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![Mauro Carvalho Chehab](/assets/img/avatar_default.png)
A section talking about demod statistics implementation was recently added, but it seemed to start from nowere, without a previous description about how a demod driver would look like. Add such description, for completeness. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
597 lines
21 KiB
ReStructuredText
597 lines
21 KiB
ReStructuredText
Digital TV (DVB) devices
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------------------------
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Digital TV devices are implemented by several different drivers:
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- A bridge driver that is responsible to talk with the bus where the other
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devices are connected (PCI, USB, SPI), bind to the other drivers and
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implement the digital demux logic (either in software or in hardware);
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- Frontend drivers that are usually implemented as two separate drivers:
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- A tuner driver that implements the logic with commands the part of the
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hardware with is reponsible to tune into a digital TV transponder or
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physical channel. The output of a tuner is usually a baseband or
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Intermediate Frequency (IF) signal;
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- A demodulator driver (a.k.a "demod") that implements the logic with
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commands the digital TV decoding hardware. The output of a demod is
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a digital stream, with multiple audio, video and data channels typically
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multiplexed using MPEG Transport Stream [#f1]_.
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On most hardware, the frontend drivers talk with the bridge driver using an
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I2C bus.
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.. [#f1] Some standards use TCP/IP for multiplexing data, like DVB-H (an
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abandoned standard, not used anymore) and ATSC version 3.0 current
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proposals. Currently, the DVB subsystem doesn't implement those standards.
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Digital TV Common functions
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---------------------------
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.. kernel-doc:: drivers/media/dvb-core/dvb_math.h
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.. kernel-doc:: drivers/media/dvb-core/dvbdev.h
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Digital TV Ring buffer
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----------------------
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Those routines implement ring buffers used to handle digital TV data and
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copy it from/to userspace.
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.. note::
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1) For performance reasons read and write routines don't check buffer sizes
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and/or number of bytes free/available. This has to be done before these
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routines are called. For example:
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.. code-block:: c
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/* write @buflen: bytes */
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free = dvb_ringbuffer_free(rbuf);
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if (free >= buflen)
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count = dvb_ringbuffer_write(rbuf, buffer, buflen);
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else
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/* do something */
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/* read min. 1000, max. @bufsize: bytes */
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avail = dvb_ringbuffer_avail(rbuf);
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if (avail >= 1000)
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count = dvb_ringbuffer_read(rbuf, buffer, min(avail, bufsize));
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else
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/* do something */
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2) If there is exactly one reader and one writer, there is no need
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to lock read or write operations.
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Two or more readers must be locked against each other.
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Flushing the buffer counts as a read operation.
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Resetting the buffer counts as a read and write operation.
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Two or more writers must be locked against each other.
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.. kernel-doc:: drivers/media/dvb-core/dvb_ringbuffer.h
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Digital TV Frontend kABI
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------------------------
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Digital TV Frontend
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~~~~~~~~~~~~~~~~~~~
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The Digital TV Frontend kABI defines a driver-internal interface for
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registering low-level, hardware specific driver to a hardware independent
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frontend layer. It is only of interest for Digital TV device driver writers.
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The header file for this API is named ``dvb_frontend.h`` and located in
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``drivers/media/dvb-core``.
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Demodulator driver
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^^^^^^^^^^^^^^^^^^
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The demodulator driver is responsible to talk with the decoding part of the
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hardware. Such driver should implement :c:type:`dvb_frontend_ops`, with
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tells what type of digital TV standards are supported, and points to a
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series of functions that allow the DVB core to command the hardware via
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the code under ``drivers/media/dvb-core/dvb_frontend.c``.
