Re: [PATCH v4] usb: xhci: quirk for data loss in ISOC transfers

["Rangoju, Raju" <raju.rangoju@amd.com>]

On 3/26/2025 12:17 PM, Michał Pecio wrote:
>>>> 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.
>

Hi Michal,
Sure, I'll take care of this in commit message when re spinning.

>>> 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.
>

If such an endpoint is implemented, it could theoretically contribute to 
scheduling conflicts similar to those caused by INT endpoints in this 
context. However, our observations and testing on affected platforms 
primarily involved periodic IN endpoints with service intervals greater 
than 32ms interfering with ISOC OUT endpoints.


>>> 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'm not completely sure about this corner case if HS OUT endpoints can 
inadvertently get affected when co-existing with long-interval LS/FS IN 
endpoints. Our IP vendor confirmed that LS/FS devices are not affected.

> (I assume that SS probably has no effect on HS schedule.)
> 
> Regards,
> Michal