linux-kernel - Re: [PATCH v2] usb: dwc2: resume root hub when device detect with suspend state

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Date:	Wed, 19 Nov 2014 11:00:06 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Julius Werner <jwerner@...omium.org>
cc:	Felipe Balbi <balbi@...com>,
	Mathias Nyman <mathias.nyman@...ux.intel.com>,
	Kever Yang <kever.yang@...k-chips.com>,
	Paul Zimmerman <paulz@...opsys.com>,
	Dinh Nguyen <dinguyen@...nsource.altera.com>,
	Romain Perier <romain.perier@...il.com>,
	Heiko Stuebner <heiko@...ech.de>,
	Douglas Anderson <dianders@...omium.org>,
	Sonny Rao <sonnyrao@...omium.org>,
	addy ke <addy.ke@...k-chips.com>,
	Eddie Cai <cf@...k-chips.com>, wulf <wulf@...k-chips.com>,
	Tao Huang <huangtao@...k-chips.com>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	Roy Li <roy.li@...k-chips.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] usb: dwc2: resume root hub when device detect with
 suspend state

On Tue, 18 Nov 2014, Julius Werner wrote:

> >> You should be aware that it's not safe to use hcd->state for anything
> >> in a host controller driver.  That field is owned by usbcore, not by
> >> the HCD, and it is not protected by any locks.
> >>
> >> Thus, for example, hcd->state does not get set to HC_STATE_SUSPENDED
> >> until some time after the bus_suspend routine has returned.  A
> >> port-change interrupt might occur during that time interval.
> 
> Looks like there is explicit code in hcd_bus_suspend() to check for
> that race condition right after setting hcd->state, or do I
> misinterpret that (the "Did we race with a root-hub wakeup event?"
> part)?

That code doesn't quite do what you think.  For example:

	CPU 1				CPU 2
	-----				-----
	hcd_bus_suspend():
	call hcd->bus_suspend():
	    root hub gets suspended

					Wakeup IRQ arrives and is
					  ignored because hcd->state
					  is still HC_STATE_QUIESCING

	set hcd->state to HC_STATE_SUSPENDED
	Did we race with a wakeup event?
	  No because usb_hcd_resume_root_hub()
	  wasn't called.

Result: the wakeup event is lost.

> Also, it seems xhci_bus_suspend() explicitly sets 'hcd->state =
> HC_STATE_SUSPENDED' before giving up the spinlock for some
> undocumented reason, maybe to avoid exactly this problem. We could
> just copy that trick if the hcd.c solution isn't enough (although the
> DWC2 bus_suspend/bus_resume in the other patch don't grab that
> spinlock right now, where I'm also not so sure if that's a good
> idea...).

It's better for HCDs to avoid testing hcd->state at all.  They should 
set it to appropriate values at the right times, because usbcore checks 
it, but they should not test it.  This is why ehci-hcd, ohci-hcd, and 
uhci-hcd all have a private rh_state variable, and they use it while 
holding their respective private spinlocks.

Alan Stern

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