Re: [PATCH] usb: xhci: Fix isochronous scheduling regression

[Michal Pecio]

On Fri, 21 Aug 2026 10:44:07 -0400, Alan Stern wrote:
> Maybe it's time to correct the hcd_periodic_completion_in_progress() 
> implementation.
> 
> For instance, we could add an atomic giveback_count field to the 
> usb_host_endpoint struct.  The HCD would increment the field (while 
> still holding its private lock) before doing a giveback, and 
> __usb_hcd_giveback_urb() would decrement the field after calling the 
> completion handler.
> 
> What do you think?

I would go as far as incrementing it on successful usb_submit_urb() and
completely doing away with those list_empty(td_list) checks in HCDs.

I wrote an xhci-only (less compilation and module reloading) prototype
which relies on hijacking completions of isoc URBs for counting, it
worked, results identical as with the standard solution in a few test
runs with snd-usb-audio.

Theoretical race condition: it seems we can't prevent new submissions
after completion releases its lock and class driver considers the pipe
idle, but before the counter is decremented to zero. That would be
another case of "scheduling to the past" unexpectedly.

Seems low probability, but this type of bug hasn't existed so far, we
generally have the opposite problem.

Or does it exist in non-BH HCDs?

Maybe the documented API guarantee just isn't feasible?

Entirely out of the box alternative: new URB flag. And really, if
only xhci-hcd existed, it wouldn't be hard to even implement explicit
start_frame requests or hints as MOTU wished for, which would make
the whole business of starting synchronized endpoints cleaner.

Regards,
Michal