Obtaining a USB Device File Path from ALSA

naftulikay

I asked this question on StackOverflow but I have had a rough time finding any answers.

I'm essentially writing a library/CLI for managing Focusrite Scarlett USB audio interfaces. I've been mostly able to do all of this using ALSA (snd_ctl_t, snd_hctl_t, snd_mixer_t, etc.) but one of the things I have not been able to do is essentially determine a device file for the USB device which provides the ALSA audio interface.

The closest thing I can get is snd_ctl_card_info_t, which lets me see that the device is a USB device, and exposes some info about it like the USB product/vendor ID and a string like usb-0000:00:14.0-2.2 within the longname of the device. In most cases, the USB product/vendor ID would be enough to uniquely identify a device, but in this case, it isn't: I myself, and certainly studios, are going to want to apply separate configuration/controls for different cards that probably have the same vendor/product IDs. Hence, I need a USB device handle, preferably a stable way to obtain that, so I can then extract the iSerial field from the device. A typical use-case with these devices is to use ADAT over optical to daisy chain devices together for more inputs/outputs, with both devices plugged in over USB to allow them to be configured.

I've been up and down the entire ALSA API but haven't found a stable way to do this, other than parsing values from snd_ctl_card_info_t and hoping that these will be stable somehow. Is there any (hopefully) stable way to take an ALSA handle of some kind and get to an actual USB device file path?