pcmtest.rst

  1.. SPDX-License-Identifier: GPL-2.0
  2
  3The Virtual PCM Test Driver
  4===========================
  5
  6The Virtual PCM Test Driver emulates a generic PCM device, and can be used for
  7testing/fuzzing of the userspace ALSA applications, as well as for testing/fuzzing of
  8the PCM middle layer. Additionally, it can be used for simulating hard to reproduce
  9problems with PCM devices.
 10
 11What can this driver do?
 12~~~~~~~~~~~~~~~~~~~~~~~~
 13
 14At this moment the driver can do the following things:
 15	* Simulate both capture and playback processes
 16	* Generate random or pattern-based capturing data
 17	* Inject delays into the playback and capturing processes
 18	* Inject errors during the PCM callbacks
 19
 20It supports up to 8 substreams and 4 channels. Also it supports both interleaved and
 21non-interleaved access modes.
 22
 23Also, this driver can check the playback stream for containing the predefined pattern,
 24which is used in the corresponding selftest (alsa/pcmtest-test.sh) to check the PCM middle
 25layer data transferring functionality. Additionally, this driver redefines the default
 26RESET ioctl, and the selftest covers this PCM API functionality as well.
 27
 28Configuration
 29-------------
 30
 31The driver has several parameters besides the common ALSA module parameters:
 32
 33	* fill_mode (bool) - Buffer fill mode (see below)
 34	* inject_delay (int)
 35	* inject_hwpars_err (bool)
 36	* inject_prepare_err (bool)
 37	* inject_trigger_err (bool)
 38
 39
 40Capture Data Generation
 41-----------------------
 42
 43The driver has two modes of data generation: the first (0 in the fill_mode parameter)
 44means random data generation, the second (1 in the fill_mode) - pattern-based
 45data generation. Let's look at the second mode.
 46
 47First of all, you may want to specify the pattern for data generation. You can do it
 48by writing the pattern to the debugfs file. There are pattern buffer debugfs entries
 49for each channel, as well as entries which contain the pattern buffer length.
 50
 51	* /sys/kernel/debug/pcmtest/fill_pattern[0-3]
 52	* /sys/kernel/debug/pcmtest/fill_pattern[0-3]_len
 53
 54To set the pattern for the channel 0 you can execute the following command:
 55
 56.. code-block:: bash
 57
 58	echo -n mycoolpattern > /sys/kernel/debug/pcmtest/fill_pattern0
 59
 60Then, after every capture action performed on the 'pcmtest' device the buffer for the
 61channel 0 will contain 'mycoolpatternmycoolpatternmycoolpatternmy...'.
 62
 63The pattern itself can be up to 4096 bytes long.
 64
 65Delay injection
 66---------------
 67
 68The driver has 'inject_delay' parameter, which has very self-descriptive name and
 69can be used for time delay/speedup simulations. The parameter has integer type, and
 70it means the delay added between module's internal timer ticks.
 71
 72If the 'inject_delay' value is positive, the buffer will be filled slower, if it is
 73negative - faster. You can try it yourself by starting a recording in any
 74audiorecording application (like Audacity) and selecting the 'pcmtest' device as a
 75source.
 76
 77This parameter can be also used for generating a huge amount of sound data in a very
 78short period of time (with the negative 'inject_delay' value).
 79
 80Errors injection
 81----------------
 82
 83This module can be used for injecting errors into the PCM communication process. This
 84action can help you to figure out how the userspace ALSA program behaves under unusual
 85circumstances.
 86
 87For example, you can make all 'hw_params' PCM callback calls return EBUSY error by
 88writing '1' to the 'inject_hwpars_err' module parameter:
 89
 90.. code-block:: bash
 91
 92	echo 1 > /sys/module/snd_pcmtest/parameters/inject_hwpars_err
 93
 94Errors can be injected into the following PCM callbacks:
 95
 96	* hw_params (EBUSY)
 97	* prepare (EINVAL)
 98	* trigger (EINVAL)
 99
100Playback test
101-------------
102
103This driver can be also used for the playback functionality testing - every time you
104write the playback data to the 'pcmtest' PCM device and close it, the driver checks the
105buffer for containing the looped pattern (which is specified in the fill_pattern
106debugfs file for each channel). If the playback buffer content represents the looped
107pattern, 'pc_test' debugfs entry is set into '1'. Otherwise, the driver sets it to '0'.
108
109ioctl redefinition test
110-----------------------
111
112The driver redefines the 'reset' ioctl, which is default for all PCM devices. To test
113this functionality, we can trigger the reset ioctl and check the 'ioctl_test' debugfs
114entry:
115
116.. code-block:: bash
117
118	cat /sys/kernel/debug/pcmtest/ioctl_test
119
120If the ioctl is triggered successfully, this file will contain '1', and '0' otherwise.