Chapter 15. Sound Subsystem

Contributed by Jean-Francois Dockes.
Table of Contents
15.1. Introduction
15.2. Files
15.3. Probing, Attaching, etc.
15.4. Interfaces

15.1. Introduction

The FreeBSD sound subsystem cleanly separates generic sound handling issues from device-specific ones. This makes it easier to add support for new hardware.

The pcm(4) framework is the central piece of the sound subsystem. It mainly implements the following elements:

  • A system call interface (read, write, ioctls) to digitized sound and mixer functions. The ioctl command set is compatible with the legacy OSS or Voxware interface, allowing common multimedia applications to be ported without modification.

  • Common code for processing sound data (format conversions, virtual channels).

  • A uniform software interface to hardware-specific audio interface modules.

  • Additional support for some common hardware interfaces (ac97), or shared hardware-specific code (ex: ISA DMA routines).

The support for specific sound cards is implemented by hardware-specific drivers, which provide channel and mixer interfaces to plug into the generic pcm code.

In this chapter, the term pcm will refer to the central, common part of the sound driver, as opposed to the hardware-specific modules.

The prospective driver writer will of course want to start from an existing module and use the code as the ultimate reference. But, while the sound code is nice and clean, it is also mostly devoid of comments. This document tries to give an overview of the framework interface and answer some questions that may arise while adapting the existing code.

As an alternative, or in addition to starting from a working example, you can find a commented driver template at

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