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A typical example of such struct in a driver ``foo`` is::
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static struct dvb_frontend_ops foo_ops = {
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.delsys = { SYS_DVBT, SYS_DVBT2, SYS_DVBC_ANNEX_A },
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.info = {
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.name = "foo DVB-T/T2/C driver",
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.caps = FE_CAN_FEC_1_2 |
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FE_CAN_FEC_2_3 |
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FE_CAN_FEC_3_4 |
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FE_CAN_FEC_5_6 |
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FE_CAN_FEC_7_8 |
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FE_CAN_FEC_AUTO |
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FE_CAN_QPSK |
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FE_CAN_QAM_16 |
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FE_CAN_QAM_32 |
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FE_CAN_QAM_64 |
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FE_CAN_QAM_128 |
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FE_CAN_QAM_256 |
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FE_CAN_QAM_AUTO |
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FE_CAN_TRANSMISSION_MODE_AUTO |
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FE_CAN_GUARD_INTERVAL_AUTO |
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FE_CAN_HIERARCHY_AUTO |
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FE_CAN_MUTE_TS |
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FE_CAN_2G_MODULATION,
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.frequency_min = 42000000, /* Hz */
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.frequency_max = 1002000000, /* Hz */
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.symbol_rate_min = 870000,
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.symbol_rate_max = 11700000
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},
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.init = foo_init,
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.sleep = foo_sleep,
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.release = foo_release,
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.set_frontend = foo_set_frontend,
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.get_frontend = foo_get_frontend,
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.read_status = foo_get_status_and_stats,
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.tune = foo_tune,
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.i2c_gate_ctrl = foo_i2c_gate_ctrl,
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.get_frontend_algo = foo_get_algo,
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};
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A typical example of such struct in a driver ``bar`` meant to be used on
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Satellite TV reception is::
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static const struct dvb_frontend_ops bar_ops = {
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.delsys = { SYS_DVBS, SYS_DVBS2 },
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.info = {
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.name = "Bar DVB-S/S2 demodulator",
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.frequency_min = 500000, /* KHz */
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.frequency_max = 2500000, /* KHz */
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.frequency_stepsize = 0,
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.symbol_rate_min = 1000000,
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.symbol_rate_max = 45000000,
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.symbol_rate_tolerance = 500,
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.caps = FE_CAN_INVERSION_AUTO |
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FE_CAN_FEC_AUTO |
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FE_CAN_QPSK,
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},
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.init = bar_init,
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.sleep = bar_sleep,
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.release = bar_release,
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.set_frontend = bar_set_frontend,
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.get_frontend = bar_get_frontend,
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.read_status = bar_get_status_and_stats,
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.i2c_gate_ctrl = bar_i2c_gate_ctrl,
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.get_frontend_algo = bar_get_algo,
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.tune = bar_tune,
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/* Satellite-specific */
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.diseqc_send_master_cmd = bar_send_diseqc_msg,
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.diseqc_send_burst = bar_send_burst,
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.set_tone = bar_set_tone,
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.set_voltage = bar_set_voltage,
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};
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.. note::
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#) For satellite digital TV standards (DVB-S, DVB-S2, ISDB-S), the
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frequencies are specified in kHz, while, for terrestrial and cable
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standards, they're specified in Hz. Due to that, if the same frontend
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supports both types, you'll need to have two separate
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:c:type:`dvb_frontend_ops` structures, one for each standard.
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#) The ``.i2c_gate_ctrl`` field is present only when the hardware has
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allows controlling an I2C gate (either directly of via some GPIO pin),
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in order to remove the tuner from the I2C bus after a channel is
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tuned.
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#) All new drivers should implement the
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:ref:`DVBv5 statistics <dvbv5_stats>` via ``.read_status``.
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Yet, there are a number of callbacks meant to get statistics for
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signal strength, S/N and UCB. Those are there to provide backward
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compatibility with legacy applications that don't support the DVBv5
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API. Implementing those callbacks are optional. Those callbacks may be
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removed in the future, after we have all existing drivers supporting
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DVBv5 stats.
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#) Other callbacks are required for satellite TV standards, in order to
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control LNBf and DiSEqC: ``.diseqc_send_master_cmd``,
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``.diseqc_send_burst``, ``.set_tone``, ``.set_voltage``.
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.. |delta| unicode:: U+00394
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The ``drivers/media/dvb-core/dvb_frontend.c`` has a kernel thread with is
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responsible for tuning the device. It supports multiple algoritms to
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detect a channel, as defined at enum :c:func:`dvbfe_algo`.
