On Sat, Jun 20, 2020 at 10:38:33PM -0500, Sid Spry wrote: > When I was using a repurposed audio device I had to set an alternate mode. Is > that related to the issue here? The alternate mode seems to be a relic of the > descriptor layout before I dropped the device class and substituted my own > driver. The current descriptors specify no alternate modes. I don't know much about FunctionFS, so I can't help with your main question about isochronous packets not being dropped. But I can explain this. In the USB-2.0 spec, at the end of section 5.6.3 the text says: All device default interface settings must not include any isochronous endpoints with non-zero data payload sizes (specified via wMaxPacketSize in the endpoint descriptor). Alternate interface settings may specify non-zero data payload sizes for isochronous endpoints. That's why you had to select an alternate setting before transferring any isochronous data. Any isochronous endpoint in altsetting 0 must have its maxpacket size set to 0. Alan Stern
