[alsa-devel] USB sound auto-suspend not working

[Takashi Iwai]

At Tue, 17 Dec 2013 14:55:48 -0800,
Sarah Sharp wrote:
> 
> Hi Oliver and Takashi,
> 
> I've noticed that in the last couple kernel releases or so, I can't get
> USB webcams to suspend.  It turns out that the USB sound interface is
> keeping the device active, even when the device is not playing sound.
> This goes back as far as 3.10, but I haven't tried older kernels.  This
> is testing on Ubuntu 13.10.
> 
> Commit 88a8516a2128a6d078a106ead48092240e8a138f "ALSA: usbaudio:
> implement USB autosuspend" went into kernel 2.6.39.  The commit message
> says the device is prevented from suspending if the pcm or midi channel
> files are open.
> 
> I plugged in a USB speaker, and ran lsof to see which files were open
> (output is attached).  AFAICT, only the USB sound device's control files
> are open, and I don't have any midi files.
> 
> Any ideas as to why USB sound auto-suspend isn't working?

I don't know of any intentional changes regarding autosuspend in
usb-audio since years.  The central points are snd_usb_autosuspend()
and snd_usb_autoresume() in sound/usb/card.c, so you can try to add
some debug prints there for tracking more deeply what's going wrong.


Takashi