linux-kernel - Re: [PATCH] USB:Fix ehci infinite suspend-resume loop issue in zhaoxin

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Date:   Tue, 5 Apr 2022 12:02:37 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     "WeitaoWang-oc@...oxin.com" <WeitaoWang-oc@...oxin.com>
Cc:     gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, CobeChen@...oxin.com,
        TimGuo@...oxin.com, tonywwang@...oxin.com, weitaowang@...oxin.com
Subject: Re: [PATCH] USB:Fix ehci infinite suspend-resume loop issue in
 zhaoxin

On Wed, Mar 16, 2022 at 10:18:39AM +0800, WeitaoWang-oc@...oxin.com wrote:
> On 2022/3/15 11:18, Alan Stern wrote:
> > On Tue, Mar 15, 2022 at 08:39:09PM +0800, WeitaoWang-oc@...oxin.com wrote:
> > > I have encountered the following situation if EHCI runtime suspend is
> > > enabled by default.

Some things about this still confuse me...

> > > 1.Wake from system to disk and boot OS.
> > 
> > You're talking about resuming after hibernation, right?
> 
> You're right.
> > > 2.EHCI will entry runtime suspend after enumerated by driver during boot
> > > phase of suspend to disk
> > 
> > I'm not sure what you mean by "boot phase of suspend to disk".  This is
> > while the restore kernel is starting up at the beginning of resume from
> > hibernation, right?
> > 
> You understood exactly what I was saying.

Okay, so we're waking up from hibernation.

> > > 3.EHCI will be placed to freeze state and ehci_resume is called after image
> > > is loaded.
> > 
> > ehci_resume is called to leave runtime suspend.  Going into the freeze
> > state doesn't require any changes.

In fact, the resume kernel doesn't call ehci_resume at all.  Here's what 
it does:

	The resume kernel boots;

	If your patch causes STS_PCD to be set at this point, the flag 
	should get cleared shortly afterward by ehci_irq;

	ehci-hcd goes into runtime suspend;

	The kernel reads the system image that was stored earlier when
	hibernation began;

	After the image is loaded, the system goes into the freeze
	state (this does not call any routines in ehci-hcd);

	The resume kernel transfers control to the image kernel;

Now the image kernel is running.  It goes into the restore state, which 
involves calling ehci_resume.  Presumably your patch cases the STS_PCD 
flag to get set at this point.

But that's all!  The system is now back up, out of hibernation, and 
running normally.  There are no more calls to check_root_hub_suspended

> > > 4.If PCD flag is set(caused by patch), then HCD_FLAG_RH_RUNNING will be set.
> > > 
> > > 5.Pci_pm_freeze_noirq is called to check ehci root hub state and return
> > > value is -EBUSY. which will cause
> > >   quiesce phase of suspend to disk fail.
> > 
> > You're talking about check_root_hub_suspended() in hcd-pci.c, right?
> > 
> It's right.

So how can this happen, given that the image kernel doesn't call 
pci_pm_freeze_noirq?

Alan Stern