Disconnecting certain USB interfaces causes the sound card to go away

[Robert Wu]

Hi ALSA devs,

I work on Android and starting from Android T, we use libusb to connect to
an MIDI peripheral when Android is in host mode. We do this by opening the
device, disconnecting the kernel MIDI interface, claiming the MIDI
interface, and using its endpoints.

However, we started getting bug reports from customers that some USB
peripherals stop playing audio once MIDI is opened.

Disconnecting the kernel MIDI interface with USBDEVFS_DISCONNECT causes the
USB sound card to disappear for some devices. I tested with 6 USB
interfaces with both MIDI and audio and I found the following.

1. I can disconnect the Audio Control, Audio Streaming, and Midi Streaming
interfaces on the Komplete Audio 6 and the Alesis iO4 without the sound
card disappearing.
2. I can disconnect the Audio Streaming and MIDI Streaming interfaces on
the PreSonus USB 96 without the sound card disappearing. Disconnecting the
Audio Control interface causes the sound card to disappear.
3. I can disconnect the Audio Streaming interface on the PreSonus Studio
26c, Yamaha AG06, and the Focusrite Scarlett 2i4 without the sound card
disappearing. Disconnecting the Audio Control or MIDI Streaming interfaces
causes the sound card to disappear.

After calling USBDEVFS_RESET, the sound cards come back up.

Does anyone know why certain USB peripherals do not handle
USBDEVFS_DISCONNECT cleanly on certain interfaces?

Thanks,
Robert