A high speed control or bulk endpoint may have bInterval set to zero,
which means it does not NAK.  If bInterval is non-zero, it means the
endpoint NAKs at a rate of 2^(bInterval - 1).

The xHCI code to compute the NAK interval does not handle the special
case of zero properly.  The current code unconditionally subtracts one
from bInterval and uses it as an exponent.  This causes a very large
bInterval to be used, and warning messages like these will be printed:

usb 1-1: ep 0x1 - rounding interval to 32768 microframes, ep desc says 0 microframes

This may cause the xHCI host hardware to reject the Configure Endpoint
command, which means the HS device will be unusable under xHCI ports.

This patch should be backported to kernels as old as 2.6.31, that contain
commit dfa49c4a "USB: xhci - fix math in
xhci_get_endpoint_interval()".
Reported-by: Vincent Pelletier <plr.vincent@gmail.com>
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org