Re: [PATCH] usb: xhci: Make usb_host_endpoint.hcpriv survive endpoint_disable()

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

On Wed, 1 Apr 2026 17:34:37 +0300, Mathias Nyman wrote:
> On 3/31/26 02:06, Michal Pecio wrote:
> > xHCI hardware maintains its endpoint state between add_endpoint()
> > and drop_endpoint() calls followed by successful check_bandwidth().
> > So does the driver.
> > 
> > Core may call endpoint_disable() during xHCI endpoint life, so don't
> > clear host_ep->hcpriv then, because this breaks endpoint_reset().
> > 
> > If a driver calls usb_set_interface(), submits URBs which make host
> > sequence state non-zero and calls usb_clear_halt(), the device clears
> > its sequence state but xhci_endpoint_reset() bails out. The next URB
> > malfunctions: USB2 loses one packet, USB3 gets Transaction Error or
> > may not complete at all on some (buggy?) HCs from ASMedia and AMD.
> > This is triggered by uvcvideo on bulk video devices.  
> 
> Were you able to trigger a usb_clear_halt() called with ep->hcpriv == NULL,
> causing a toggle/seq mismatch?
> 
> The ep->hcpriv should be set back correctly in usb_set_interface():
> 
> usb_set_interface()
>    usb_hcd_alloc_bandwidth()
>      hcd->driver->add_endpoint()
>        xhci_add_endpoint()
>          ep->hcpriv = udev;

right, and later:

     usb_disable_interface(dev, iface, true)
       usb_disable_endpoint(dev, ..., true)
         usb_hcd_disable_endpoint(dev, ep)
           hcd->driver->endpoint_disable(hcd, ep)
     usb_enable_interface(dev, iface, true)
       usb_enable_endpoint(dev, ..., true)
         usb_hcd_reset_endpoint(dev, ep)
           hcd->driver->endpoint_reset(hcd, ep)

So it seems set_interface() is broken and doesn't actually reset host
sequence state (while the device is supposed to reset its own).

This alone is rarely a problem because the endpoint is usually "fresh".

But uvcvideo calls usb_clear_halt() *after* stopping a bulk stream,
because that's what Windows does. Then sequence state is random and
gets cleared only on the device, because hcpriv is still NULL.

The next URB gets Transaction Error, the host endpoint is reset and
another URB succeeds (because we restart the endpoint unconditionally).

Regards,
Michal