Documentation/sound/cards/pcmtest.rst

Source file repositories/reference/linux-study-clean/Documentation/sound/cards/pcmtest.rst

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Documentation/sound/cards/pcmtest.rst
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Support Tooling And Documentation
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Documentation
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Support Tooling And Documentation: documentation
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Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

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.. SPDX-License-Identifier: GPL-2.0

The Virtual PCM Test Driver
===========================

The Virtual PCM Test Driver emulates a generic PCM device, and can be used for
testing/fuzzing of the userspace ALSA applications, as well as for testing/fuzzing of
the PCM middle layer. Additionally, it can be used for simulating hard to reproduce
problems with PCM devices.

What can this driver do?
~~~~~~~~~~~~~~~~~~~~~~~~

At this moment the driver can do the following things:
	* Simulate both capture and playback processes
	* Generate random or pattern-based capturing data
	* Inject delays into the playback and capturing processes
	* Inject errors during the PCM callbacks

It supports up to 8 substreams and 4 channels. Also it supports both interleaved and
non-interleaved access modes.

Also, this driver can check the playback stream for containing the predefined pattern,
which is used in the corresponding selftest (alsa/pcmtest-test.sh) to check the PCM middle
layer data transferring functionality. Additionally, this driver redefines the default
RESET ioctl, and the selftest covers this PCM API functionality as well.

Configuration
-------------

The driver has several parameters besides the common ALSA module parameters:

	* fill_mode (bool) - Buffer fill mode (see below)
	* inject_delay (int)
	* inject_hwpars_err (bool)
	* inject_prepare_err (bool)
	* inject_trigger_err (bool)


Capture Data Generation
-----------------------

The driver has two modes of data generation: the first (0 in the fill_mode parameter)
means random data generation, the second (1 in the fill_mode) - pattern-based
data generation. Let's look at the second mode.

First of all, you may want to specify the pattern for data generation. You can do it
by writing the pattern to the debugfs file. There are pattern buffer debugfs entries
for each channel, as well as entries which contain the pattern buffer length.

	* /sys/kernel/debug/pcmtest/fill_pattern[0-3]
	* /sys/kernel/debug/pcmtest/fill_pattern[0-3]_len

To set the pattern for the channel 0 you can execute the following command:

.. code-block:: bash

	echo -n mycoolpattern > /sys/kernel/debug/pcmtest/fill_pattern0

Then, after every capture action performed on the 'pcmtest' device the buffer for the
channel 0 will contain 'mycoolpatternmycoolpatternmycoolpatternmy...'.

The pattern itself can be up to 4096 bytes long.

Delay injection
---------------

The driver has 'inject_delay' parameter, which has very self-descriptive name and
can be used for time delay/speedup simulations. The parameter has integer type, and
it means the delay added between module's internal timer ticks.

Annotation

Implementation Notes