A couple days ago, I found this excellent informational post, and that Mike Oliphant's patch is in mainline now: http://blog.nostatic.org/2020/01/getting-boss-gt-1-effects-processor-to.html

Since I was fairly certain it applies to the Roland devices I own, I added my USB IDs. Wonderfully, now USB audio capture seems to be working perfectly from the three devices I can test: the INTEGRA-7, Boutique D-05, & VG-99!

I don't know if the D-05 and VG-99 have digital audio playback/output, as I only tried it with the INTEGRA-7. It does work, but only when the INTEGRA-7's "Sampling Rate" is set to "44.1 kHz" and "Ext Part Source Select" is set to "USB AUDIO".

I searched a bit for other Roland devices people have had USB digital audio issues with over the years, and took the liberty of adding them to the patch too. Likely there are still many more, especially in the Boutique line, but I couldn't find their USB IDs.

I hope this can be added to the mainline kernel too, as this support should help many that have struggled. I, certainly, have been hoping somebody would figure this out for quite a while, and even bought a USB through/monitoring device to try to develop it myself (sadly, the software supporting that has its own bugs, so I probably should've just used wireshark). Suffice it to say, I'm quite happy to have this figured out, and to not have to delve into stuff I know nothing about! :)

Thanks,

Lucas