Adding upstream version 4.19.249.upstream/4.19.249

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

1 files changed, 299 insertions, 0 deletions

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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _raw-vbi:
+
+**********************
+Raw VBI Data Interface
+**********************
+
+VBI is an abbreviation of Vertical Blanking Interval, a gap in the
+sequence of lines of an analog video signal. During VBI no picture
+information is transmitted, allowing some time while the electron beam
+of a cathode ray tube TV returns to the top of the screen. Using an
+oscilloscope you will find here the vertical synchronization pulses and
+short data packages ASK modulated [#f1]_ onto the video signal. These are
+transmissions of services such as Teletext or Closed Caption.
+
+Subject of this interface type is raw VBI data, as sampled off a video
+signal, or to be added to a signal for output. The data format is
+similar to uncompressed video images, a number of lines times a number
+of samples per line, we call this a VBI image.
+
+Conventionally V4L2 VBI devices are accessed through character device
+special files named ``/dev/vbi`` and ``/dev/vbi0`` to ``/dev/vbi31``
+with major number 81 and minor numbers 224 to 255. ``/dev/vbi`` is
+typically a symbolic link to the preferred VBI device. This convention
+applies to both input and output devices.
+
+To address the problems of finding related video and VBI devices VBI
+capturing and output is also available as device function under
+``/dev/video``. To capture or output raw VBI data with these devices
+applications must call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
+Accessed as ``/dev/vbi``, raw VBI capturing or output is the default
+device function.
+
+
+Querying Capabilities
+=====================
+
+Devices supporting the raw VBI capturing or output API set the
+``V4L2_CAP_VBI_CAPTURE`` or ``V4L2_CAP_VBI_OUTPUT`` flags, respectively,
+in the ``capabilities`` field of struct
+:c:type:`v4l2_capability` returned by the
+:ref:`VIDIOC_QUERYCAP` ioctl. At least one of the
+read/write, streaming or asynchronous I/O methods must be supported. VBI
+devices may or may not have a tuner or modulator.
+
+
+Supplemental Functions
+======================
+
+VBI devices shall support :ref:`video input or output <video>`,
+:ref:`tuner or modulator <tuner>`, and :ref:`controls <control>`
+ioctls as needed. The :ref:`video standard <standard>` ioctls provide
+information vital to program a VBI device, therefore must be supported.
+
+
+Raw VBI Format Negotiation
+==========================
+
+Raw VBI sampling abilities can vary, in particular the sampling
+frequency. To properly interpret the data V4L2 specifies an ioctl to
+query the sampling parameters. Moreover, to allow for some flexibility
+applications can also suggest different parameters.
+
+As usual these parameters are *not* reset at :ref:`open() <func-open>`
+time to permit Unix tool chains, programming a device and then reading
+from it as if it was a plain file. Well written V4L2 applications should
+always ensure they really get what they want, requesting reasonable
+parameters and then checking if the actual parameters are suitable.
+
+To query the current raw VBI capture parameters applications set the
+``type`` field of a struct :c:type:`v4l2_format` to
+``V4L2_BUF_TYPE_VBI_CAPTURE`` or ``V4L2_BUF_TYPE_VBI_OUTPUT``, and call
+the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer to this
+structure. Drivers fill the struct
+:c:type:`v4l2_vbi_format` ``vbi`` member of the
+``fmt`` union.
+
+To request different parameters applications set the ``type`` field of a
+struct :c:type:`v4l2_format` as above and initialize all
+fields of the struct :c:type:`v4l2_vbi_format`
+``vbi`` member of the ``fmt`` union, or better just modify the results
+of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
+ioctl with a pointer to this structure. Drivers return an ``EINVAL`` error
+code only when the given parameters are ambiguous, otherwise they modify
+the parameters according to the hardware capabilities and return the
+actual parameters. When the driver allocates resources at this point, it
+may return an ``EBUSY`` error code to indicate the returned parameters are
+valid but the required resources are currently not available. That may
+happen for instance when the video and VBI areas to capture would
+overlap, or when the driver supports multiple opens and another process
+already requested VBI capturing or output. Anyway, applications must
+expect other resource allocation points which may return ``EBUSY``, at the
+:ref:`VIDIOC_STREAMON` ioctl and the first :ref:`read() <func-read>`
+, :ref:`write() <func-write>` and :ref:`select() <func-select>` calls.
