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?
Sarah Sharp