Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 659 insertions, 0 deletions

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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+.. c:namespace:: V4L
+
+.. _diff-v4l:
+
+********************************
+Differences between V4L and V4L2
+********************************
+
+The Video For Linux API was first introduced in Linux 2.1 to unify and
+replace various TV and radio device related interfaces, developed
+independently by driver writers in prior years. Starting with Linux 2.5
+the much improved V4L2 API replaces the V4L API. The support for the old
+V4L calls were removed from Kernel, but the library :ref:`libv4l`
+supports the conversion of a V4L API system call into a V4L2 one.
+
+Opening and Closing Devices
+===========================
+
+For compatibility reasons the character device file names recommended
+for V4L2 video capture, overlay, radio and raw vbi capture devices did
+not change from those used by V4L. They are listed in :ref:`devices`
+and below in :ref:`v4l-dev`.
+
+The teletext devices (minor range 192-223) have been removed in V4L2 and
+no longer exist. There is no hardware available anymore for handling
+pure teletext. Instead raw or sliced VBI is used.
+
+The V4L ``videodev`` module automatically assigns minor numbers to
+drivers in load order, depending on the registered device type. We
+recommend that V4L2 drivers by default register devices with the same
+numbers, but the system administrator can assign arbitrary minor numbers
+using driver module options. The major device number remains 81.
+
+.. _v4l-dev:
+
+.. flat-table:: V4L Device Types, Names and Numbers
+    :header-rows:  1
+    :stub-columns: 0
+
+    * - Device Type
+      - File Name
+      - Minor Numbers
+    * - Video capture and overlay
+      - ``/dev/video`` and ``/dev/bttv0``\  [#f1]_, ``/dev/video0`` to
+	``/dev/video63``
+      - 0-63
+    * - Radio receiver
+      - ``/dev/radio``\  [#f2]_, ``/dev/radio0`` to ``/dev/radio63``
+      - 64-127
+    * - Raw VBI capture
+      - ``/dev/vbi``, ``/dev/vbi0`` to ``/dev/vbi31``
+      - 224-255
+
+V4L prohibits (or used to prohibit) multiple opens of a device file.
+V4L2 drivers *may* support multiple opens, see :ref:`open` for details
+and consequences.
+
+V4L drivers respond to V4L2 ioctls with an ``EINVAL`` error code.
+
+Querying Capabilities
+=====================
+
+The V4L ``VIDIOCGCAP`` ioctl is equivalent to V4L2's
+:ref:`VIDIOC_QUERYCAP`.
+
+The ``name`` field in struct ``video_capability`` became
+``card`` in struct :c:type:`v4l2_capability`, ``type``
+was replaced by ``capabilities``. Note V4L2 does not distinguish between
+device types like this, better think of basic video input, video output
+and radio devices supporting a set of related functions like video
+capturing, video overlay and VBI capturing. See :ref:`open` for an
+introduction.
+
+.. tabularcolumns:: |p{5.5cm}|p{6.5cm}|p{5.5cm}
+
+.. cssclass:: longtable
+
+.. flat-table::
+    :header-rows:  1
+    :stub-columns: 0
+
+    * - ``struct video_capability`` ``type``
+      - struct :c:type:`v4l2_capability`
+	``capabilities`` flags
+      - Purpose
+    * - ``VID_TYPE_CAPTURE``
+      - ``V4L2_CAP_VIDEO_CAPTURE``
+      - The :ref:`video capture <capture>` interface is supported.
+    * - ``VID_TYPE_TUNER``
+      - ``V4L2_CAP_TUNER``
+      - The device has a :ref:`tuner or modulator <tuner>`.
+    * - ``VID_TYPE_TELETEXT``
+      - ``V4L2_CAP_VBI_CAPTURE``
+      - The :ref:`raw VBI capture <raw-vbi>` interface is supported.
+    * - ``VID_TYPE_OVERLAY``
+      - ``V4L2_CAP_VIDEO_OVERLAY``
+      - The :ref:`video overlay <overlay>` interface is supported.
