[PATCH v2] USB: xhci: reducing HS port auto-resume latency.

[anshuman . gupta]

From: Anshuman Gupta <anshuman.gupta@xxxxxxxxx>

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

When xhci gets a HS 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_RESUME_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(when HZ=1000).

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

xhci bug behind the higher latency:
When HS port is resuming, xhci passes the resume event to hub event
thread after USB_RESUME_TIMEOUT delay. it modify rh_timer with
USB_RESUME_TIMEOUT delay, and set the polling mode flag and invoke
usb_hcd_poll_rh_status() function manually.
This eventually makes hub event thread to read the HS port status
before it resumes, since polling mode flag is set, host controller driver
modify rh_timer again with (HZ/4)ms delay, This makes hub event thread to
wait for (HZ/4)ms. which causes higher auto-resume latency.

This variable resume latency can be optimized, as in case of HS port
resume event rh_timer has already been modified with USB_RESUME_TIMEOUT
(40ms) delay, leaving the rest to GetPortStatus.
We can avoid calling usb_hcd_poll_rh_status() function manually in case
of HS port resume event.

This patch set the HCD_FLAG_POLL_RH to hcd->flags after modification of
rh_timer, and avoid calling usb_hcd_poll_rh_status(), This makes to call
usb_hcd_poll_rh_status() by rh_timer and passing the HS port resume event
to hub event thread after USB_RESUME_TIMEOUT delay.

Signed-off-by: Anshuman Gupta <anshuman.gupta@xxxxxxxxx>
---
Changes in v2
	- Make the commit message more clearer.
	- Changing the comment.

 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..0bccff1 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 calling usb_hcd_poll_rh_status before that,so
+			 * an usb port can auto-resume after resume time delay.
+			 */
+			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