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diff --git a/Documentation/userspace-api/media/v4l/diff-v4l.rst b/Documentation/userspace-api/media/v4l/diff-v4l.rst new file mode 100644 index 000000000000..37644d26c4ae --- /dev/null +++ b/Documentation/userspace-api/media/v4l/diff-v4l.rst @@ -0,0 +1,693 @@ +.. Permission is granted to copy, distribute and/or modify this +.. document under the terms of the GNU Free Documentation License, +.. Version 1.1 or any later version published by the Free Software +.. Foundation, with no Invariant Sections, no Front-Cover Texts +.. and no Back-Cover Texts. A copy of the license is included at +.. Documentation/userspace-api/media/fdl-appendix.rst. +.. +.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections + +.. _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 :ref:`read() <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 +:ref:`select() <func-select>` and :ref:`poll() <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 :ref:`mmap() <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, + :ref:`select() <func-select>` or :ref:`poll() <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 :ref:`open() <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. |