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media: Documentation: vidtv: Add ReST documentation for vidtv
Add documentation for the Virtual Digital TV driver (vidtv) in the Restructured Text (ReST) format. This discusses: - What is vidtv - Why vidtv is needed - How to build and run vidtv - How vidtv is structured - How to test vidtv - How to improve vidtv [mchehab: fixed bad whitespaces] Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com> Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
This commit is contained in:
Daniel W. S. Almeida
Mauro Carvalho Chehab
parent
f90cf6079b
commit
1f9a704601
@ -35,4 +35,5 @@ Digital TV drivers
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dvb-usb
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frontends
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vidtv
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contributors
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417
Documentation/driver-api/media/drivers/vidtv.rst
Normal file
417
Documentation/driver-api/media/drivers/vidtv.rst
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@ -0,0 +1,417 @@
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.. SPDX-License-Identifier: GPL-2.0
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================================
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vidtv: Virtual Digital TV driver
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================================
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Author: Daniel W. S. Almeida <dwlsalmeida@gmail.com>, June 2020.
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Background
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----------
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Vidtv is a virtual DVB driver that aims to serve as a reference for driver
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writers by serving as a template. It also validates the existing media DVB
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APIs, thus helping userspace application writers.
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Currently, it consists of:
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- A fake tuner driver, which will report a bad signal quality if the chosen
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frequency is too far away from a table of valid frequencies for a
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particular delivery system.
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- A fake demod driver, which will constantly poll the fake signal quality
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returned by the tuner, simulating a device that can lose/reacquire a lock
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on the signal depending on the CNR levels.
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- A fake bridge driver, which is the module responsible for modprobing the
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fake tuner and demod modules and implementing the demux logic. This module
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takes parameters at initialization that will dictate how the simulation
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behaves.
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- Code reponsible for encoding a valid MPEG Transport Stream, which is then
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passed to the bridge driver. This fake stream contains some hardcoded content.
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For now, we have a single, audio-only channel containing a single MPEG
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Elementary Stream, which in turn contains a SMPTE 302m encoded sine-wave.
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Note that this particular encoder was chosen because it is the easiest
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way to encode PCM audio data in a MPEG Transport Stream.
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Building vidtv
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--------------
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vidtv is a test driver and thus is **not** enabled by default when
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compiling the kernel.
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In order to enable compilation of vidtv:
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- Enable **DVB_TEST_DRIVERS**, then
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- Enable **DVB_VIDTV**
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When compiled as a module, expect the following .ko files:
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- dvb_vidtv_tuner.ko
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- dvb_vidtv_demod.ko
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- dvb_vidtv_bridge.ko
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Running vidtv
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-------------
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When compiled as a module, run::
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modprobe dvb_vidtv_bridge
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That's it! The bridge driver will initialize the tuner and demod drivers as
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part of its own initialization.
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You can optionally define some command-line arguments to vidtv.
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Command-line arguments to vidtv
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-------------------------------
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Below is a list of all arguments that can be supplied to vidtv:
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drop_tslock_prob_on_low_snr
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Probability of losing the TS lock if the signal quality is bad.
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This probability be used by the fake demodulator driver to
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eventually return a status of 0 when the signal quality is not
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good.
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recover_tslock_prob_on_good_snr:
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Probability recovering the TS lock when the signal improves. This
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probability be used by the fake demodulator driver to eventually
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return a status of 0x1f when/if the signal quality improves.
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mock_power_up_delay_msec
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Simulate a power up delay. Default: 0.
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mock_tune_delay_msec
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Simulate a tune delay. Default 0.
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vidtv_valid_dvb_t_freqs
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Valid DVB-T frequencies to simulate.
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vidtv_valid_dvb_c_freqs
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Valid DVB-C frequencies to simulate.
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vidtv_valid_dvb_s_freqs
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Valid DVB-C frequencies to simulate.
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max_frequency_shift_hz,
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Maximum shift in HZ allowed when tuning in a channel.
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si_period_msec
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How often to send SI packets. Default: 40ms.
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pcr_period_msec
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How often to send PCR packets. Default: 40ms.
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mux_rate_kbytes_sec
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Attempt to maintain this bit rate by inserting TS null packets, if
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necessary. Default: 4096.
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pcr_pid,
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PCR PID for all channels. Default: 0x200.
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mux_buf_sz_pkts,
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Size for the mux buffer in multiples of 188 bytes.
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vidtv internal structure
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------------------------
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The kernel modules are split in the following way:
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vidtv_tuner.[ch]
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Implements a fake tuner DVB driver.
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vidtv_demod.[ch]
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Implements a fake demodulator DVB driver.
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vidtv_bridge.[ch]
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Implements a bridge driver.
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The MPEG related code is split in the following way:
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vidtv_ts.[ch]
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Code to work with MPEG TS packets, such as TS headers, adaptation
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fields, PCR packets and NULL packets.