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The algorithm to be used is obtained via ``.get_frontend_algo``. If the driver
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doesn't fill its field at struct :c:type:`dvb_frontend_ops`, it will default to
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``DVBFE_ALGO_SW``, meaning that the dvb-core will do a zigzag when tuning,
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e. g. it will try first to use the specified center frequency ``f``,
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then, it will do ``f`` + |delta|, ``f`` - |delta|, ``f`` + 2 x |delta|,
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``f`` - 2 x |delta| and so on.
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If the hardware has internally a some sort of zigzag algorithm, you should
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define a ``.get_frontend_algo`` function that would return ``DVBFE_ALGO_HW``.
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.. note::
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The core frontend support also supports
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a third type (``DVBFE_ALGO_CUSTOM``), in order to allow the driver to
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define its own hardware-assisted algorithm. Very few hardware need to
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use it nowadays. Using ``DVBFE_ALGO_CUSTOM`` require to provide other
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function callbacks at struct :c:type:`dvb_frontend_ops`.
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Attaching frontend driver to the bridge driver
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Before using the Digital TV frontend core, the bridge driver should attach
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the frontend demod, tuner and SEC devices and call
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:c:func:`dvb_register_frontend()`,
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in order to register the new frontend at the subsystem. At device
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detach/removal, the bridge driver should call
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:c:func:`dvb_unregister_frontend()` to
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remove the frontend from the core and then :c:func:`dvb_frontend_detach()`
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to free the memory allocated by the frontend drivers.
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The drivers should also call :c:func:`dvb_frontend_suspend()` as part of
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their handler for the :c:type:`device_driver`.\ ``suspend()``, and
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:c:func:`dvb_frontend_resume()` as
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part of their handler for :c:type:`device_driver`.\ ``resume()``.
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A few other optional functions are provided to handle some special cases.
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.. _dvbv5_stats:
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Digital TV Frontend statistics
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Introduction
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^^^^^^^^^^^^
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Digital TV frontends provide a range of
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:ref:`statistics <frontend-stat-properties>` meant to help tuning the device
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and measuring the quality of service.
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For each statistics measurement, the driver should set the type of scale used,
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or ``FE_SCALE_NOT_AVAILABLE`` if the statistics is not available on a given
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time. Drivers should also provide the number of statistics for each type.
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that's usually 1 for most video standards [#f2]_.
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Drivers should initialize each statistic counters with length and
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scale at its init code. For example, if the frontend provides signal
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strength, it should have, on its init code::
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struct dtv_frontend_properties *c = &state->fe.dtv_property_cache;
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c->strength.len = 1;
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c->strength.stat[0].scale = FE_SCALE_NOT_AVAILABLE;
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And, when the statistics got updated, set the scale::
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c->strength.stat[0].scale = FE_SCALE_DECIBEL;
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c->strength.stat[0].uvalue = strength;
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.. [#f2] For ISDB-T, it may provide both a global statistics and a per-layer
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set of statistics. On such cases, len should be equal to 4. The first
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value corresponds to the global stat; the other ones to each layer, e. g.:
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- c->cnr.stat[0] for global S/N carrier ratio,
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- c->cnr.stat[1] for Layer A S/N carrier ratio,
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- c->cnr.stat[2] for layer B S/N carrier ratio,
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- c->cnr.stat[3] for layer C S/N carrier ratio.
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.. note:: Please prefer to use ``FE_SCALE_DECIBEL`` instead of
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``FE_SCALE_RELATIVE`` for signal strength and CNR measurements.
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Groups of statistics
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^^^^^^^^^^^^^^^^^^^^
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There are several groups of statistics currently supported:
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Signal strength (:ref:`DTV-STAT-SIGNAL-STRENGTH`)
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- Measures the signal strength level at the analog part of the tuner or
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demod.
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- Typically obtained from the gain applied to the tuner and/or frontend
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in order to detect the carrier. When no carrier is detected, the gain is
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at the maximum value (so, strength is on its minimal).
