One of the Linux ALSA Sound Card Drivers: ALSA Architecture Overview

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I. Overview

ALSA is short for advanced Linux sound architecture and has become the mainstream audio architecture of Linux. I would like to learn more about ALSA's open-source project, for more information, see http://www.alsa-project.org /.

In the driver layer of the internal device, ALSA provides ALSA-driver. At the application layer, ALSA provides ALSA-lib and applications.ProgramYou only need to call the API provided by ALSA-lib to control the underlying audio hardware.

 

Figure 1.1 ALSA Software Architecture

As shown in Figure 1.1, ALSA-lib of the user space provides a unified API interface for the application, which hides the implementation details of the driver layer and simplifies the implementation of the application. In kernel space, ALSA-Soc is actually further encapsulation of ALSA-driver. It provides some column-enhanced functions for embedded devices. This series of blog posts only discuss ALSA-driver and ALSA-SOC in embedded systems.

Ii. ALSA device file structure

Let's start our ALSA journey from the device file structure of ALSA in Linux. Let's take a look at the structure of the device file of the Alsa driver in my computer:

$ CD/dev/SND
$ LS-l

CrW-RW ---- + 1 root audio 116, 8 controlc0
CrW-RW ---- + 1 root audio 116, 4 midic0d0
CrW-RW ---- + 1 root audio 116, 7 pcmc0d0c
CrW-RW ---- + 1 root audio 116, 6 pcmc0d0p
CrW-RW ---- + 1 root audio 116, 5 pcmc0d1p
CrW-RW ---- + 1 root audio 116, 3 seq
CrW-RW ---- + 1 root audio 116, 2 Timer
$

We can see the following device files:

• controlc0 --> used for sound card control, such as channel selection, sound mixing, and microphone Control
• midic0d0 --> used to play MIDI audio
• pcmc0d0c --> PCM device used for recording
• pcmc0d0p --> PCM device used for playback
• seq --> sequencer
• timer --> timer

c0d0 indicates the device 0 in sound card 0, pcmc0d0c the last C indicates capture, and pcmc0d0p The Last P indicates playback. These are naming rules in ALSA-driver. From the list above, we can see that my sound card has 6 devices mounted. According to the actual capacity of the Sound Card, more types of devices can be mounted to the driver, in the include/sound/core. h defines the following device types:

# Definesndrc v_dev_toplevel (_ force snd_device_type_t) 0) <br/> # definesndrc v_dev_control (_ force snd_device_type_t) 1) <br/> # define (_ force snd_device_type_t) 2) <br/> # definesndrc v_dev_lowlevel_normal (_ force snd_device_type_t) 0x1000) <br/> # definesndrc v_dev_pcm (_ force snd_device_type_t) 0x1001) <br/> # definesndrc v_dev_rawmidi (_ force snd_device_type_t) 0x1002) <br/> # definesndrc v_dev_timer (_ force snd_device_type_t) 0x1003) <br/> # definesndrc v_dev_sequencer (_ force snd_device_type_t) 0x1004) <br/> # definesndrc v_dev_hwdep (_ force snd_device_type_t) 0x1005) <br/> # definesndrc v_dev_info (_ force snd_device_type_t) 0x1006) <br/> # definesndrc v_dev_bus (_ force snd_device_type_t) 0x1007) <br/> # definesndrc v_dev_codec (_ force snd_device_type_t) 0x1008) <br/> # definesndrc v_dev_jack (_ force snd_device_type_t) 0x1009) <br/> # definesndrc v_dev_lowlevel (_ force snd_device_type_t) 0x2000)

Generally, we are more concerned with PCM and control devices.

 

3. Driver Code File structure

In the linux2.6 code tree, the structure of the Alsa code file is as follows:

Sound
/Core
/OSS
/Seq
/Ioctl32
/Include
/Drivers
/I2C
/Synth
/Emux
/PCI
/(CARDS)
/Isa
/(CARDS)
/ARM
/PPC
/ISCSI
/USB
/PCMCIA/(CARDS)
/OSS
/SOC
/Codecs

• Core this directory contains the intermediate layer of the Alsa driver, which is the core part of the entire ALSA driver
• Core/OSS contains PCM and mixer modules that simulate the old OSS Architecture
• Core/seq Code related to sequencer
• Include the public header file directory of the Alsa driver. The header file of this directory must be exported to applications in the user space. Generally, the private header file of the driver module should not be placed here.
• Drivers places some public code irrelevant to the CPU and bus architecture
• I2C ALSA's own I2C control code
• The top-level directory of the PCI sound card. The subdirectory contains various PCI sound card codes.
• The top-level directory of the ISA sound card. The subdirectory contains various ISA sound card codes.
• SOC's intermediate layer Code for the System-on-chip system
• SOC/codecs code for codec of SoC system, independent of Platform