+
+VBI devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and
+:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all requests
+and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does.
+:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional.
+
+.. tabularcolumns:: |p{2.4cm}|p{4.4cm}|p{10.7cm}|
+
+.. c:type:: v4l2_vbi_format
+
+.. cssclass:: longtable
+
+.. flat-table:: struct v4l2_vbi_format
+    :header-rows:  0
+    :stub-columns: 0
+    :widths:       1 1 2
+
+    * - __u32
+      - ``sampling_rate``
+      - Samples per second, i. e. unit 1 Hz.
+    * - __u32
+      - ``offset``
+      - Horizontal offset of the VBI image, relative to the leading edge
+	of the line synchronization pulse and counted in samples: The
+	first sample in the VBI image will be located ``offset`` /
+	``sampling_rate`` seconds following the leading edge. See also
+	:ref:`vbi-hsync`.
+    * - __u32
+      - ``samples_per_line``
+      -
+    * - __u32
+      - ``sample_format``
+      - Defines the sample format as in :ref:`pixfmt`, a
+	four-character-code. [#f2]_ Usually this is ``V4L2_PIX_FMT_GREY``,
+	i. e. each sample consists of 8 bits with lower values oriented
+	towards the black level. Do not assume any other correlation of
+	values with the signal level. For example, the MSB does not
+	necessarily indicate if the signal is 'high' or 'low' because 128
+	may not be the mean value of the signal. Drivers shall not convert
+	the sample format by software.
+    * - __u32
+      - ``start``\ [#f2]_
+      - This is the scanning system line number associated with the first
+	line of the VBI image, of the first and the second field
+	respectively. See :ref:`vbi-525` and :ref:`vbi-625` for valid
+	values. The ``V4L2_VBI_ITU_525_F1_START``,
+	``V4L2_VBI_ITU_525_F2_START``, ``V4L2_VBI_ITU_625_F1_START`` and
+	``V4L2_VBI_ITU_625_F2_START`` defines give the start line numbers
+	for each field for each 525 or 625 line format as a convenience.
+	Don't forget that ITU line numbering starts at 1, not 0. VBI input
+	drivers can return start values 0 if the hardware cannot reliable
+	identify scanning lines, VBI acquisition may not require this
+	information.
+    * - __u32
+      - ``count``\ [#f2]_
+      - The number of lines in the first and second field image,
+	respectively.
+    * - :cspan:`2`
+
+	Drivers should be as flexibility as possible. For example, it may
+	be possible to extend or move the VBI capture window down to the
+	picture area, implementing a 'full field mode' to capture data
+	service transmissions embedded in the picture.
+
+	An application can set the first or second ``count`` value to zero
+	if no data is required from the respective field; ``count``\ [1]
+	if the scanning system is progressive, i. e. not interlaced. The
+	corresponding start value shall be ignored by the application and
+	driver. Anyway, drivers may not support single field capturing and
+	return both count values non-zero.
+
+	Both ``count`` values set to zero, or line numbers are outside the
+	bounds depicted\ [#f4]_, or a field image covering lines of two
+	fields, are invalid and shall not be returned by the driver.
+
+	To initialize the ``start`` and ``count`` fields, applications
+	must first determine the current video standard selection. The
+	:ref:`v4l2_std_id <v4l2-std-id>` or the ``framelines`` field
+	of struct :c:type:`v4l2_standard` can be evaluated
+	for this purpose.
+    * - __u32
+      - ``flags``
+      - See :ref:`vbifmt-flags` below. Currently only drivers set flags,
+	applications must set this field to zero.
+    * - __u32
+      - ``reserved``\ [#f2]_
+      - This array is reserved for future extensions. Drivers and
+	applications must set it to zero.