+    * - ``VID_TYPE_CHROMAKEY``
+      - ``V4L2_FBUF_CAP_CHROMAKEY`` in field ``capability`` of struct
+	:c:type:`v4l2_framebuffer`
+      - Whether chromakey overlay is supported. For more information on
+	overlay see :ref:`overlay`.
+    * - ``VID_TYPE_CLIPPING``
+      - ``V4L2_FBUF_CAP_LIST_CLIPPING`` and
+	``V4L2_FBUF_CAP_BITMAP_CLIPPING`` in field ``capability`` of
+	struct :c:type:`v4l2_framebuffer`
+      - Whether clipping the overlaid image is supported, see
+	:ref:`overlay`.
+    * - ``VID_TYPE_FRAMERAM``
+      - ``V4L2_FBUF_CAP_EXTERNOVERLAY`` *not set* in field ``capability``
+	of struct :c:type:`v4l2_framebuffer`
+      - Whether overlay overwrites frame buffer memory, see
+	:ref:`overlay`.
+    * - ``VID_TYPE_SCALES``
+      - ``-``
+      - This flag indicates if the hardware can scale images. The V4L2 API
+	implies the scale factor by setting the cropping dimensions and
+	image size with the :ref:`VIDIOC_S_CROP <VIDIOC_G_CROP>` and
+	:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, respectively. The
+	driver returns the closest sizes possible. For more information on
+	cropping and scaling see :ref:`crop`.
+    * - ``VID_TYPE_MONOCHROME``
+      - ``-``
+      - Applications can enumerate the supported image formats with the
+	:ref:`VIDIOC_ENUM_FMT` ioctl to determine if
+	the device supports grey scale capturing only. For more
+	information on image formats see :ref:`pixfmt`.
+    * - ``VID_TYPE_SUBCAPTURE``
+      - ``-``
+      - Applications can call the :ref:`VIDIOC_G_CROP <VIDIOC_G_CROP>`
+	ioctl to determine if the device supports capturing a subsection
+	of the full picture ("cropping" in V4L2). If not, the ioctl
+	returns the ``EINVAL`` error code. For more information on cropping
+	and scaling see :ref:`crop`.
+    * - ``VID_TYPE_MPEG_DECODER``
+      - ``-``
+      - Applications can enumerate the supported image formats with the
+	:ref:`VIDIOC_ENUM_FMT` ioctl to determine if
+	the device supports MPEG streams.
+    * - ``VID_TYPE_MPEG_ENCODER``
+      - ``-``
+      - See above.
+    * - ``VID_TYPE_MJPEG_DECODER``
+      - ``-``
+      - See above.
+    * - ``VID_TYPE_MJPEG_ENCODER``
+      - ``-``
+      - See above.
+
+The ``audios`` field was replaced by ``capabilities`` flag
+``V4L2_CAP_AUDIO``, indicating *if* the device has any audio inputs or
+outputs. To determine their number applications can enumerate audio
+inputs with the :ref:`VIDIOC_G_AUDIO <VIDIOC_G_AUDIO>` ioctl. The
+audio ioctls are described in :ref:`audio`.
+
+The ``maxwidth``, ``maxheight``, ``minwidth`` and ``minheight`` fields
+were removed. Calling the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` or
+:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl with the desired
+dimensions returns the closest size possible, taking into account the
+current video standard, cropping and scaling limitations.
+
+Video Sources
+=============
+
+V4L provides the ``VIDIOCGCHAN`` and ``VIDIOCSCHAN`` ioctl using struct
+``video_channel`` to enumerate the video inputs of a V4L
+device. The equivalent V4L2 ioctls are
+:ref:`VIDIOC_ENUMINPUT`,
+:ref:`VIDIOC_G_INPUT <VIDIOC_G_INPUT>` and
+:ref:`VIDIOC_S_INPUT <VIDIOC_G_INPUT>` using struct
+:c:type:`v4l2_input` as discussed in :ref:`video`.
