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diff --git a/Documentation/userspace-api/media/v4l/dev-raw-vbi.rst b/Documentation/userspace-api/media/v4l/dev-raw-vbi.rst new file mode 100644 index 000000000..3f43a01ba --- /dev/null +++ b/Documentation/userspace-api/media/v4l/dev-raw-vbi.rst @@ -0,0 +1,292 @@ +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later +.. c:namespace:: V4L + +.. _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 :c: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 :c:func:`read()` +, :c:func:`write()` and :c: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{1.6cm}|p{4.2cm}|p{11.7cm}| + +.. 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{4.4cm}|p{1.5cm}|p{11.6cm}| + +.. _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 :c:func:`read()`, :c:func:`write()` and +:c: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`. |