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

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Date:   Wed, 6 Apr 2022 10:38:28 +0800
From:   "WeitaoWang-oc@...oxin.com" <WeitaoWang-oc@...oxin.com>
To:     Alan Stern <stern@...land.harvard.edu>
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 2022/4/6 00:02, Alan Stern wrote:
> 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);
On this phase, pci_pm_freeze will be called for pci device. In this
function, pm_runtime_resume will be called to resume already
runtime-suspend devices. which will cause ehci_resume to be called.
Thus STS_PCD flag will be set in ehci_resume function.

Thanks
Weitao Wang
> 
> 	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.
> o
> 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
> .