======================
Writing an ALSA Driver
======================

:Author: Takashi Iwai <tiwai@suse.de>

Preface
=======

This document describes how to write an `ALSA (Advanced Linux Sound
Architecture) <http://www.alsa-project.org/>`__ driver. The document
focuses mainly on PCI soundcards. In the case of other device types, the
API might be different, too. However, at least the ALSA kernel API is
consistent, and therefore it would be still a bit help for writing them.

This document targets people who already have enough C language skills
and have basic linux kernel programming knowledge. This document doesn't
explain the general topic of linux kernel coding and doesn't cover
low-level driver implementation details. It only describes the standard
way to write a PCI sound driver on ALSA.

File Tree Structure
===================

General
-------

The file tree structure of ALSA driver is depicted below::

            sound
                    /core
                            /oss
                            /seq
                                    /oss
                    /include
                    /drivers
                            /mpu401
                            /opl3
                    /i2c
                    /synth
                            /emux
                    /pci
                            /(cards)
                    /isa
                            /(cards)
                    /arm
                    /ppc
                    /sparc
                    /usb
                    /pcmcia /(cards)
                    /soc
                    /oss


core directory
--------------

This directory contains the middle layer which is the heart of ALSA
drivers. In this directory, the native ALSA modules are stored. The
sub-directories contain different modules and are dependent upon the
kernel config.

core/oss
~~~~~~~~

The code for OSS PCM and mixer emulation modules is stored in this
directory. The OSS rawmidi emulation is included in the ALSA rawmidi
code since it's quite small. The sequencer code is stored in
``core/seq/oss`` directory (see `below <core/seq/oss_>`__).

core/seq
~~~~~~~~

This directory and its sub-directories are for the ALSA sequencer. This
directory contains the sequencer core and primary sequencer modules such
as snd-seq-midi, snd-seq-virmidi, etc. They are compiled only when
``CONFIG_SND_SEQUENCER`` is set in the kernel config.

core/seq/oss
~~~~~~~~~~~~

This contains the OSS sequencer emulation code.

include directory
-----------------

This is the place for the pub                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     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