446 строки
15 KiB
ReStructuredText
446 строки
15 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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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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``include/media/``.
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Demodulator driver
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^^^^^^^^^^^^^^^^^^
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The demodulator driver is responsible for talking with the decoding part of the
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hardware. Such driver should implement :c:type:`dvb_frontend_ops`, which
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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 ``include/media/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 ``include/media/dvb_frontend.c`` has a kernel thread which is
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responsible for tuning the device. It supports multiple algorithms 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 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 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 these 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 there is 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, it 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 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 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 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 collection
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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 programmable), 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;
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}
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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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u64 bits;
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/* Check if time for stats was elapsed */
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if (!time_after(jiffies, state->per_jiffies_stats))
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return 0;
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/* Next stat should be collected in 1000 ms */
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state->per_jiffies_stats = jiffies + msecs_to_jiffies(1000);
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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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/*
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* On this particular frontend, there's no register that
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* would provide the number of bits per 1000ms sample. So,
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* some function would calculate it based on DTV properties
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*/
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bits = get_number_of_bits_per_1000ms(fe);
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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 += bits;
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return 0;
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}
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Please notice that, on both cases, we're getting the statistics using the
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:c:type:`dvb_frontend_ops` ``.read_status`` callback. The rationale is that
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the frontend core will automatically call this function periodically
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(usually, 3 times per second, when the frontend is locked).
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That warrants that we won't miss to collect a counter and increment the
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monotonic stats at the right time.
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Digital TV Frontend functions and types
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: include/media/dvb_frontend.h
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