提交 f91df2e4 编写于 作者: M Mauro Carvalho Chehab

media: dtv-core.rst: complete description of a demod driver

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: NMauro Carvalho Chehab <mchehab@s-opensource.com>
上级 f495ab86
......@@ -80,8 +80,141 @@ Digital TV Frontend
The Digital TV Frontend kABI defines a driver-internal interface for
registering low-level, hardware specific driver to a hardware independent
frontend layer. It is only of interest for Digital TV device driver writers.
The header file for this API is named dvb_frontend.h and located in
drivers/media/dvb-core.
The header file for this API is named ``dvb_frontend.h`` and located in
``drivers/media/dvb-core``.
Demodulator driver
^^^^^^^^^^^^^^^^^^
The demodulator driver is responsible to talk with the decoding part of the
hardware. Such driver should implement :c:type:`dvb_frontend_ops`, with
tells what type of digital TV standards are supported, and points to a
series of functions that allow the DVB core to command the hardware via
the code under ``drivers/media/dvb-core/dvb_frontend.c``.
A typical example of such struct in a driver ``foo`` is::
static struct dvb_frontend_ops foo_ops = {
.delsys = { SYS_DVBT, SYS_DVBT2, SYS_DVBC_ANNEX_A },
.info = {
.name = "foo DVB-T/T2/C driver",
.caps = FE_CAN_FEC_1_2 |
FE_CAN_FEC_2_3 |
FE_CAN_FEC_3_4 |
FE_CAN_FEC_5_6 |
FE_CAN_FEC_7_8 |
FE_CAN_FEC_AUTO |
FE_CAN_QPSK |
FE_CAN_QAM_16 |
FE_CAN_QAM_32 |
FE_CAN_QAM_64 |
FE_CAN_QAM_128 |
FE_CAN_QAM_256 |
FE_CAN_QAM_AUTO |
FE_CAN_TRANSMISSION_MODE_AUTO |
FE_CAN_GUARD_INTERVAL_AUTO |
FE_CAN_HIERARCHY_AUTO |
FE_CAN_MUTE_TS |
FE_CAN_2G_MODULATION,
.frequency_min = 42000000, /* Hz */
.frequency_max = 1002000000, /* Hz */
.symbol_rate_min = 870000,
.symbol_rate_max = 11700000
},
.init = foo_init,
.sleep = foo_sleep,
.release = foo_release,
.set_frontend = foo_set_frontend,
.get_frontend = foo_get_frontend,
.read_status = foo_get_status_and_stats,
.tune = foo_tune,
.i2c_gate_ctrl = foo_i2c_gate_ctrl,
.get_frontend_algo = foo_get_algo,
};
A typical example of such struct in a driver ``bar`` meant to be used on
Satellite TV reception is::
static const struct dvb_frontend_ops bar_ops = {
.delsys = { SYS_DVBS, SYS_DVBS2 },
.info = {
.name = "Bar DVB-S/S2 demodulator",
.frequency_min = 500000, /* KHz */
.frequency_max = 2500000, /* KHz */
.frequency_stepsize = 0,
.symbol_rate_min = 1000000,
.symbol_rate_max = 45000000,
.symbol_rate_tolerance = 500,
.caps = FE_CAN_INVERSION_AUTO |
FE_CAN_FEC_AUTO |
FE_CAN_QPSK,
},
.init = bar_init,
.sleep = bar_sleep,
.release = bar_release,
.set_frontend = bar_set_frontend,
.get_frontend = bar_get_frontend,
.read_status = bar_get_status_and_stats,
.i2c_gate_ctrl = bar_i2c_gate_ctrl,
.get_frontend_algo = bar_get_algo,
.tune = bar_tune,
/* Satellite-specific */
.diseqc_send_master_cmd = bar_send_diseqc_msg,
.diseqc_send_burst = bar_send_burst,
.set_tone = bar_set_tone,
.set_voltage = bar_set_voltage,
};
.. note::
#) For satellite digital TV standards (DVB-S, DVB-S2, ISDB-S), the
frequencies are specified in kHz, while, for terrestrial and cable
standards, they're specified in Hz. Due to that, if the same frontend
supports both types, you'll need to have two separate
:c:type:`dvb_frontend_ops` structures, one for each standard.
#) The ``.i2c_gate_ctrl`` field is present only when the hardware has
allows controlling an I2C gate (either directly of via some GPIO pin),
in order to remove the tuner from the I2C bus after a channel is
tuned.
#) All new drivers should implement the
:ref:`DVBv5 statistics <dvbv5_stats>` via ``.read_status``.
Yet, there are a number of callbacks meant to get statistics for
signal strength, S/N and UCB. Those are there to provide backward
compatibility with legacy applications that don't support the DVBv5
API. Implementing those callbacks are optional. Those callbacks may be
removed in the future, after we have all existing drivers supporting
DVBv5 stats.
#) Other callbacks are required for satellite TV standards, in order to
control LNBf and DiSEqC: ``.diseqc_send_master_cmd``,
``.diseqc_send_burst``, ``.set_tone``, ``.set_voltage``.
.. |delta| unicode:: U+00394
The ``drivers/media/dvb-core/dvb_frontend.c`` has a kernel thread with is
responsible for tuning the device. It supports multiple algoritms to
detect a channel, as defined at enum :c:func:`dvbfe_algo`.
The algorithm to be used is obtained via ``.get_frontend_algo``. If the driver
doesn't fill its field at struct :c:type:`dvb_frontend_ops`, it will default to
``DVBFE_ALGO_SW``, meaning that the dvb-core will do a zigzag when tuning,
e. g. it will try first to use the specified center frequency ``f``,
then, it will do ``f`` + |delta|, ``f`` - |delta|, ``f`` + 2 x |delta|,
``f`` - 2 x |delta| and so on.
If the hardware has internally a some sort of zigzag algorithm, you should
define a ``.get_frontend_algo`` function that would return ``DVBFE_ALGO_HW``.
.. note::
The core frontend support also supports
a third type (``DVBFE_ALGO_CUSTOM``), in order to allow the driver to
define its own hardware-assisted algorithm. Very few hardware need to
use it nowadays. Using ``DVBFE_ALGO_CUSTOM`` require to provide other
function callbacks at struct :c:type:`dvb_frontend_ops`.
Attaching frontend driver to the bridge driver
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Before using the Digital TV frontend core, the bridge driver should attach
the frontend demod, tuner and SEC devices and call
......@@ -99,6 +232,8 @@ part of their handler for :c:type:`device_driver`.\ ``resume()``.
A few other optional functions are provided to handle some special cases.
.. _dvbv5_stats:
Digital TV Frontend statistics
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
......
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