Thanks to Andreas, I got my hands on a Playback Design MPD-3 and hacked up some small hanges necessary to make it fully work with snd-usb.
The device reports 0x80000000 in the bmFormats field of two of three of its alternate settings, which wrongly made the driver believe it's a usual PCM endpoint (actually due to a fix-up fallback). However, bit 31 of this mask in fact denotes 'raw data'.
The effect of this issue is that in addition to the first altsetting with a bit depth of 24, the driver exposed 8-bit and 16-bit native audio formats on the raw data endpoints, which do not work as expected.
Also, those devices need a 50ms delay after switching the USB interface.
Thanks, Daniel
Daniel Mack (3): ALSA: snd-usb: handle the bmFormats field as unsigned int ALSA: snd-usb: handle raw data format of UAC2 devices ALSA: snd-usb: add delay quirk for "Playback Design" products
include/linux/usb/audio-v2.h | 2 ++ sound/usb/format.c | 20 ++++++++++++-------- sound/usb/format.h | 2 +- sound/usb/pcm.c | 7 +++++++ sound/usb/stream.c | 2 +- 5 files changed, 23 insertions(+), 10 deletions(-)