+
+The ``channel`` field counting inputs was renamed to ``index``, the
+video input types were renamed as follows:
+
+
+.. flat-table::
+    :header-rows:  1
+    :stub-columns: 0
+
+    * - struct ``video_channel`` ``type``
+      - struct :c:type:`v4l2_input` ``type``
+    * - ``VIDEO_TYPE_TV``
+      - ``V4L2_INPUT_TYPE_TUNER``
+    * - ``VIDEO_TYPE_CAMERA``
+      - ``V4L2_INPUT_TYPE_CAMERA``
+
+Unlike the ``tuners`` field expressing the number of tuners of this
+input, V4L2 assumes each video input is connected to at most one tuner.
+However a tuner can have more than one input, i. e. RF connectors, and a
+device can have multiple tuners. The index number of the tuner
+associated with the input, if any, is stored in field ``tuner`` of
+struct :c:type:`v4l2_input`. Enumeration of tuners is
+discussed in :ref:`tuner`.
+
+The redundant ``VIDEO_VC_TUNER`` flag was dropped. Video inputs
+associated with a tuner are of type ``V4L2_INPUT_TYPE_TUNER``. The
+``VIDEO_VC_AUDIO`` flag was replaced by the ``audioset`` field. V4L2
+considers devices with up to 32 audio inputs. Each set bit in the
+``audioset`` field represents one audio input this video input combines
+with. For information about audio inputs and how to switch between them
+see :ref:`audio`.
+
+The ``norm`` field describing the supported video standards was replaced
+by ``std``. The V4L specification mentions a flag ``VIDEO_VC_NORM``
+indicating whether the standard can be changed. This flag was a later
+addition together with the ``norm`` field and has been removed in the
+meantime. V4L2 has a similar, albeit more comprehensive approach to
+video standards, see :ref:`standard` for more information.
+
+Tuning
+======
+
+The V4L ``VIDIOCGTUNER`` and ``VIDIOCSTUNER`` ioctl and struct
+``video_tuner`` can be used to enumerate the tuners of a
+V4L TV or radio device. The equivalent V4L2 ioctls are
+:ref:`VIDIOC_G_TUNER <VIDIOC_G_TUNER>` and
+:ref:`VIDIOC_S_TUNER <VIDIOC_G_TUNER>` using struct
+:c:type:`v4l2_tuner`. Tuners are covered in :ref:`tuner`.
+
+The ``tuner`` field counting tuners was renamed to ``index``. The fields
+``name``, ``rangelow`` and ``rangehigh`` remained unchanged.
+
+The ``VIDEO_TUNER_PAL``, ``VIDEO_TUNER_NTSC`` and ``VIDEO_TUNER_SECAM``
+flags indicating the supported video standards were dropped. This
+information is now contained in the associated struct
+:c:type:`v4l2_input`. No replacement exists for the
+``VIDEO_TUNER_NORM`` flag indicating whether the video standard can be
+switched. The ``mode`` field to select a different video standard was
+replaced by a whole new set of ioctls and structures described in
+:ref:`standard`. Due to its ubiquity it should be mentioned the BTTV
+driver supports several standards in addition to the regular
+``VIDEO_MODE_PAL`` (0), ``VIDEO_MODE_NTSC``, ``VIDEO_MODE_SECAM`` and
+``VIDEO_MODE_AUTO`` (3). Namely N/PAL Argentina, M/PAL, N/PAL, and NTSC
+Japan with numbers 3-6 (sic).
+
+The ``VIDEO_TUNER_STEREO_ON`` flag indicating stereo reception became
+``V4L2_TUNER_SUB_STEREO`` in field ``rxsubchans``. This field also
+permits the detection of monaural and bilingual audio, see the
+definition of struct :c:type:`v4l2_tuner` for details.
+Presently no replacement exists for the ``VIDEO_TUNER_RDS_ON`` and
+``VIDEO_TUNER_MBS_ON`` flags.