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- As the gain is visible through the set of registers that adjust the gain,
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typically, this statistics is always available [#f3]_.
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- Drivers should try to make it available all the times, as this statistics
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can be used when adjusting an antenna position and to check for troubles
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at the cabling.
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.. [#f3] On a few devices, the gain keeps floating if no carrier.
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On such devices, strength report should check first if carrier is
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detected at the tuner (``FE_HAS_CARRIER``, see :c:type:`fe_status`),
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and otherwise return the lowest possible value.
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Carrier Signal to Noise ratio (:ref:`DTV-STAT-CNR`)
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- Signal to Noise ratio for the main carrier.
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- Signal to Noise measurement depends on the device. On some hardware, is
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available when the main carrier is detected. On those hardware, CNR
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measurement usually comes from the tuner (e. g. after ``FE_HAS_CARRIER``,
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see :c:type:`fe_status`).
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On other devices, it requires inner FEC decoding,
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as the frontend measures it indirectly from other parameters (e. g. after
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``FE_HAS_VITERBI``, see :c:type:`fe_status`).
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Having it available after inner FEC is more common.
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Bit counts post-FEC (:ref:`DTV-STAT-POST-ERROR-BIT-COUNT` and :ref:`DTV-STAT-POST-TOTAL-BIT-COUNT`)
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- Those counters measure the number of bits and bit errors errors after
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the forward error correction (FEC) on the inner coding block
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(after Viterbi, LDPC or other inner code).
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- Due to its nature, those statistics depend on full coding lock
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(e. g. after ``FE_HAS_SYNC`` or after ``FE_HAS_LOCK``,
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see :c:type:`fe_status`).
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Bit counts pre-FEC (:ref:`DTV-STAT-PRE-ERROR-BIT-COUNT` and :ref:`DTV-STAT-PRE-TOTAL-BIT-COUNT`)
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- Those counters measure the number of bits and bit errors errors before
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the forward error correction (FEC) on the inner coding block
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(before Viterbi, LDPC or other inner code).
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- Not all frontends provide this kind of statistics.
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- Due to its nature, those statistics depend on inner coding lock (e. g.
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after ``FE_HAS_VITERBI``, see :c:type:`fe_status`).
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Block counts (:ref:`DTV-STAT-ERROR-BLOCK-COUNT` and :ref:`DTV-STAT-TOTAL-BLOCK-COUNT`)
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- Those counters measure the number of blocks and block errors errors after
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the forward error correction (FEC) on the inner coding block
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(before Viterbi, LDPC or other inner code).
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- Due to its nature, those statistics depend on full coding lock
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(e. g. after ``FE_HAS_SYNC`` or after
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``FE_HAS_LOCK``, see :c:type:`fe_status`).
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.. note:: All counters should be monotonically increased as they're
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collected from the hardware.
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A typical example of the logic that handle status and statistics is::
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static int foo_get_status_and_stats(struct dvb_frontend *fe)
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{
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struct foo_state *state = fe->demodulator_priv;
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struct dtv_frontend_properties *c = &fe->dtv_property_cache;
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int rc;
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enum fe_status *status;
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/* Both status and strength are always available */
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rc = foo_read_status(fe, &status);
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if (rc < 0)
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return rc;
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rc = foo_read_strength(fe);
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if (rc < 0)
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return rc;
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/* Check if CNR is available */
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if (!(fe->status & FE_HAS_CARRIER))
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return 0;
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rc = foo_read_cnr(fe);
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if (rc < 0)
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return rc;
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/* Check if pre-BER stats are available */
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if (!(fe->status & FE_HAS_VITERBI))
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return 0;
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rc = foo_get_pre_ber(fe);
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if (rc < 0)
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return rc;
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/* Check if post-BER stats are available */
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if (!(fe->status & FE_HAS_SYNC))
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return 0;
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rc = foo_get_post_ber(fe);
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if (rc < 0)
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return rc;
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}
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static const struct dvb_frontend_ops ops = {
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/* ... */
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.read_status = foo_get_status_and_stats,
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};
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Statistics collect
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^^^^^^^^^^^^^^^^^^
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On almost all frontend hardware, the bit and byte counts are stored by
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the hardware after a certain amount of time or after the total bit/block
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counter reaches a certain value (usually programable), for example, on
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every 1000 ms or after receiving 1,000,000 bits.
