This patch set is an updated version of patches by Timo Wischer: https://mailman.alsa-project.org/pipermail/alsa-devel/2019-March/146871.html
This patch set is required for forwarding audio data between a HW sound card and an aloop device without the usage of an asynchronous sample rate converter.
Most of sound and timers related code is kept the same as in previous set. The code, related to snd_pcm_link() functionality and its using for timer source setting, is removed (as rejected earlier). The changes in this update are mainly related to the parameters handling and some cleanup.
The timer source can be initially selected by "timer_source" kernel module parameter. It is supposed to have the following format: [<pref>:](<card name>|<card idx>)[{.,}<dev idx>[{.,}<subdev idx>]] For example: "hw:I82801AAICH.1.0", or "1.1", or just "I82801AAICH". (Prefix is ignored, just allowed here to be able to use the strings, the user got used to). Although the parsing function recognizes both '.' and ',' as a separator, module parameters handling routines use ',' to separate parameters for different module instances (array elements), so we have to use '.' to separate device and subdevice numbers from the card name or number in module parameters. Empty string indicates using jiffies as a timer source.
Besides "static" selection of timer source at module load time, it is possible to dynamically change it via sound "info" interface (using "/proc/asound/<card>/timer_source" file in read-write mode. The contents of this file is used as a timer source string for a particular loopback card, e.g. "hw:0,0,0" (and here ',' can be used as a separator).
The timer source string value can be changed at any time, but it is latched by PCM substream open callback "loopback_open()" (the first one for a particular cable). At this point it is actually used, that is the string is parsed, and the timer is looked up and opened. This seems to be a good trade-off between flexibility of updates and synchronizations or racing complexity.
The timer source is set for a loopback card (the same as initial setting by module parameter), but every cable uses the value, current at the moment of opening. Theoretically, it's possible to set the timer source for each cable independently (via separate files), but it would be inconsistent with the initial setting via module parameters on a per-card basis.
v2: https://mailman.alsa-project.org/pipermail/alsa-devel/2019-November/157961.h...
v3: - Change sound card lookup to use snd_card_ref() and avoid direct access to sound cards array - Squash commits on returning error codes for timer start and stop - Some locking related fixes - Some code cleanup
Andrew Gabbasov (1): ALSA: aloop: Support runtime change of snd_timer via info interface
Timo Wischer (6): ALSA: aloop: Describe units of variables ALSA: aloop: Support return of error code for timer start and stop ALSA: aloop: Use callback functions for timer specific implementations ALSA: aloop: Rename all jiffies timer specific functions ALSA: aloop: Move CABLE_VALID_BOTH to the top of file ALSA: aloop: Support selection of snd_timer instead of jiffies
sound/drivers/aloop.c | 672 +++++++++++++++++++++++++++++++++++++++--- 1 file changed, 637 insertions(+), 35 deletions(-)