+
+The ``VIDEO_TUNER_LOW`` flag was renamed to ``V4L2_TUNER_CAP_LOW`` in
+the struct :c:type:`v4l2_tuner` ``capability`` field.
+
+The ``VIDIOCGFREQ`` and ``VIDIOCSFREQ`` ioctl to change the tuner
+frequency where renamed to
+:ref:`VIDIOC_G_FREQUENCY <VIDIOC_G_FREQUENCY>` and
+:ref:`VIDIOC_S_FREQUENCY <VIDIOC_G_FREQUENCY>`. They take a pointer
+to a struct :c:type:`v4l2_frequency` instead of an
+unsigned long integer.
+
+.. _v4l-image-properties:
+
+Image Properties
+================
+
+V4L2 has no equivalent of the ``VIDIOCGPICT`` and ``VIDIOCSPICT`` ioctl
+and struct ``video_picture``. The following fields where
+replaced by V4L2 controls accessible with the
+:ref:`VIDIOC_QUERYCTRL`,
+:ref:`VIDIOC_G_CTRL <VIDIOC_G_CTRL>` and
+:ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` ioctls:
+
+
+.. flat-table::
+    :header-rows:  1
+    :stub-columns: 0
+
+    * - struct ``video_picture``
+      - V4L2 Control ID
+    * - ``brightness``
+      - ``V4L2_CID_BRIGHTNESS``
+    * - ``hue``
+      - ``V4L2_CID_HUE``
+    * - ``colour``
+      - ``V4L2_CID_SATURATION``
+    * - ``contrast``
+      - ``V4L2_CID_CONTRAST``
+    * - ``whiteness``
+      - ``V4L2_CID_WHITENESS``
+
+The V4L picture controls are assumed to range from 0 to 65535 with no
+particular reset value. The V4L2 API permits arbitrary limits and
+defaults which can be queried with the
+:ref:`VIDIOC_QUERYCTRL` ioctl. For general
+information about controls see :ref:`control`.
+
+The ``depth`` (average number of bits per pixel) of a video image is
+implied by the selected image format. V4L2 does not explicitly provide
+such information assuming applications recognizing the format are aware
+of the image depth and others need not know. The ``palette`` field moved
+into the struct :c:type:`v4l2_pix_format`:
+
+
+.. flat-table::
+    :header-rows:  1
+    :stub-columns: 0
+
+    * - struct ``video_picture`` ``palette``
+      - struct :c:type:`v4l2_pix_format` ``pixfmt``
+    * - ``VIDEO_PALETTE_GREY``
+      - :ref:`V4L2_PIX_FMT_GREY <V4L2-PIX-FMT-GREY>`
+    * - ``VIDEO_PALETTE_HI240``
+      - :ref:`V4L2_PIX_FMT_HI240 <pixfmt-reserved>` [#f3]_
+    * - ``VIDEO_PALETTE_RGB565``
+      - :ref:`V4L2_PIX_FMT_RGB565 <pixfmt-rgb>`
+    * - ``VIDEO_PALETTE_RGB555``
+      - :ref:`V4L2_PIX_FMT_RGB555 <pixfmt-rgb>`
+    * - ``VIDEO_PALETTE_RGB24``
+      - :ref:`V4L2_PIX_FMT_BGR24 <pixfmt-rgb>`
+    * - ``VIDEO_PALETTE_RGB32``
+      - :ref:`V4L2_PIX_FMT_BGR32 <pixfmt-rgb>` [#f4]_
+    * - ``VIDEO_PALETTE_YUV422``
+      - :ref:`V4L2_PIX_FMT_YUYV <V4L2-PIX-FMT-YUYV>`
+    * - ``VIDEO_PALETTE_YUYV``\  [#f5]_
+      - :ref:`V4L2_PIX_FMT_YUYV <V4L2-PIX-FMT-YUYV>`
+    * - ``VIDEO_PALETTE_UYVY``
+      - :ref:`V4L2_PIX_FMT_UYVY <V4L2-PIX-FMT-UYVY>`
+    * - ``VIDEO_PALETTE_YUV420``
+      - None
+    * - ``VIDEO_PALETTE_YUV411``
