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Chapter 15. Sound Subsystem
Sound Subsystem
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 man: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 [.filename]#pcm# code.
In this chapter, the term [.filename]#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 https://people.FreeBSD.org/~cg/template.c[ https://people.FreeBSD.org/~cg/template.c]
All the relevant code lives in [.filename]#/usr/src/sys/dev/sound/#, except for the public ioctl interface definitions, found in [.filename]#/usr/src/sys/sys/soundcard.h#
Under [.filename]#/usr/src/sys/dev/sound/#, the [.filename]#pcm/# directory holds the central code, while the [.filename]#pci/#, [.filename]#isa/# and [.filename]#usb/# directories have the drivers for PCI and ISA boards, and for USB audio devices.
Probing, Attaching, etc.
Sound drivers probe and attach in almost the same way as any hardware driver module. You might want to look at the <<isa-driver,ISA>> or <<pci,PCI>> specific sections of the handbook for more information.
However, sound drivers differ in some ways:
They declare themselves as [.filename]#pcm# class devices, with a `struct snddev_info` device private structure: