CODEWITHSUNDEEP
androidaudiolinux-device-driveralsa
Dongguo
Dongguo

Reputation: 379

About ALSA driver: How to handle Audio Devices correctly

Recently ,I am trying to develop a alsa driver for Linux(actually ,Android TV).I came across a problem about alsa(alsa refers to Advanced Linux Sound Architecture) devices handling

Actually ,I have studied part of source code of alsa-lib-1.0.27.1, and tinyalsa(for android devices). And I studied audio driver of Samsung Devices.

User space application uses /dev/snd/pcmXXXXX to play/capture audio data. However, there is a problem.If there is only one audio devices,it is fine;

/dev/snd/
├── by-path
│   └── pci-0000:02:02.0 -> ../controlC0
├── controlC0
├── midiC0D0
├── pcmC0D0c
├── pcmC0D0p
├── pcmC0D1p
└── timer

But ,if there is more than one device(for instance,when I plug in a usb audio device).

/dev/snd/
├── by-path
│   └── pci-0000:02:02.0 -> ../controlC0
├── controlC0    
├── controlC1    //new Control channel
├── midiC0D0
├── pcmC0D0c
├── pcmC0D0p
├── pcmC0D1p
├── pcmC1D0c     //new card,new capture channel
├── pcmC1D0p     //new card,new playback channel
└── timer

How can user application handle these devices correctly.

I am really confused by Samsung Driver. ==>linux-3.5.4/device/samsung/crespo/libaudio/AudioHardware.cpp

status_t AudioHardware::AudioStreamInALSA::open_l()
{
//.....
    ALOGV("open pcm_in driver");
    TRACE_DRIVER_IN(DRV_PCM_OPEN)
    mPcm = pcm_open(0, 0, flags, &config); 
          //Why it always uses Card 0 and device 0.
          //does this mean it can not support multi-devices

Further more, I would really appreciate if you could share me a piece of source code for more information(for instance ,other drivers...)

thank you very much indeed.

Upvotes: 1

Views: 12396

Answers (1)

Michael
Michael

Reputation: 58427

mPcm = pcm_open(0, 0, flags, &config); 
      //Why it always uses Card 0 and device 0.
      //does this mean it can not support multi-devices

On the platforms I've worked with, all audio devices except for USB were located on card 0. I'm not saying that it's a good idea to hardcode card and device numbers - just that it's usually not going to be a real problem in practice. Until you start adding more cards..

On those platforms, the ALSA card/device number of the USB audio accessory would be picked up by the WiredAccessoryManager (or the UsbDeviceManager in the case of host-mode accessories) (this is in the Java layers of Android). So the information is available if you want to avoid having the audio HAL hardcoding the card/device numbers for USB audio accessories (but I suspect that there are implementations that simply assume card 1 for USB audio accessories).

Here's a link to the audio HAL implementation at the CodeAurora Forum, which serves as the sort-of reference implementation for a lot of Qualcomm's platforms. And here's the driver for host-mode USB audio accessories (Android Open Accessory).

Upvotes: 5

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