+      - :ref:`V4L2_PIX_FMT_Y41P <V4L2-PIX-FMT-Y41P>` [#f6]_
+    * - ``VIDEO_PALETTE_RAW``
+      - None [#f7]_
+    * - ``VIDEO_PALETTE_YUV422P``
+      - :ref:`V4L2_PIX_FMT_YUV422P <V4L2-PIX-FMT-YUV422P>`
+    * - ``VIDEO_PALETTE_YUV411P``
+      - :ref:`V4L2_PIX_FMT_YUV411P <V4L2-PIX-FMT-YUV411P>` [#f8]_
+    * - ``VIDEO_PALETTE_YUV420P``
+      - :ref:`V4L2_PIX_FMT_YVU420 <V4L2-PIX-FMT-YVU420>`
+    * - ``VIDEO_PALETTE_YUV410P``
+      - :ref:`V4L2_PIX_FMT_YVU410 <V4L2-PIX-FMT-YVU410>`
+
+V4L2 image formats are defined in :ref:`pixfmt`. The image format can
+be selected with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
+
+Audio
+=====
+
+The ``VIDIOCGAUDIO`` and ``VIDIOCSAUDIO`` ioctl and struct
+``video_audio`` are used to enumerate the audio inputs
+of a V4L device. The equivalent V4L2 ioctls are
+:ref:`VIDIOC_G_AUDIO <VIDIOC_G_AUDIO>` and
+:ref:`VIDIOC_S_AUDIO <VIDIOC_G_AUDIO>` using struct
+:c:type:`v4l2_audio` as discussed in :ref:`audio`.
+
+The ``audio`` "channel number" field counting audio inputs was renamed
+to ``index``.
+
+On ``VIDIOCSAUDIO`` the ``mode`` field selects *one* of the
+``VIDEO_SOUND_MONO``, ``VIDEO_SOUND_STEREO``, ``VIDEO_SOUND_LANG1`` or
+``VIDEO_SOUND_LANG2`` audio demodulation modes. When the current audio
+standard is BTSC ``VIDEO_SOUND_LANG2`` refers to SAP and
+``VIDEO_SOUND_LANG1`` is meaningless. Also undocumented in the V4L
+specification, there is no way to query the selected mode. On
+``VIDIOCGAUDIO`` the driver returns the *actually received* audio
+programmes in this field. In the V4L2 API this information is stored in
+the struct :c:type:`v4l2_tuner` ``rxsubchans`` and
+``audmode`` fields, respectively. See :ref:`tuner` for more
+information on tuners. Related to audio modes struct
+:c:type:`v4l2_audio` also reports if this is a mono or
+stereo input, regardless if the source is a tuner.
+
+The following fields where replaced by V4L2 controls accessible with the
+:ref:`VIDIOC_QUERYCTRL`,
+:ref:`VIDIOC_G_CTRL <VIDIOC_G_CTRL>` and
+:ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` ioctls:
+
+
+.. flat-table::
+    :header-rows:  1
+    :stub-columns: 0
+
+    * - struct ``video_audio``
+      - V4L2 Control ID
+    * - ``volume``
+      - ``V4L2_CID_AUDIO_VOLUME``
+    * - ``bass``
+      - ``V4L2_CID_AUDIO_BASS``
+    * - ``treble``
+      - ``V4L2_CID_AUDIO_TREBLE``
+    * - ``balance``
+      - ``V4L2_CID_AUDIO_BALANCE``
+
+To determine which of these controls are supported by a driver V4L
+provides the ``flags`` ``VIDEO_AUDIO_VOLUME``, ``VIDEO_AUDIO_BASS``,
+``VIDEO_AUDIO_TREBLE`` and ``VIDEO_AUDIO_BALANCE``. In the V4L2 API the
+:ref:`VIDIOC_QUERYCTRL` ioctl reports if the
+respective control is supported. Accordingly the ``VIDEO_AUDIO_MUTABLE``
+and ``VIDEO_AUDIO_MUTE`` flags where replaced by the boolean
+``V4L2_CID_AUDIO_MUTE`` control.
