[PATCH] usb: reducing an usb-port auto-resume latency.

[anshuman.gupta@intel.com]

From: Anshuman Gupta <anshuman.gupta@intel.com>

This patch will improve the variable auto-resume latency of an usb-port.

When xhci gets a port status change event interrupt due to PORT_PLC
(port link state transition), linux Host controller driver drives the
resume signalling on the bus for the amount of time defined by
USB_REUME_TIMEOUT(40ms) macro.

This 40ms delay for resume signalling is in acceptable limit, but
it get worse when xhci goes for polling mode in order to detect other
events on its ports and modify rh_timer timer with a variable time out of
1ms to (HZ/4)ms.

drivers/usb/core/hcd.c line 799
mod_timer (&hcd->rh_timer, (jiffies/(HZ/4) + 1) * (HZ/4)).

Due to above variable timeout usb auto-resume latency varies from
40ms to ~300ms.

Log Snippet:
~128ms latency
[   53.112049] hub 1-0:1.0: state 7 ports 12 chg 0000 evt 0000
[   53.229200] hub 1-0:1.0: state 7 ports 12 chg 0000 evt 0004
[   53.240177] usb 1-2: usb wakeup-resume
[   53.240195] usb 1-2: finish resume
[   53.240357] usb usb1-port2: resume, status 0
-----------------------------------------------------------------
~300ms latency
[   59.946620] hub 1-0:1.0: state 7 ports 12 chg 0000 evt 0000
[   59.979341] hub 1-0:1.0: state 7 ports 12 chg 0000 evt 0000
[   60.229342] hub 1-0:1.0: state 7 ports 12 chg 0000 evt 0004
[   60.251321] usb 1-2: usb wakeup-resume
[   60.251335] usb 1-2: finish resume
[   60.251539] usb usb1-port2: resume, status 0

This variable resume latency can be optimized, as in case of PORT_PLC
change event rh_timer has already been modified with USB_RESUME_TIMEOUT
(40ms) delay,leaving the rest to GetPortStatus and started polling for
root hub status (invoking usb_hcd_poll_rh_status).
We can avoid polling as we have already modified rh_timer with
delay of 40ms.

This patch set the HCD_FLAG_POLL_RH to hcd->flags after modification of
rh_timer, and avoids polling of root hub status. so rh_timer can fire
after 40ms and usb device auto-resuem latency will be around 40ms.

Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
---
 drivers/usb/host/xhci-ring.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index cc368ad..3b7349a 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -1677,9 +1677,14 @@ static void handle_port_status(struct xhci_hcd *xhci,
 			bus_state->resume_done[faked_port_index] = jiffies +
 				msecs_to_jiffies(USB_RESUME_TIMEOUT);
 			set_bit(faked_port_index, &bus_state->resuming_ports);
+			/* Do the rest in GetPortStatus after resume time delay.
+			 * Avoid polling roothub status before that so that a
+			 * usb device auto-resume latency around ~40ms.
+			 */
+			set_bit(HCD_FLAG_POLL_RH, &hcd->flags);
 			mod_timer(&hcd->rh_timer,
 				  bus_state->resume_done[faked_port_index]);
-			/* Do the rest in GetPortStatus */
+			bogus_port_status = true;
 		}
 	}
 
-- 
2.7.4