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So, if you read the registers too soon, you'll end by reading the same
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value as in the previous reading, causing the monotonic value to be
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incremented too often.
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Drivers should take the responsibility to avoid too often reads. That
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can be done using two approaches:
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if the driver have a bit that indicates when a collected data is ready
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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Driver should check such bit before making the statistics available.
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An example of such behavior can be found at this code snippet (adapted
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from mb86a20s driver's logic)::
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static int foo_get_pre_ber(struct dvb_frontend *fe)
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{
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struct foo_state *state = fe->demodulator_priv;
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struct dtv_frontend_properties *c = &fe->dtv_property_cache;
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int rc, bit_error;
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/* Check if the BER measures are already available */
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rc = foo_read_u8(state, 0x54);
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if (rc < 0)
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return rc;
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if (!rc)
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return 0;
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/* Read Bit Error Count */
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bit_error = foo_read_u32(state, 0x55);
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if (bit_error < 0)
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return bit_error;
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/* Read Total Bit Count */
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rc = foo_read_u32(state, 0x51);
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if (rc < 0)
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return rc;
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c->pre_bit_error.stat[0].scale = FE_SCALE_COUNTER;
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c->pre_bit_error.stat[0].uvalue += bit_error;
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c->pre_bit_count.stat[0].scale = FE_SCALE_COUNTER;
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c->pre_bit_count.stat[0].uvalue += rc;
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return 0;
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}
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If the driver doesn't provide a statistics available check bit
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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A few devices, however, may not provide a way to check if the stats are
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available (or the way to check it is unknown). They may not even provide
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a way to directly read the total number of bits or blocks.
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On those devices, the driver need to ensure that it won't be reading from
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the register too often and/or estimate the total number of bits/blocks.
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On such drivers, a typical routine to get statistics would be like
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(adapted from dib8000 driver's logic)::
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struct foo_state {
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/* ... */
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unsigned long per_jiffies_stats;
|
|
}
|
|
|
|
static int foo_get_pre_ber(struct dvb_frontend *fe)
|
|
{
|
|
struct foo_state *state = fe->demodulator_priv;
|
|
struct dtv_frontend_properties *c = &fe->dtv_property_cache;
|
|
int rc, bit_error;
|
|
u64 bits;
|
|
|
|
/* Check if time for stats was elapsed */
|
|
if (!time_after(jiffies, state->per_jiffies_stats))
|
|
return 0;
|
|
|
|
/* Next stat should be collected in 1000 ms */
|
|
state->per_jiffies_stats = jiffies + msecs_to_jiffies(1000);
|
|
|
|
/* Read Bit Error Count */
|
|
bit_error = foo_read_u32(state, 0x55);
|
|
if (bit_error < 0)
|
|
return bit_error;
|
|
|
|
/*
|
|
* On this particular frontend, there's no register that
|
|
* would provide the number of bits per 1000ms sample. So,
|
|
* some function would calculate it based on DTV properties
|
|
*/
|
|
bits = get_number_of_bits_per_1000ms(fe);
|
|
|
|
c->pre_bit_error.stat[0].scale = FE_SCALE_COUNTER;
|
|
c->pre_bit_error.stat[0].uvalue += bit_error;
|
|
c->pre_bit_count.stat[0].scale = FE_SCALE_COUNTER;
|
|
c->pre_bit_count.stat[0].uvalue += bits;
|
|
|
|
return 0;
|
|
}
|
|
|
|
Please notice that, on both cases, we're getting the statistics using the
|
|
:c:type:`dvb_frontend_ops` ``.read_status`` callback. The rationale is that
|
|
the frontend core will automatically call this function periodically
|
|
(usually, 3 times per second, when the frontend is locked).
