Writing an ALSA Driver
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Writing an ALSA Driver

Takashi Iwai

<tiwai@suse.de>

Copyright (c) 2002-2005 Takashi Iwai <tiwai@suse.de>

This document is free; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This document is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

Abstract

This document describes how to write an ALSA (Advanced Linux Sound Architecture) driver.

Table of Contents

1. File Tree Structure

core directory

core/oss
core/ioctl32
core/seq
core/seq/oss
core/seq/instr

include directory

drivers directory

drivers/mpu401
drivers/opl3 and opl4

i2c directory

i2c/l3

synth directory

pci directory

isa directory

arm, ppc, and sparc directories

usb directory

pcmcia directory

oss directory

2. Basic Flow for PCI Drivers

Full Code Example

Constructor

1) Check and increment the device index.
2) Create a card instance
3) Create a main component
4) Set the driver ID and name strings.
5) Create other components, such as mixer, MIDI, etc.
6) Register the card instance.
7) Set the PCI driver data and return zero.

Header Files

3. Management of Cards and Components

Card Instance

Chip-Specific Data

1. Allocating via snd_card_new().
2. Allocating an extra device.

Registration and Release

4. PCI Resource Management

Full Code Example
Some Hafta's
Resource Allocation
PCI Entries

5. PCM Interface

Full Code Example

Constructor

... And the Destructor?

Runtime Pointer - The Chest of PCM Information

Hardware Description
PCM Configurations
DMA Buffer Information
Running Status
Private Data
Interrupt Callbacks

open callback
close callback
ioctl callback
hw_params callback
hw_free callback
prepare callback
trigger callback
pointer callback
copy and silence callbacks
ack callback
page callback

Interrupt Handler

Interrupts at the period (fragment) boundary
High frequency timer interrupts
On calling snd_pcm_period_elapsed()

Constraints

6. Control Interface

Definition of Controls

Control Names

Global capture and playback
Tone-controls
3D controls
Mic boost

Access Flags

info callback
get callback
put callback
Callbacks are not atomic

Constructor

Change Notification

7. API for AC97 Codec

General
Full Code Example
Constructor
Callbacks
Updating Registers in The Driver
Clock Adjustment
Proc Files
Multiple Codecs

8. MIDI (MPU401-UART) Interface

General
Constructor
Interrupt Handler

9. RawMIDI Interface

Constructor

open callback
close callback
trigger callback for output substreams
trigger callback for input substreams
drain callback

10. Miscellaneous Devices

FM OPL3
Hardware-Dependent Devices
IEC958 (S/PDIF)

11. Buffer and Memory Management

Buffer Types
External Hardware Buffers
Non-Contiguous Buffers
Vmalloc'ed Buffers

12. Proc Interface

13. Power Management

14. Module Parameters

15. How To Put Your Driver Into ALSA Tree

General
Driver with A Single Source File
Drivers with Several Source Files

16. Useful Functions

snd_printk() and friends
snd_BUG()
snd_BUG_ON()

17. Acknowledgments

List of Examples

1.1. ALSA File Tree Structure
2.1. Basic Flow for PCI Drivers - Example
4.1. PCI Resource Management Example
5.1. PCM Example Code
5.2. PCM Instance with a Destructor
5.3. Interrupt Handler Case #1
5.4. Interrupt Handler Case #2
5.5. Example of Hardware Constraints
5.6. Example of Hardware Constraints for Channels
5.7. Example of Hardware Constraints for Formats
6.1. Definition of a Control
6.2. Example of info callback
6.3. Example of get callback
6.4. Example of put callback
7.1. Example of AC97 Interface
15.1. Sample Makefile for a driver xyz

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