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Subject: media: dtv-core.rst: add an introduction to FE kAPI
Author: Mauro Carvalho Chehab <mche...@s-opensource.com>
Date: Sun Jun 25 12:40:41 2017 -0300
Instead of just start describing the kAPI functions, add
an introduction giving a general line about a DVB driver's
structure.
Signed-off-by: Mauro Carvalho Chehab <mche...@s-opensource.com>
Documentation/media/kapi/dtv-core.rst | 33 +++++++++++++++++++++++++++++----
1 file changed, 29 insertions(+), 4 deletions(-)
---
diff --git a/Documentation/media/kapi/dtv-core.rst
b/Documentation/media/kapi/dtv-core.rst
index bec7875a7e2e..1430f0b7e615 100644
--- a/Documentation/media/kapi/dtv-core.rst
+++ b/Documentation/media/kapi/dtv-core.rst
@@ -1,6 +1,31 @@
Digital TV (DVB) devices
------------------------
+Digital TV devices are implemented by several different drivers:
+
+- A bridge driver that is responsible to talk with the bus where the other
+ devices are connected (PCI, USB, SPI), bind to the other drivers and
+ implement the digital demux logic (either in software or in hardware);
+
+- Frontend drivers that are usually implemented as two separate drivers:
+
+ - A tuner driver that implements the logic with commands the part of the
+ hardware with is reponsible to tune into a digital TV transponder or
+ physical channel. The output of a tuner is usually a baseband or
+ Intermediate Frequency (IF) signal;
+
+ - A demodulator driver (a.k.a "demod") that implements the logic with
+ commands the digital TV decoding hardware. The output of a demod is
+ a digital stream, with multiple audio, video and data channels typically
+ multiplexed using MPEG Transport Stream [#f1]_.
+
+On most hardware, the frontend drivers talk with the bridge driver using an
+I2C bus.
+
+.. [#f1] Some standards use TCP/IP for multiplexing data, like DVB-H (an
+ abandoned standard, not used anymore) and ATSC version 3.0 current
+ proposals. Currently, the DVB subsystem doesn't implement those standards.
+
Digital TV Common functions
---------------------------
@@ -87,7 +112,7 @@ and measuring the quality of service.
For each statistics measurement, the driver should set the type of scale used,
or ``FE_SCALE_NOT_AVAILABLE`` if the statistics is not available on a given
time. Drivers should also provide the number of statistics for each type.
-that's usually 1 for most video standards [#f1]_.
+that's usually 1 for most video standards [#f2]_.
Drivers should initialize each statistic counters with length and
scale at its init code. For example, if the frontend provides signal
@@ -103,7 +128,7 @@ And, when the statistics got updated, set the scale::
c->strength.stat[0].scale = FE_SCALE_DECIBEL;
c->strength.stat[0].uvalue = strength;
-.. [#f1] For ISDB-T, it may provide both a global statistics and a per-layer
+.. [#f2] For ISDB-T, it may provide both a global statistics and a per-layer
set of statistics. On such cases, len should be equal to 4. The first
value corresponds to the global stat; the other ones to each layer, e. g.:
@@ -129,13 +154,13 @@ Signal strength (:ref:`DTV-STAT-SIGNAL-STRENGTH`)
at the maximum value (so, strength is on its minimal).
- As the gain is visible through the set of registers that adjust the gain,
- typically, this statistics is always available [#f2]_.
+ typically, this statistics is always available [#f3]_.
- Drivers should try to make it available all the times, as this statistics
can be used when adjusting an antenna position and to check for troubles
at the cabling.
- .. [#f2] On a few devices, the gain keeps floating if no carrier.
+ .. [#f3] On a few devices, the gain keeps floating if no carrier.
On such devices, strength report should check first if carrier is
detected at the tuner (``FE_HAS_CARRIER``, see :c:type:`fe_status`),
and otherwise return the lowest possible value.
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