From: "Daniel W. S. Almeida" <dwlsalmeida@xxxxxxxxx> 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 Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@xxxxxxxxx> --- .../driver-api/media/drivers/index.rst | 1 + .../driver-api/media/drivers/vidtv.rst | 415 ++++++++++++++++++ 2 files changed, 416 insertions(+) create mode 100644 Documentation/driver-api/media/drivers/vidtv.rst diff --git a/Documentation/driver-api/media/drivers/index.rst b/Documentation/driver-api/media/drivers/index.rst index 0df85fc966056..5f340cfdb4cc2 100644 --- a/Documentation/driver-api/media/drivers/index.rst +++ b/Documentation/driver-api/media/drivers/index.rst @@ -35,4 +35,5 @@ Digital TV drivers dvb-usb frontends + vidtv contributors diff --git a/Documentation/driver-api/media/drivers/vidtv.rst b/Documentation/driver-api/media/drivers/vidtv.rst new file mode 100644 index 0000000000000..698be9350e56b --- /dev/null +++ b/Documentation/driver-api/media/drivers/vidtv.rst @@ -0,0 +1,415 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================================ +vidtv: Virtual Digital TV driver +================================ + +Author: Daniel W. S. Almeida <dwlsalmeida@xxxxxxxxx>, June 2020. + +Background +---------- + +Vidtv is a virtual DVB driver that aims to serve as a reference for driver +writers by serving as a template. It also validates the existing media DVB +APIs, thus helping userspace application writers. + +Currently, it consists of: + +- A fake tuner driver, which will report a bad signal quality if the chosen + frequency is too far away from a table of valid frequencies for a + particular delivery system. + +- A fake demod driver, which will constantly poll the fake signal quality + returned by the tuner, simulating a device that can lose/reacquire a lock + on the signal depending on the CNR levels. + +- A fake bridge driver, which is the module responsible for modprobing the + fake tuner and demod modules and implementing the demux logic. This module + takes parameters at initialization that will dictate how the simulation + behaves. + +- Code reponsible for encoding a valid MPEG Transport Stream, which is then + passed to the bridge driver. This fake stream contains some hardcoded content. + For now, we have a single, audio-only channel containing a single MPEG + Elementary Stream, which in turn contains a SMPTE 302m encoded sine-wave. + Note that this particular encoder was chosen because it is the easiest + way to encode PCM audio data in a MPEG Transport Stream. + +Building vidtv +-------------- +vidtv is a test driver and thus is **not** enabled by default when +compiling the kernel. + +In order to enable compilation of vidtv: + +- Enable **DVB_TEST_DRIVERS**, then +- Enable **DVB_VIDTV** + +When compiled as a module, expect the following .ko files: + +- dvb_vidtv_tuner.ko + +- dvb_vidtv_demod.ko + +- dvb_vidtv_bridge.ko + +Running vidtv +------------- +When compiled as a module, run:: + + modprobe dvb_vidtv_bridge + +That's it! The bridge driver will initialize the tuner and demod drivers as +part of its own initialization. + +You can optionally define some command-line arguments to vidtv. + +Command-line arguments to vidtv +------------------------------- +Below is a list of all arguments that can be supplied to vidtv: + +drop_tslock_prob_on_low_snr + Probability of losing the TS lock if the signal quality is bad. + This probability be used by the fake demodulator driver to + eventually return a status of 0 when the signal quality is not + good. + +recover_tslock_prob_on_good_snr: + Probability recovering the TS lock when the signal improves. This + probability be used by the fake demodulator driver to eventually + return a status of 0x1f when/if the signal quality improves. + +mock_power_up_delay_msec + Simulate a power up delay. Default: 0. + +mock_tune_delay_msec + Simulate a tune delay. Default 0. + +vidtv_valid_dvb_t_freqs + Valid DVB-T frequencies to simulate. + +vidtv_valid_dvb_c_freqs + Valid DVB-C frequencies to simulate. + +vidtv_valid_dvb_s_freqs + Valid DVB-C frequencies to simulate. + +max_frequency_shift_hz, + Maximum shift in HZ allowed when tuning in a channel. + +si_period_msec + How often to send SI packets. Default: 40ms. + +pcr_period_msec + How often to send PCR packets. Default: 40ms. + +mux_rate_kbytes_sec + Attempt to maintain this bit rate by inserting TS null packets, if + necessary. Default: 4096. + +pcr_pid, + PCR PID for all channels. Default: 0x200. + +ts_buf_sz_pkts, + Size for the TS buffer in multiples of 188 bytes. Default: 20 + packets. + +vidtv internal structure +------------------------ +The kernel modules are split in the following way: + +vidtv_tuner.[ch] + Implements a fake tuner DVB driver. + +vidtv_demod.[ch] + Implements a fake demodulator DVB driver. + +vidtv_bridge.