Driving One's Own Audio Device

In this article Alessandro will show the design and implementation of a custom audio device, paying particular attention to the software driver. The driver, as usual, is developed as a kernel module. Even though Linux 2.2 will be out by the time you read this, the software described here works only with Linux 2.0 and the first few decades of 2.1 versions.
Other Device Methods

The device driver interface offers other device methods in addition to the open/close and read/write pairs. While none of them is critical to device operation, I usually add a few lines of code to implement select and lseek. The former is needed by those programs which multiplex several input/output channels or use non-blocking operations to read and write data. Its role is quite needed if you run real programs, and the implementation is straightforward enough that I won't show it here. The implementation of lseek, on the other hand, consists of the one line return -ESPIPE; and is meant to tell any program that tries to lseek the device that this “is a pipe” (reported to user space as “Illegal seek”).

Related Stuff

My aversion to computer sound makes me a novice in the field, and I really don't know anything about programs that play audio, or sites where audio files can be retrieved. Although Linus Torvalds offered an interesting “I pronounce Linux as Linux”, the file was not enough to test my device, and I needed to generate some audio data. The result is the sad distribution includes a program that plays sinusoidal waves, one that plays square waves and a not-so-good piano implementation. These tools work with any /dev/audio you happen to run and can be fun to play with, especially if you have a scope near your Linux box.

All code for the sad program is available by anonymous download in the file ftp.linuxjournal.com/pub/lj/listings/issue53/2997.tgz.

Alessandro Rubini tries to develop Open Source software/hardware for a living and that's why he and other hackers founded “Prosa Srl”. He can be reached at rubini@prosa.it, in addition to the usual addresses rubini@linux.it and rubini@systemy.it.

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