Re: [Query] checking hub port status while USB 2.0 port is resuming.

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On Tue, 22 May 2018, Anshuman Gupta wrote:

> On Tue, May 22, 2018 at 09:53:09AM -0400, Alan Stern wrote:
> > On Tue, 22 May 2018, Anshuman Gupta wrote:
> >
> Thanks Alan for your quick response. 
> > > Hi,
> > > 
> > > When a xhci USB2.0 port is resuming and waiting for resume signaling, completion of
> > > USB_RESUME_TIMEOUT, it just reports the port status as USB_PORT_STAT_SUSPEND,
> > > and let the usbcore to check the port status again.
> > > 
> > > https://elixir.bootlin.com/linux/v4.17-rc6/source/drivers/usb/host/xhci-hub.c#L960
> > > 
> > > 
> > > USB core just checks the port status in usb_port_resume function
> > > and if port status is in suspended state, it clears the port suspend feature. 
> > > 
> > > https://elixir.bootlin.com/linux/v4.17-rc6/source/drivers/usb/core/hub.c#L3441
> > > 
> > > But when system resumes due to a USB remote wakeup event from suspend state,
> > > and port which causes remote wakeup is still waiting for resume signaling,
> > > usb_port_resume function in this particular case clears the port suspend feature,
> > > and wait for 40ms again(USB_RESUME_TIMEOUT).
> > > 
> > > Query regarding above case:
> > > Here in this case USB core will miss the wake up event from the port which causes
> > > the remote wake.
> > > https://elixir.bootlin.com/linux/v4.17-rc6/source/drivers/usb/core/hub.c#L3444
> > 
> > What do you mean?  You said above: "system resumes due to a USB remote
> > wakeup event".  The fact that the system is resuming indicates that it
> > did _not_ miss the wakeup event.
> > 
> What was i meaning, if system come out of suspend due to a key pressed on usb keyboard.
> For the above mention case, it will not raise any pm_wakeup_event so the notification
> of the pm_wakeup_event for usb keyboard will get missed.

Okay, let's say you're right.  What difference does it make if the
pm_wakeup_event gets lost?  Will that cause a problem?

Alan Stern

> > > If a port is already resuming and waiting for resume signaling, does it make
> > > sense to clear the port suspend feature and wait for 40ms again.
> > 
> > When the system resumes from a suspend state, it wakes up _all_ the
> > suspended USB ports.  Waiting an extra 40 ms for one port won't make
> > much difference.
> > 
> > Alan Stern

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