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ALSA PCM proc commands

By default, ALSA emulates OSS PCM with the so-called plugin layer, i.e. tries to convert the sample format, rate or channels automatically when the card doesn't support it natively. This will lead to some problems for some applications like quake or wine, especially if they use the card only in the MMAP mode.

In such a case, you can change the behavior of the PCM per application by writing a command to the relavent /proc file. There is a proc file for each PCM stream,

  • X is the card number (starting from zero; the first card is card 0, the second card is card 1, etc.)
  • Y is the PCM device number (also starting from zero)
  • p is for playback and c for capture, respectively.

Note that this proc file exists only after snd-pcm-oss module is loaded.

The command sequence has the following syntax:

app_name fragments fragment_size [options]
  • app_name is the name of application with (higher priority) or without path.
  • fragments specifies the number of fragments or zero if no specific number is given.
  • fragment_size is the size of fragment in bytes or zero if not given.
  • options is the optional parameters.

The following options are available:

  • disable -- The application tries to open a pcm device for this channel but does not want to use it.
  • direct -- Don't use plugins
  • block -- Force block open mode
  • non-block -- Force non-block open mode

The disable option is useful when one stream direction (playback or capture) is not handled correctly by the application although the hardware itself does support both directions.

The direct option is used, as mentioned above, to bypass the automatic conversion and useful for MMAP-applications. For example, to playback the first PCM device without plugins for quake, send a command to the relevant /proc "file" via echo like the following:

% echo "quake 0 0 direct" > /proc/asound/card0/pcm0p/oss

While quake wants only playback, you may append the second command to notify the driver that only this direction is about to be allocated:

% echo "quake 0 0 disable" > /proc/asound/card0/pcm0c/oss

The permissions of /proc files depend on the module options of ((snd)). As a default it's set as root, so you'll likely need to be superuser for sending the command above.

The block and non-block options are used to change the behavior of opening the device file. As default, ALSA behaves as defined in POSIX, i.e. blocks the file when it's busy until the device becomes free (unless O_NONBLOCK is specified). Some applications assume the non-block open behavior, which are actually implemented in some real OSS drivers.

This blocking behavior can be changed globally via nonblock_open module option of snd-pcm-oss. For using the non-block mode as default for OSS devices, define like the following:

options snd-pcm-oss nonblock_open=1

You can check the currently defined configuration by reading the proc file. The read image can be sent to the proc file again, hence you can save the current configuration

% cat /proc/asound/card0/pcm0p/oss > /somewhere/oss-cfg

and restore it like

% cat /somewhere/oss-cfg > /proc/asound/card0/pcm0p/oss

Also, for clearing all the current configuration, send erase command as below:

% echo "erase" > /proc/asound/card0/pcm0p/oss