+
+All V4L2 controls have a ``step`` attribute replacing the struct
+``video_audio`` ``step`` field. The V4L audio controls
+are assumed to range from 0 to 65535 with no particular reset value. The
+V4L2 API permits arbitrary limits and defaults which can be queried with
+the :ref:`VIDIOC_QUERYCTRL` ioctl. For general
+information about controls see :ref:`control`.
+
+Frame Buffer Overlay
+====================
+
+The V4L2 ioctls equivalent to ``VIDIOCGFBUF`` and ``VIDIOCSFBUF`` are
+:ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` and
+:ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>`. The ``base`` field of struct
+``video_buffer`` remained unchanged, except V4L2 defines
+a flag to indicate non-destructive overlays instead of a ``NULL``
+pointer. All other fields moved into the struct
+:c:type:`v4l2_pix_format` ``fmt`` substructure of
+struct :c:type:`v4l2_framebuffer`. The ``depth``
+field was replaced by ``pixelformat``. See :ref:`pixfmt-rgb` for a
+list of RGB formats and their respective color depths.
+
+Instead of the special ioctls ``VIDIOCGWIN`` and ``VIDIOCSWIN`` V4L2
+uses the general-purpose data format negotiation ioctls
+:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and
+:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`. They take a pointer to a struct
+:c:type:`v4l2_format` as argument. Here the ``win`` member
+of the ``fmt`` union is used, a struct
+:c:type:`v4l2_window`.
+
+The ``x``, ``y``, ``width`` and ``height`` fields of struct
+``video_window`` moved into struct
+:c:type:`v4l2_rect` substructure ``w`` of struct
+:c:type:`v4l2_window`. The ``chromakey``, ``clips``, and
+``clipcount`` fields remained unchanged. Struct
+``video_clip`` was renamed to struct
+:c:type:`v4l2_clip`, also containing a struct
+:c:type:`v4l2_rect`, but the semantics are still the same.
+
+The ``VIDEO_WINDOW_INTERLACE`` flag was dropped. Instead applications
+must set the ``field`` field to ``V4L2_FIELD_ANY`` or
+``V4L2_FIELD_INTERLACED``. The ``VIDEO_WINDOW_CHROMAKEY`` flag moved
+into struct :c:type:`v4l2_framebuffer`, under the new
+name ``V4L2_FBUF_FLAG_CHROMAKEY``.
+
+In V4L, storing a bitmap pointer in ``clips`` and setting ``clipcount``
+to ``VIDEO_CLIP_BITMAP`` (-1) requests bitmap clipping, using a fixed
+size bitmap of 1024 × 625 bits. Struct :c:type:`v4l2_window`
+has a separate ``bitmap`` pointer field for this purpose and the bitmap
+size is determined by ``w.width`` and ``w.height``.
+
+The ``VIDIOCCAPTURE`` ioctl to enable or disable overlay was renamed to
+:ref:`VIDIOC_OVERLAY`.
+
+Cropping
+========
+
+To capture only a subsection of the full picture V4L defines the
+``VIDIOCGCAPTURE`` and ``VIDIOCSCAPTURE`` ioctls using struct
+``video_capture``. The equivalent V4L2 ioctls are
+:ref:`VIDIOC_G_CROP <VIDIOC_G_CROP>` and
+:ref:`VIDIOC_S_CROP <VIDIOC_G_CROP>` using struct
+:c:type:`v4l2_crop`, and the related
+:ref:`VIDIOC_CROPCAP` ioctl. This is a rather
+complex matter, see :ref:`crop` for details.
+
+The ``x``, ``y``, ``width`` and ``height`` fields moved into struct
+:c:type:`v4l2_rect` substructure ``c`` of struct
+:c:type:`v4l2_crop`. The ``decimation`` field was dropped. In
+the V4L2 API the scaling factor is implied by the size of the cropping
+rectangle and the size of the captured or overlaid image.
