usb mic stops working in Alsa after soft reboot, but will still work if pulseaudio started

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Summary

The USB microphone stops working with Alsa after a soft reboot, but works fine if PulseAudio is started. This issue does not occur after a cold boot or when the USB cable is unplugged and replugged.

Root Cause

The root cause of this issue is likely due to the USB bus not being properly reset after a soft reboot, causing the Alsa driver to fail to initialize the microphone correctly. The possible causes include:

  • Incomplete USB bus reset: The USB bus may not be fully reset after a soft reboot, leading to inconsistent device state.
  • Driver initialization issues: The Alsa driver may not be properly initializing the microphone device after a soft reboot.

Why This Happens in Real Systems

This issue can occur in real systems due to the following reasons:

  • Soft reboot: A soft reboot may not fully reset the system’s hardware, leading to inconsistent device state.
  • USB device persistence: The USB device may retain its state after a soft reboot, causing issues with device initialization.
  • Driver limitations: The Alsa driver may have limitations in handling device initialization after a soft reboot.

Real-World Impact

The impact of this issue includes:

  • Inconsistent audio recording: The microphone may not work consistently after a soft reboot, leading to failed audio recordings.
  • System instability: The issue may cause system instability, requiring manual intervention to resolve.
  • Difficulty in remote management: The issue can make it challenging to manage the system remotely, as the microphone may not work as expected.

Example or Code

arecord -l
arecord -d5 -D hw:1,0 -c2 -r16000 -fS16_LE /tmp/t.wav
pulseaudio &
arecord -d5 -D default -c2 -r16000 -fS16_LE /tmp/t.wav

How Senior Engineers Fix It

Senior engineers can fix this issue by:

  • Implementing a proper USB bus reset: Ensuring that the USB bus is fully reset after a soft reboot.
  • Modifying the Alsa driver: Updating the Alsa driver to properly handle device initialization after a soft reboot.
  • Using a workaround: Using a workaround, such as starting PulseAudio after a soft reboot, to ensure the microphone works correctly.

Why Juniors Miss It

Junior engineers may miss this issue due to:

  • Lack of understanding of USB bus dynamics: Not fully understanding how the USB bus works and how it can affect device state.
  • Insufficient knowledge of Alsa driver internals: Not being familiar with the Alsa driver’s internal workings and how it handles device initialization.
  • Inadequate testing: Not thoroughly testing the system after a soft reboot to identify potential issues.