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vidtv_psi.[ch]
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This is the PSI generator. PSI packets contain general information
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about a MPEG Transport Stream. A PSI generator is needed so
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userspace apps can retrieve information about the Transport Stream
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and eventually tune into a (dummy) channel.
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Because the generator is implemented in a separate file, it can be
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reused elsewhere in the media subsystem.
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Currently vidtv supports working with 3 PSI tables: PAT, PMT and
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SDT.
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The specification for PAT and PMT can be found in *ISO 13818-1:
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Systems*, while the specification for the SDT can be found in *ETSI
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EN 300 468: Specification for Service Information (SI) in DVB
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systems*.
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It isn't strictly necessary, but using a real TS file helps when
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debugging PSI tables. Vidtv currently tries to replicate the PSI
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structure found in this file: `TS1Globo.ts
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<https://tsduck.io/streams/brazil-isdb-tb/TS1globo.ts>`_.
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A good way to visualize the structure of streams is by using
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`DVBInspector <https://sourceforge.net/projects/dvbinspector/>`_.
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vidtv_pes.[ch]
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Implements the PES logic to convert encoder data into MPEG TS
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packets. These can then be fed into a TS multiplexer and eventually
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into userspace.
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vidtv_encoder.h
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An interface for vidtv encoders. New encoders can be added to this
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driver by implementing the calls in this file.
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vidtv_s302m.[ch]
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Implements a S302M encoder to make it possible to insert PCM audio
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data in the generated MPEG Transport Stream. The relevant
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specification is available online as *SMPTE 302M-2007: Television -
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Mapping of AES3 Data into MPEG-2 Transport Stream*.
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The resulting MPEG Elementary Stream is conveyed in a private
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stream with a S302M registration descriptor attached.
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This shall enable passing an audio signal into userspace so it can
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be decoded and played by media software. The corresponding decoder
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in ffmpeg is located in 'libavcodec/s302m.c' and is experimental.
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vidtv_channel.[ch]
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Implements a 'channel' abstraction.
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When vidtv boots, it will create some hardcoded channels:
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#. Their services will be concatenated to populate the SDT.
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#. Their programs will be concatenated to populate the PAT
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#. For each program in the PAT, a PMT section will be created
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#. The PMT section for a channel will be assigned its streams.
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#. Every stream will have its corresponding encoder polled in a
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loop to produce TS packets.
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These packets may be interleaved by the muxer and then delivered
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to the bridge.
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vidtv_mux.[ch]
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Implements a MPEG TS mux, loosely based on the ffmpeg
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implementation in "libavcodec/mpegtsenc.c"
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The muxer runs a loop which is responsible for:
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#. Keeping track of the amount of time elapsed since the last
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iteration.
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#. Polling encoders in order to fetch 'elapsed_time' worth of data.
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#. Inserting PSI and/or PCR packets, if needed.
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#. Padding the resulting stream with NULL packets if
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necessary in order to maintain the chosen bit rate.
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#. Delivering the resulting TS packets to the bridge
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driver so it can pass them to the demux.
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Testing vidtv with v4l-utils
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----------------------------
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Using the tools in v4l-utils is a great way to test and inspect the output of
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vidtv. It is hosted here: `v4l-utils Documentation
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<https://linuxtv.org/wiki/index.php/V4l-utils>`_.
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From its webpage::
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The v4l-utils are a series of packages for handling media devices.
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It is hosted at http://git.linuxtv.org/v4l-utils.git, and packaged
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on most distributions.
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It provides a series of libraries and utilities to be used to
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control several aspect of the media boards.
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Start by installing v4l-utils and then modprobing vidtv::
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modprobe dvb_vidtv_bridge
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If the driver is OK, it should load and its probing code will run. This will
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pull in the tuner and demod drivers.
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Using dvb-fe-tool
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~~~~~~~~~~~~~~~~~
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The first step to check whether the demod loaded successfully is to run::
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$ dvb-fe-tool
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This should return what is currently set up at the demod struct, i.e.::
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static const struct dvb_frontend_ops vidtv_demod_ops = {
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.delsys = {
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SYS_DVBT,
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SYS_DVBT2,
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SYS_DVBC_ANNEX_A,
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SYS_DVBS,
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SYS_DVBS2,
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},
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.info = {
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.name = "Dummy demod for DVB-T/T2/C/S/S2",
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.frequency_min_hz = 51 * MHz,
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.frequency_max_hz = 2150 * MHz,
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.frequency_stepsize_hz = 62500,
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.frequency_tolerance_hz = 29500 * kHz,
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.symbol_rate_min = 1000000,
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.symbol_rate_max = 45000000,
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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_4_5 |
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FE_CAN_FEC_5_6 |
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FE_CAN_FEC_6_7 |
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FE_CAN_FEC_7_8 |
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FE_CAN_FEC_8_9 |
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FE_CAN_QAM_16 |
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FE_CAN_QAM_64 |
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FE_CAN_QAM_32 |
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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_QPSK |
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FE_CAN_FEC_AUTO |
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FE_CAN_INVERSION_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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}
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....