+
+The ``VIDEO_CAPTURE_ODD`` and ``VIDEO_CAPTURE_EVEN`` flags to capture
+only the odd or even field, respectively, were replaced by
+``V4L2_FIELD_TOP`` and ``V4L2_FIELD_BOTTOM`` in the field named
+``field`` of struct :c:type:`v4l2_pix_format` and
+struct :c:type:`v4l2_window`. These structures are used to
+select a capture or overlay format with the
+:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
+
+Reading Images, Memory Mapping
+==============================
+
+Capturing using the read method
+-------------------------------
+
+There is no essential difference between reading images from a V4L or
+V4L2 device using the :c:func:`read()` function, however V4L2
+drivers are not required to support this I/O method. Applications can
+determine if the function is available with the
+:ref:`VIDIOC_QUERYCAP` ioctl. All V4L2 devices
+exchanging data with applications must support the
+:c:func:`select()` and :c:func:`poll()`
+functions.
+
+To select an image format and size, V4L provides the ``VIDIOCSPICT`` and
+``VIDIOCSWIN`` ioctls. V4L2 uses the general-purpose data format
+negotiation ioctls :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and
+:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`. They take a pointer to a struct
+:c:type:`v4l2_format` as argument, here the struct
+:c:type:`v4l2_pix_format` named ``pix`` of its
+``fmt`` union is used.
+
+For more information about the V4L2 read interface see :ref:`rw`.
+
+Capturing using memory mapping
+------------------------------
+
+Applications can read from V4L devices by mapping buffers in device
+memory, or more often just buffers allocated in DMA-able system memory,
+into their address space. This avoids the data copying overhead of the
+read method. V4L2 supports memory mapping as well, with a few
+differences.
+
+
+.. flat-table::
+    :header-rows:  1
+    :stub-columns: 0
+
+    * - V4L
+      - V4L2
+    * -
+      - The image format must be selected before buffers are allocated,
+	with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl. When no
+	format is selected the driver may use the last, possibly by
+	another application requested format.
+    * - Applications cannot change the number of buffers. The it is built
+	into the driver, unless it has a module option to change the
+	number when the driver module is loaded.
+      - The :ref:`VIDIOC_REQBUFS` ioctl allocates the
+	desired number of buffers, this is a required step in the
+	initialization sequence.
+    * - Drivers map all buffers as one contiguous range of memory. The
+	``VIDIOCGMBUF`` ioctl is available to query the number of buffers,
+	the offset of each buffer from the start of the virtual file, and
+	the overall amount of memory used, which can be used as arguments
+	for the :c:func:`mmap()` function.
+      - Buffers are individually mapped. The offset and size of each
+	buffer can be determined with the
+	:ref:`VIDIOC_QUERYBUF` ioctl.
+    * - The ``VIDIOCMCAPTURE`` ioctl prepares a buffer for capturing. It
+	also determines the image format for this buffer. The ioctl
+	returns immediately, eventually with an ``EAGAIN`` error code if no
+	video signal had been detected. When the driver supports more than
+	one buffer applications can call the ioctl multiple times and thus
+	have multiple outstanding capture requests.
+
+	The ``VIDIOCSYNC`` ioctl suspends execution until a particular
+	buffer has been filled.
+      - Drivers maintain an incoming and outgoing queue.
+	:ref:`VIDIOC_QBUF` enqueues any empty buffer into
+	the incoming queue. Filled buffers are dequeued from the outgoing
+	queue with the :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. To wait
+	until filled buffers become available this function,
+	:c:func:`select()` or :c:func:`poll()` can
+	be used. The :ref:`VIDIOC_STREAMON` ioctl
+	must be called once after enqueuing one or more buffers to start
+	capturing. Its counterpart
+	:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` stops capturing and
+	dequeues all buffers from both queues. Applications can query the
+	signal status, if known, with the
+	:ref:`VIDIOC_ENUMINPUT` ioctl.
