🎙️Mikey, an audio loopback, virtual linux soundcard driver.

Warning

This is a work-in-progress project.

Inspired by v4l2loopback, Mikey functions as a loopback device that is capable of receiving audio playback and outputting it to other applications as if it were a microphone. It can be used to test userspace audio programs as well, which is a common use of virtual drivers.

In-tree kernel module snd_aloop has richer functionality, but I think I can do it in a simpler way so that anyone can grab the code and modify it. Mikey is a work-in-progress project that takes some references from in-tree snd_pcmtest module and vsnd. Currently a playback device and a capture device can be listed from aplay -l and arecord -l after inserting the module.

Testing

This kernel module is not battle-tested yet so I recommend using a virtual environment like vritme-ng to test it until it is reliable enough. If snd_pcm module has not been loaded, use modprobe snd_pcm to load it first. Test Mikey with aplay -D plughw:CARD=mikey,DEV=0 /path/to/audio/file.

ToDo

• Use timer interrupt to simulate hardware interrupt for updating substream buffer (Currently it would hang the kernel)
• Loopback pcm data
• Make capture and playback device share the same buffer (SPSC queue) to reduce data copying
• Accept control options through ioctl

Writing an ALSA soundcard driver

Since there is a lack of materials on this topic on the internet, here is my note on writing Mikey. Notice that it may not be as complete as a document like the one in kernel, but it should be more beginner friendly.

(To be continued...)