The root cause of the MSE is attributed to the ISOC OUT endpoint being omitted from scheduling. This can happen either when an IN endpoint with a 64ms service interval is pre-scheduled prior to the ISOC OUT endpoint or when the interval of the ISOC OUT endpoint is shorter than that of the IN endpoint.
To me this reads like the condition is
(IN ESIT >= 64ms && IN pre-scheduled before OUT) || (OUT ESIT < IN ESIT)
but I suspect it really is
(IN ESIT >= 64ms) && (IN pre-scheduled before OUT || OUT ESIT < IN ESIT)
because otherwise this workaround wouldn't really help: ISOC OUT ESIT < INT IN ESIT is almost always true in practice.
Moving "either" later maybe makes it more clear:
This can happen when an IN endpoint with a 64ms service interval either is pre-scheduled prior to the ISOC OUT endpoint or the interval of the ISOC OUT endpoint is shorter than that of the IN endpoint.
This code limits interval to 32ms for Interrupt endpoints (any speed), should it be isoc instead?
The affected transfer is ISOC. However, due to INT EP service interval of 64ms causing the ISO EP to be skipped, the WA is to reduce the INT EP service to be less than 64ms (32ms).
What if there is an ISOC IN endpoint with 64ms ESIT? I haven't yet seen such a slow isoc endpoint, but I think they are allowed by the spec. Your changelog suggests any periodic IN endpoint can trigger this bug.
Are Full-/Low-speed devices really also affected?
No, Full-/Low-speed devices are not affected.
The interesting question here is whether LS/FS devices with long interval IN endpoints can disrupt a HS OUT endpoint or not, because the patch solves the problem from the IN endpoint's side.
(I assume that SS probably has no effect on HS schedule.)
Regards, Michal