+
+For a more in-depth discussion of memory mapping and examples, see
+:ref:`mmap`.
+
+Reading Raw VBI Data
+====================
+
+Originally the V4L API did not specify a raw VBI capture interface, only
+the device file ``/dev/vbi`` was reserved for this purpose. The only
+driver supporting this interface was the BTTV driver, de-facto defining
+the V4L VBI interface. Reading from the device yields a raw VBI image
+with the following parameters:
+
+
+.. flat-table::
+    :header-rows:  1
+    :stub-columns: 0
+
+    * - struct :c:type:`v4l2_vbi_format`
+      - V4L, BTTV driver
+    * - sampling_rate
+      - 28636363 Hz NTSC (or any other 525-line standard); 35468950 Hz PAL
+	and SECAM (625-line standards)
+    * - offset
+      - ?
+    * - samples_per_line
+      - 2048
+    * - sample_format
+      - V4L2_PIX_FMT_GREY. The last four bytes (a machine endianness
+	integer) contain a frame counter.
+    * - start[]
+      - 10, 273 NTSC; 22, 335 PAL and SECAM
+    * - count[]
+      - 16, 16 [#f9]_
+    * - flags
+      - 0
+
+Undocumented in the V4L specification, in Linux 2.3 the
+``VIDIOCGVBIFMT`` and ``VIDIOCSVBIFMT`` ioctls using struct
+``vbi_format`` were added to determine the VBI image
+parameters. These ioctls are only partially compatible with the V4L2 VBI
+interface specified in :ref:`raw-vbi`.
+
+An ``offset`` field does not exist, ``sample_format`` is supposed to be
+``VIDEO_PALETTE_RAW``, equivalent to ``V4L2_PIX_FMT_GREY``. The
+remaining fields are probably equivalent to struct
+:c:type:`v4l2_vbi_format`.
+
+Apparently only the Zoran (ZR 36120) driver implements these ioctls. The
+semantics differ from those specified for V4L2 in two ways. The
+parameters are reset on :c:func:`open()` and
+``VIDIOCSVBIFMT`` always returns an ``EINVAL`` error code if the parameters
+are invalid.
+
+Miscellaneous
+=============
+
+V4L2 has no equivalent of the ``VIDIOCGUNIT`` ioctl. Applications can
+find the VBI device associated with a video capture device (or vice
+versa) by reopening the device and requesting VBI data. For details see
+:ref:`open`.
+
+No replacement exists for ``VIDIOCKEY``, and the V4L functions for
+microcode programming. A new interface for MPEG compression and playback
+devices is documented in :ref:`extended-controls`.
+
+.. [#f1]
+   According to Documentation/admin-guide/devices.rst these should be symbolic links
+   to ``/dev/video0``. Note the original bttv interface is not
+   compatible with V4L or V4L2.
+
+.. [#f2]
+   According to ``Documentation/admin-guide/devices.rst`` a symbolic link to
+   ``/dev/radio0``.
+
+.. [#f3]
+   This is a custom format used by the BTTV driver, not one of the V4L2
+   standard formats.
+
+.. [#f4]
+   Presumably all V4L RGB formats are little-endian, although some
+   drivers might interpret them according to machine endianness. V4L2
+   defines little-endian, big-endian and red/blue swapped variants. For
+   details see :ref:`pixfmt-rgb`.
+
+.. [#f5]
+   ``VIDEO_PALETTE_YUV422`` and ``VIDEO_PALETTE_YUYV`` are the same
+   formats. Some V4L drivers respond to one, some to the other.
+
+.. [#f6]
+   Not to be confused with ``V4L2_PIX_FMT_YUV411P``, which is a planar
+   format.
+
+.. [#f7]
+   V4L explains this as: "RAW capture (BT848)"
+
+.. [#f8]
+   Not to be confused with ``V4L2_PIX_FMT_Y41P``, which is a packed
+   format.
+
+.. [#f9]
+   Old driver versions used different values, eventually the custom
+   ``BTTV_VBISIZE`` ioctl was added to query the correct values.