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.

The code was copied from ehci_endpoint_disable() but it isn't needed
here - hcpriv should only be NULL on emulated root hub endpoints.
It might prevent resetting and inadvertently enabling a disabled and
dropped endpoint, but core shouldn't try to reset dropped endpoints.

Document xhci requirements regarding hcpriv. They are currently met.

Fixes: 18b74067ac78 ("xhci: Fix use-after-free regression in xhci clear hub TT implementation")
Cc: stable@vger.kernel.org
Signed-off-by: Michal Pecio <michal.pecio@gmail.com>
---
 drivers/usb/host/xhci.c | 1 -
 include/linux/usb.h     | 3 ++-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
index 2f7e6544e5ae..849a568d0e63 100644
--- a/drivers/usb/host/xhci.c
+++ b/drivers/usb/host/xhci.c
@@ -3353,7 +3353,6 @@ static void xhci_endpoint_disable(struct usb_hcd *hcd,
 		xhci_dbg(xhci, "endpoint disable with ep_state 0x%x\n",
 			 ep->ep_state);
 done:
-	host_ep->hcpriv = NULL;
 	spin_unlock_irqrestore(&xhci->lock, flags);
 }
 
diff --git a/include/linux/usb.h b/include/linux/usb.h
index 04277af4bb9d..27e95eade121 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -54,7 +54,8 @@ struct ep_device;
  * @eusb2_isoc_ep_comp: eUSB2 isoc companion descriptor for this endpoint
  * @urb_list: urbs queued to this endpoint; maintained by usbcore
  * @hcpriv: for use by HCD; typically holds hardware dma queue head (QH)
- *	with one or more transfer descriptors (TDs) per urb
+ *	with one or more transfer descriptors (TDs) per urb; must be preserved
+ *	by core while BW is allocated for the endpoint
  * @ep_dev: ep_device for sysfs info
  * @extra: descriptors following this endpoint in the configuration
  * @extralen: how many bytes of "extra" are valid
-- 
2.48.1