ref:`streaming <mmap>` I/O method.
    * - ``V4L2_CAP_META_OUTPUT``
      - 0x08000000
      - The device supports the :ref:`metadata` output interface.
    * - ``V4L2_CAP_TOUCH``
      - 0x10000000
      - This is a touch device.
    * - ``V4L2_CAP_IO_MC``
      - 0x20000000
      - There is only one input and/or output seen from userspace. The whole
        video topology configuration, including which I/O entity is routed to
        the input/output, is configured by userspace via the Media Controller.
        See :ref:`media_controller`.
    * - ``V4L2_CAP_DEVICE_CAPS``
      - 0x80000000
      - The driver fills the ``device_caps`` field. This capability can
	only appear in the ``capabilities`` field and never in the
	``device_caps`` field.

반환값과 overlay 각주

성공하면 0, 오류이면 -1을 반환하고 `errno`를 설정합니다. 명세와 호환되지 않는 장치는 `EINVAL`일 수 있습니다.

Overlay type은 `v4l2_framebuffer` 구조체에 명시 필드가 없으므로 `V4L2_CAP_VIDEO_OVERLAY` 또는 `V4L2_CAP_VIDEO_OUTPUT_OVERLAY` capability로 판별합니다.

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.

.. [#f1]
   The struct :c:type:`v4l2_framebuffer` lacks an
   enum :c:type:`v4l2_buf_type` field, therefore the
   type of overlay is implied by the driver capabilities.