|
|
|
|
That warrants that we won't miss to collect a counter and increment the
|
|
monotonic stats at the right time.
|
|
|
|
Digital TV Frontend functions and types
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. kernel-doc:: drivers/media/dvb-core/dvb_frontend.h
|
|
|
|
|
|
Digital TV Demux kABI
|
|
---------------------
|
|
|
|
Digital TV Demux
|
|
~~~~~~~~~~~~~~~~
|
|
|
|
The Kernel Digital TV Demux kABI defines a driver-internal interface for
|
|
registering low-level, hardware specific driver to a hardware independent
|
|
demux layer. It is only of interest for Digital TV device driver writers.
|
|
The header file for this kABI is named demux.h and located in
|
|
drivers/media/dvb-core.
|
|
|
|
The demux kABI should be implemented for each demux in the system. It is
|
|
used to select the TS source of a demux and to manage the demux resources.
|
|
When the demux client allocates a resource via the demux kABI, it receives
|
|
a pointer to the kABI of that resource.
|
|
|
|
Each demux receives its TS input from a DVB front-end or from memory, as
|
|
set via this demux kABI. In a system with more than one front-end, the kABI
|
|
can be used to select one of the DVB front-ends as a TS source for a demux,
|
|
unless this is fixed in the HW platform.
|
|
|
|
The demux kABI only controls front-ends regarding to their connections with
|
|
demuxes; the kABI used to set the other front-end parameters, such as
|
|
tuning, are devined via the Digital TV Frontend kABI.
|
|
|
|
The functions that implement the abstract interface demux should be defined
|
|
static or module private and registered to the Demux core for external
|
|
access. It is not necessary to implement every function in the struct
|
|
&dmx_demux. For example, a demux interface might support Section filtering,
|
|
but not PES filtering. The kABI client is expected to check the value of any
|
|
function pointer before calling the function: the value of ``NULL`` means
|
|
that the function is not available.
|
|
|
|
Whenever the functions of the demux API modify shared data, the
|
|
possibilities of lost update and race condition problems should be
|
|
addressed, e.g. by protecting parts of code with mutexes.
|
|
|
|
Note that functions called from a bottom half context must not sleep.
|
|
Even a simple memory allocation without using ``GFP_ATOMIC`` can result in a
|
|
kernel thread being put to sleep if swapping is needed. For example, the
|
|
Linux Kernel calls the functions of a network device interface from a
|
|
bottom half context. Thus, if a demux kABI function is called from network
|
|
device code, the function must not sleep.
|
|
|
|
|
|
|
|
Demux Callback API
|
|
------------------
|
|
|
|
Demux Callback
|
|
~~~~~~~~~~~~~~
|
|
|
|
This kernel-space API comprises the callback functions that deliver filtered
|
|
data to the demux client. Unlike the other DVB kABIs, these functions are
|
|
provided by the client and called from the demux code.
|
|
|
|
The function pointers of this abstract interface are not packed into a
|
|
structure as in the other demux APIs, because the callback functions are
|
|
registered and used independent of each other. As an example, it is possible
|
|
for the API client to provide several callback functions for receiving TS
|
|
packets and no callbacks for PES packets or sections.
|
|
|
|
The functions that implement the callback API need not be re-entrant: when
|
|
a demux driver calls one of these functions, the driver is not allowed to
|
|
call the function again before the original call returns. If a callback is
|
|
triggered by a hardware interrupt, it is recommended to use the Linux
|
|
bottom half mechanism or start a tasklet instead of making the callback
|
|
function call directly from a hardware interrupt.
|
|
|
|
This mechanism is implemented by :c:func:`dmx_ts_cb()` and :c:func:`dmx_section_cb()`
|
|
callbacks.
|
|
|
|
.. kernel-doc:: drivers/media/dvb-core/demux.h
|
|
|
|
Digital TV Conditional Access kABI
|
|
----------------------------------
|
|
|
|
.. kernel-doc:: drivers/media/dvb-core/dvb_ca_en50221.h
|