[ch] + Implements a bridge driver. + +The MPEG related code is split in the following way: + +vidtv_ts.[ch] + Code to work with MPEG TS packets, such as TS headers, adaptation + fields, PCR packets and NULL packets. + +vidtv_psi.[ch] + This is the PSI generator. PSI packets contain general information + about a MPEG Transport Stream. A PSI generator is needed so + userspace apps can retrieve information about the Transport Stream + and eventually tune into a (dummy) channel. + + Because the generator is implemented in a separate file, it can be + reused elsewhere in the media subsystem. + + Currently vidtv supports working with 3 PSI tables: PAT, PMT and + SDT. + + The specification for PAT and PMT can be found in *ISO 13818-1: + Systems*, while the specification for the SDT can be found in *ETSI + EN 300 468: Specification for Service Information (SI) in DVB + systems*. + + It isn't strictly necessary, but using a real TS file helps when + debugging PSI tables. Vidtv currently tries to replicate the PSI + structure found in this file: `TS1Globo.ts + <https://tsduck.io/streams/brazil-isdb-tb/TS1globo.ts>`_. + + A good way to visualize the structure of streams is by using + `DVBInspector <https://sourceforge.net/projects/dvbinspector/>`_. + +vidtv_pes.[ch] + Implements the PES logic to convert encoder data into MPEG TS + packets. These can then be fed into a TS multiplexer and eventually + into userspace. + +vidtv_encoder.h + An interface for vidtv encoders. New encoders can be added to this + driver by implementing the calls in this file. + +vidtv_s302m.[ch] + Implements a S302M encoder to make it possible to insert PCM audio + data in the generated MPEG Transport Stream. The relevant + specification is available online as *SMPTE 302M-2007: Television - + Mapping of AES3 Data into MPEG-2 Transport Stream*. + + + The resulting MPEG Elementary Stream is conveyed in a private + stream with a S302M registration descriptor attached. + + This shall enable passing an audio signal into userspace so it can + be decoded and played by media software. The corresponding decoder + in ffmpeg is located in 'libavcodec/s302m.c' and is experimental. + +vidtv_channel.[ch] + Implements a 'channel' abstraction. + + When vidtv boots, it will create some hardcoded channels: + + #. Their services will be concatenated to populate the SDT. + + #. Their programs will be concatenated to populate the PAT + + #. For each program in the PAT, a PMT section will be created + + #. The PMT section for a channel will be assigned its streams. + + #. Every stream will have its corresponding encoder polled in a + loop to produce TS packets. + These packets may be interleaved by the muxer and then delivered + to the bridge. + +vidtv_mux.[ch] + Implements a MPEG TS mux, loosely based on the ffmpeg + implementation in "libavcodec/mpegtsenc.c" + + The muxer runs a loop which is responsible for: + + #. Keeping track of the amount of time elapsed since the last + iteration. + + #. Polling encoders in order to fetch 'elapsed_time' worth of data. + + #. Inserting PSI and/or PCR packets, if needed. + + #. Padding the resulting stream with NULL packets if + necessary in order to maintain the chosen bit rate. + + #. Delivering the resulting TS packets to the bridge + driver so it can pass them to the demux. + +Testing vidtv with v4l-utils +============================ + +Using the tools in v4l-utils is a great way to test and inspect the output of +vidtv. It is hosted here: `v4l-utils Documentation +<https://linuxtv.org/wiki/index.php/V4l-utils>`_. + +From its webpage:: + + The v4l-utils are a series of packages for handling media devices. + + It is hosted at http://git.linuxtv.org/v4l-utils.git, and packaged + on most distributions. + + It provides a series of libraries and utilities to be used to + control several aspect of the media boards. + + +Start by installing v4l-utils and then modprobing vidtv:: + + modprobe dvb_vidtv_bridge + +If the driver is OK, it should load and its probing code will run. This will +pull in the tuner and demod drivers. + +Using dvb-fe-tool +----------------- + +The first step to check whether the demod loaded successfully is to run:: + + $ dvb-fe-tool + +This should return what is currently set up at the demod struct, i.e.:: + + static const struct dvb_frontend_ops vidtv_demod_ops = { + .delsys = { + SYS_DVBT, + SYS_DVBT2, + SYS_DVBC_ANNEX_A, + SYS_DVBS, + SYS_DVBS2, + }, + + .info = { + .name = "Dummy demod for DVB-T/T2/C/S/S2", + .frequency_min_hz = 51 * MHz, + .frequency_max_hz = 2150 * MHz, + .frequency_stepsize_hz = 62500, + .frequency_tolerance_hz = 29500 * kHz, + .symbol_rate_min = 1000000, + .symbol_rate_max = 45000000, + + .caps = FE_CAN_FEC_1_2 | + FE_CAN_FEC_2_3 | + FE_CAN_FEC_3_4 | + FE_CAN_FEC_4_5 | + FE_CAN_FEC_5_6 | + FE_CAN_FEC_6_7 | + FE_CAN_FEC_7_8 | + FE_CAN_FEC_8_9 | + FE_CAN_QAM_16 | + FE_CAN_QAM_64 | + FE_CAN_QAM_32 | + FE_CAN_QAM_128 | + FE_CAN_QAM_256 | + FE_CAN_QAM_AUTO | + FE_CAN_QPSK | + FE_CAN_FEC_AUTO | + FE_CAN_INVERSION_AUTO | + FE_CAN_TRANSMISSION_MODE_AUTO | + FE_CAN_GUARD_INTERVAL_AUTO | + FE_CAN_HIERARCHY_AUTO, + } + + .... + +For more information on dvb-fe-tools check its online documentation here: +`dvb-fe-tool Documentation +<https://www.linuxtv.org/wiki/index.php/Dvb-fe-tool>`_. + +Using dvb-scan +-------------- + +In order to tune into a channel and read the PSI tables, we can use dvb-scan. + +For this, one should provide a configuration file known as a 'scan file', +here's an example:: + + [Channel] + FREQUENCY = 330000000 + MODULATION = QAM/AUTO + SYMBOL_RATE = 6940000 + INNER_FEC = AUTO + DELIVERY_SYSTEM = DVBC/ANNEX_A + +.. note:: + The parameters depend on the video standard you're testing. + +.. note:: + Vidtv is a fake driver and does not validate much of the information + in the scan file. Just specifying 'FREQUENCY' and 'DELIVERY_SYSTEM' + should be enough for DVB-T/DVB-T2. For DVB-S/DVB-C however, you + should also provide 'SYMBOL_RATE'. + +You can browse scan tables online here: `dvb-scan-tables +<https://git.linuxtv.org/dtv-scan-tables.git>`_. + +Assuming this channel is named 'channel.conf', you can then run:: + + $ dvbv5-scan dresden_dvbc_channel.conf + +For more information on dvb-scan, check its documentation online here: +`dvb-scan Documentation <https://www.linuxtv.org/wiki/index.php/Dvbscan>`_. + +Using dvb-zap +------------- + +dvbv5-zap is a command line tool that can be used to record MPEG-TS to disk. The +typical use is to tune into a channel and put it into record mode. The example +below - which is taken from the documentation - illustrates that:: + + $ dvbv5-zap -c dvb_channel.conf "trilhas sonoras" -r + using demux '/dev/dvb/adapter0/demux0' + reading channels from file 'dvb_channel.conf' + service has pid type 05: 204 + tuning to 573000000 Hz + audio pid 104 + dvb_set_pesfilter 104 + Lock (0x1f) Quality= Good Signal= 100.00% C/N= -13.80dB UCB= 70 postBER= 3.14x10^-3 PER= 0 + DVR interface '/dev/dvb/adapter0/dvr0' can now be opened + +The channel can be watched by playing the contents of the DVR interface, with +some player that recognizes the MPEG-TS format, such as *mplayer* or *vlc*. + +By playing the contents of the stream one can visually inspect the workings of +vidtv, e.g.:: + + $ mplayer /dev/dvb/adapter0/dvr0 + +For more information on dvb-zap check its online documentation here: +`dvb-zap Documentation +<https://www.linuxtv.org/wiki/index.php/Dvbv5-zap>`_. +See also: `zap <https://www.linuxtv.org/wiki/index.php/Zap>`_. + + +What can still be improved in vidtv +=================================== + +Add *debugfs* integration +-------------------------- +Although frontend drivers provide DVBv5 statistics via the .read_status +call, a nice addition would be to make additional statistics available to +userspace via debugfs, which is a simple-to-use, RAM-based filesystem +specifically designed for debug purposes. + +The logic for this would be implemented on a separate file so as not to +pollute the frontend driver. These statistics are driver-specific and can +be useful during tests. + +The Siano driver is one example of a driver using +debugfs to convey driver-specific statistics to userspace and it can be +used as a reference. + +This should be further enabled and disabled via a Kconfig +option for convenience. + +Add a way to test video +----------------------- +Currently, vidtv can only encode PCM audio. It would be great to implement +a barebones version of MPEG-2 video encoding so we can also test video. The +first place to look into is *ISO 13818-2: Information technology — Generic +coding of moving pictures and associated audio information — Part 2: Video*, +which covers the encoding of compressed video in MPEG Transport Streams. + +This might optionally use the Video4Linux2 Test Pattern Generator, v4l2-tpg, +which resides at:: + + drivers/media/common/v4l2-tpg/ + + +Add white noise simulation +-------------------------- +The vidtv tuner already has code to identify whether the chosen frequency +is too far away from a table of valid frequencies. For now, this means that +the demodulator can eventually lose the lock on the signal, since the tuner will +report a bad signal quality. + +A nice addition is to simulate some noise when the signal quality is bad by: + +- Randomly dropping some TS packets. This will trigger a continuity error if the + continuity counter is updated but the packet is not passed on to the demux. + +- Updating the error statistics accordingly (e.g. BER, etc). + +- Simulating some noise in the encoded data. -- 2.27.0
