[Linux-kernel-mentees] [v10 0/4] media: vidtv: Implement a virtual DVB driver

Daniel W. S. Almeida dwlsalmeida at gmail.com
Fri Aug 21 12:58:44 UTC 2020

From: "Daniel W. S. Almeida" <dwlsalmeida at gmail.com>

This series is work in progress. It represents the current work done on a
virtual DVB driver for the Linux media subsystem. I am new to the media
subsystem and to kernel development in general.

This driver aims to:
	- Serve as template for new DVB driver writers
	- Help userspace application writers in general
	- Push fake audio/video to userspace for testing
	- Push debug information to userspace via debugfs

Current state for this driver:
	- Driver generates PSI information (PAT, PMT, SDT)
	- Driver generates PCR packets
	- Driver generates NULL packets for padding
	- PCM audio stream is decoded by ffmpeg, but no audio is heard yet.

changes in v10:
	s302m encoder got reworked

The Virtual Digital TV Driver (vidtv)


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

- 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, see the
documentation for more info.

Testing vidtv with v4l-utils

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 = {

		.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 |


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::

	FREQUENCY = 330000000
	SYMBOL_RATE = 6940000

	The parameters depend on the video standard you're testing.

	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'.

Assuming this channel is named 'channel.conf', you can then run::

        $ dvbv5-scan dresden_dvbc_channel.conf

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

Daniel W. S. Almeida (4):
  media: vidtv: implement a tuner driver
  media: vidtv: implement a demodulator driver
  media: vidtv: add a bridge driver
  media: Documentation: vidtv: Add ReST documentation for vidtv

 .../driver-api/media/drivers/index.rst        |    1 +
 .../driver-api/media/drivers/vidtv.rst        |  417 +++++
 MAINTAINERS                                   |    8 +
 drivers/media/test-drivers/Kconfig            |   16 +
 drivers/media/test-drivers/Makefile           |    1 +
 drivers/media/test-drivers/vidtv/Kconfig      |   11 +
 drivers/media/test-drivers/vidtv/Makefile     |    9 +
 .../media/test-drivers/vidtv/vidtv_bridge.c   |  546 +++++++
 .../media/test-drivers/vidtv/vidtv_bridge.h   |   60 +
 .../media/test-drivers/vidtv/vidtv_channel.c  |  306 ++++
 .../media/test-drivers/vidtv/vidtv_channel.h  |   76 +
 .../media/test-drivers/vidtv/vidtv_common.c   |   89 ++
 .../media/test-drivers/vidtv/vidtv_common.h   |   33 +
 .../media/test-drivers/vidtv/vidtv_demod.c    |  440 ++++++
 .../media/test-drivers/vidtv/vidtv_demod.h    |   73 +
 .../media/test-drivers/vidtv/vidtv_encoder.h  |   96 ++
 drivers/media/test-drivers/vidtv/vidtv_mux.c  |  479 ++++++
 drivers/media/test-drivers/vidtv/vidtv_mux.h  |  160 ++
 drivers/media/test-drivers/vidtv/vidtv_pes.c  |  398 +++++
 drivers/media/test-drivers/vidtv/vidtv_pes.h  |  189 +++
 drivers/media/test-drivers/vidtv/vidtv_psi.c  | 1352 +++++++++++++++++
 drivers/media/test-drivers/vidtv/vidtv_psi.h  |  593 ++++++++
 .../media/test-drivers/vidtv/vidtv_s302m.c    |  552 +++++++
 .../media/test-drivers/vidtv/vidtv_s302m.h    |   90 ++
 drivers/media/test-drivers/vidtv/vidtv_ts.c   |  137 ++
 drivers/media/test-drivers/vidtv/vidtv_ts.h   |  130 ++
 .../media/test-drivers/vidtv/vidtv_tuner.c    |  427 ++++++
 .../media/test-drivers/vidtv/vidtv_tuner.h    |   43 +
 28 files changed, 6732 insertions(+)
 create mode 100644 Documentation/driver-api/media/drivers/vidtv.rst
 create mode 100644 drivers/media/test-drivers/vidtv/Kconfig
 create mode 100644 drivers/media/test-drivers/vidtv/Makefile
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_bridge.c
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_bridge.h
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_channel.c
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_channel.h
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_common.c
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_common.h
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_demod.c
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_demod.h
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_encoder.h
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_mux.c
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_mux.h
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_pes.c
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_pes.h
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_psi.c
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_psi.h
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_s302m.c
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_s302m.h
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_ts.c
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_ts.h
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_tuner.c
 create mode 100644 drivers/media/test-drivers/vidtv/vidtv_tuner.h


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