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For more information on dvb-fe-tools check its online documentation here:
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`dvb-fe-tool Documentation
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<https://www.linuxtv.org/wiki/index.php/Dvb-fe-tool>`_.
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Using dvb-scan
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~~~~~~~~~~~~~~
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In order to tune into a channel and read the PSI tables, we can use dvb-scan.
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For this, one should provide a configuration file known as a 'scan file',
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here's an example::
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[Channel]
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FREQUENCY = 330000000
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MODULATION = QAM/AUTO
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SYMBOL_RATE = 6940000
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INNER_FEC = AUTO
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DELIVERY_SYSTEM = DVBC/ANNEX_A
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.. note::
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The parameters depend on the video standard you're testing.
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.. note::
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Vidtv is a fake driver and does not validate much of the information
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in the scan file. Just specifying 'FREQUENCY' and 'DELIVERY_SYSTEM'
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should be enough for DVB-T/DVB-T2. For DVB-S/DVB-C however, you
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should also provide 'SYMBOL_RATE'.
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You can browse scan tables online here: `dvb-scan-tables
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<https://git.linuxtv.org/dtv-scan-tables.git>`_.
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Assuming this channel is named 'channel.conf', you can then run::
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$ dvbv5-scan channel.conf
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For more information on dvb-scan, check its documentation online here:
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`dvb-scan Documentation <https://www.linuxtv.org/wiki/index.php/Dvbscan>`_.
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Using dvb-zap
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~~~~~~~~~~~~~
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dvbv5-zap is a command line tool that can be used to record MPEG-TS to disk. The
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typical use is to tune into a channel and put it into record mode. The example
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below - which is taken from the documentation - illustrates that::
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$ dvbv5-zap -c dvb_channel.conf "trilhas sonoras" -r
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using demux '/dev/dvb/adapter0/demux0'
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reading channels from file 'dvb_channel.conf'
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service has pid type 05: 204
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tuning to 573000000 Hz
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audio pid 104
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dvb_set_pesfilter 104
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Lock (0x1f) Quality= Good Signal= 100.00% C/N= -13.80dB UCB= 70 postBER= 3.14x10^-3 PER= 0
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DVR interface '/dev/dvb/adapter0/dvr0' can now be opened
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The channel can be watched by playing the contents of the DVR interface, with
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some player that recognizes the MPEG-TS format, such as *mplayer* or *vlc*.
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By playing the contents of the stream one can visually inspect the workings of
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vidtv, e.g.::
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$ mplayer /dev/dvb/adapter0/dvr0
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For more information on dvb-zap check its online documentation here:
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`dvb-zap Documentation
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<https://www.linuxtv.org/wiki/index.php/Dvbv5-zap>`_.
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See also: `zap <https://www.linuxtv.org/wiki/index.php/Zap>`_.
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What can still be improved in vidtv
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-----------------------------------
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Add *debugfs* integration
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~~~~~~~~~~~~~~~~~~~~~~~~~
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Although frontend drivers provide DVBv5 statistics via the .read_status
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call, a nice addition would be to make additional statistics available to
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userspace via debugfs, which is a simple-to-use, RAM-based filesystem
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specifically designed for debug purposes.
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The logic for this would be implemented on a separate file so as not to
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pollute the frontend driver. These statistics are driver-specific and can
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be useful during tests.
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The Siano driver is one example of a driver using
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debugfs to convey driver-specific statistics to userspace and it can be
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used as a reference.
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This should be further enabled and disabled via a Kconfig
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option for convenience.
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Add a way to test video
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~~~~~~~~~~~~~~~~~~~~~~~
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Currently, vidtv can only encode PCM audio. It would be great to implement
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a barebones version of MPEG-2 video encoding so we can also test video. The
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first place to look into is *ISO 13818-2: Information technology — Generic
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coding of moving pictures and associated audio information — Part 2: Video*,
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which covers the encoding of compressed video in MPEG Transport Streams.
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This might optionally use the Video4Linux2 Test Pattern Generator, v4l2-tpg,
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which resides at::
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drivers/media/common/v4l2-tpg/
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Add white noise simulation
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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The vidtv tuner already has code to identify whether the chosen frequency
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is too far away from a table of valid frequencies. For now, this means that
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the demodulator can eventually lose the lock on the signal, since the tuner will
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report a bad signal quality.
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A nice addition is to simulate some noise when the signal quality is bad by:
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- Randomly dropping some TS packets. This will trigger a continuity error if the
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continuity counter is updated but the packet is not passed on to the demux.
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- Updating the error statistics accordingly (e.g. BER, etc).
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- Simulating some noise in the encoded data.
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