Re: [RFT PATCH] xhci: fix frame id calculation for isoc transfer

[Michal Pecio <michal.pecio@gmail.com>]

On Fri, 15 May 2026 14:13:14 -0400, Dylan Robinson wrote:
> On Fri, May 15, 2026 at 12:32 AM Michal Pecio wrote:
> > Did you try with short enough playback buffer to get occasional xHCI
> > ring underruns?
> >
> > Is Frame ID still correct then?  
> 
> Good point, no, it does not remain correct.
> 
> I repeated the test using 1-microframe-long URBs in order to induce
> ring underruns/overruns.

Underruns are the most important thing to test here, both with CFC and
without. Under normal conditions these patches have no effect (besides
possibility of blatant bugs), but when the ring underruns we want to
know how the HW and SW will react to it.

> When the URB_ISO_ASAP flag is not set, urb->start_frame is initially
> accurate, but drifts further and further off following each ring
> underrun/overrun event. In the completion handler, urb->start_frame
> appears to remain continuous with the previous URB even after
> underrun/overrun events occur. The URB completion status and the
> isochronous frame descriptor statuses always report success.

Yes, this is very annoying because it's undetectable (without CFC).
It seems we need to handle Ring Underrun/Overrun events, somehow.

So far the only successful handling I found was to delay completion of
the next URB by 1ms, which caused snd-usb-audio to underrun its buffer
and restart the stream :)

I also tried:

1. Violate the 1.0 spec and write Frame IDs instead of SIA to all TDs.
HCs respond oddly, some skip more than they should, some get stuck when
Frame ID is in the past (probably wait for MFINDEX overflow).

2. Queue initial TDs of the next URB as No-Op TRBs and use Frame ID in
the first TD which meets IST. But HCs handle isoc No-Op TDs oddly too:
- Renesas ignores Frame ID in an Isoch TD following a No-Op TD
- Fresco executes only one No-Op per interval
- VIA re-runs the No-Os sequence each interval until Frame ID is met
  in the subsequent Isoch TD

3. Don't queue inital TDs at all. This turns out to violate assumptions
   made in various parts of the driver. PITA, WiP, kernel panics etc.

Regards,
Michal