+
+
+.. tabularcolumns:: |p{4.0cm}|p{1.5cm}|p{12.0cm}|
+
+.. _vbifmt-flags:
+
+.. flat-table:: Raw VBI Format Flags
+    :header-rows:  0
+    :stub-columns: 0
+    :widths:       3 1 4
+
+    * - ``V4L2_VBI_UNSYNC``
+      - 0x0001
+      - This flag indicates hardware which does not properly distinguish
+	between fields. Normally the VBI image stores the first field
+	(lower scanning line numbers) first in memory. This may be a top
+	or bottom field depending on the video standard. When this flag is
+	set the first or second field may be stored first, however the
+	fields are still in correct temporal order with the older field
+	first in memory. [#f3]_
+    * - ``V4L2_VBI_INTERLACED``
+      - 0x0002
+      - By default the two field images will be passed sequentially; all
+	lines of the first field followed by all lines of the second field
+	(compare :ref:`field-order` ``V4L2_FIELD_SEQ_TB`` and
+	``V4L2_FIELD_SEQ_BT``, whether the top or bottom field is first in
+	memory depends on the video standard). When this flag is set, the
+	two fields are interlaced (cf. ``V4L2_FIELD_INTERLACED``). The
+	first line of the first field followed by the first line of the
+	second field, then the two second lines, and so on. Such a layout
+	may be necessary when the hardware has been programmed to capture
+	or output interlaced video images and is unable to separate the
+	fields for VBI capturing at the same time. For simplicity setting
+	this flag implies that both ``count`` values are equal and
+	non-zero.
+
+
+
+.. _vbi-hsync:
+
+.. kernel-figure:: vbi_hsync.svg
+    :alt:   vbi_hsync.svg
+    :align: center
+
+    **Figure 4.1. Line synchronization**
+
+
+.. _vbi-525:
+
+.. kernel-figure:: vbi_525.svg
+    :alt:   vbi_525.svg
+    :align: center
+
+    **Figure 4.2. ITU-R 525 line numbering (M/NTSC and M/PAL)**
+
+.. _vbi-625:
+
+.. kernel-figure:: vbi_625.svg
+    :alt:   vbi_625.svg
+    :align: center
+
+    **Figure 4.3. ITU-R 625 line numbering**
+
+Remember the VBI image format depends on the selected video standard,
+therefore the application must choose a new standard or query the
+current standard first. Attempts to read or write data ahead of format
+negotiation, or after switching the video standard which may invalidate
+the negotiated VBI parameters, should be refused by the driver. A format
+change during active I/O is not permitted.
+
+
+Reading and writing VBI images
+==============================
+
+To assure synchronization with the field number and easier
+implementation, the smallest unit of data passed at a time is one frame,
+consisting of two fields of VBI images immediately following in memory.
+
+The total size of a frame computes as follows:
+
+
+.. code-block:: c
+
+    (count[0] + count[1]) * samples_per_line * sample size in bytes
+
+The sample size is most likely always one byte, applications must check
+the ``sample_format`` field though, to function properly with other
+drivers.
+
+A VBI device may support :ref:`read/write <rw>` and/or streaming
+(:ref:`memory mapping <mmap>` or :ref:`user pointer <userp>`) I/O.
+The latter bears the possibility of synchronizing video and VBI data by
+using buffer timestamps.
+
+Remember the :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` ioctl and the
+first :ref:`read() <func-read>`, :ref:`write() <func-write>` and
+:ref:`select() <func-select>` call can be resource allocation
+points returning an ``EBUSY`` error code if the required hardware resources
+are temporarily unavailable, for example the device is already in use by
+another process.
+
+.. [#f1]
+   ASK: Amplitude-Shift Keying. A high signal level represents a '1'
+   bit, a low level a '0' bit.
+
+.. [#f2]
+   A few devices may be unable to sample VBI data at all but can extend
+   the video capture window to the VBI region.
+
+.. [#f3]
+   Most VBI services transmit on both fields, but some have different
+   semantics depending on the field number. These cannot be reliable
+   decoded or encoded when ``V4L2_VBI_UNSYNC`` is set.
+
+.. [#f4]
+   The valid values ar shown at :ref:`vbi-525` and :ref:`vbi-625`.