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

[Mathias Nyman <mathias.nyman@linux.intel.com>]

On 23.08.2017 09:18, anshuman.gupta@intel.com wrote:
> 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>
> ---

Thanks,
adding and sending forward after 4.14-rc1

-Mathias