linux-kernel - Re: [PATCH] usb: core: Add runtime resume checking

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Message-ID: <CAMz4kuLXJeYBTPXP+ZS8pHei3pygdjiL0KvrrF_2AUsRRG4TTw@mail.gmail.com>
Date:	Thu, 11 Aug 2016 16:07:21 +0800
From:	Baolin Wang <baolin.wang@...aro.org>
To:	Peter Chen <hzpeterchen@...il.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Greg KH <gregkh@...uxfoundation.org>, stefan.koch10@...il.com,
	Oliver Neukum <oneukum@...e.com>,
	Muhammad Falak R Wani <falakreyaz@...il.com>,
	Mark Brown <broonie@...nel.org>,
	USB <linux-usb@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: core: Add runtime resume checking

Hi Peter,

On 11 August 2016 at 14:54, Peter Chen <hzpeterchen@...il.com> wrote:
> On Thu, Aug 11, 2016 at 11:08:52AM +0800, Baolin Wang wrote:
>> Hi Alan,
>>
>> On 10 August 2016 at 22:25, Alan Stern <stern@...land.harvard.edu> wrote:
>> > On Wed, 10 Aug 2016, Baolin Wang wrote:
>> >
>> >> >> >> For example: No slave attached----> usb interface runtime suspend
>> >> >> >> ----> usb device runtime suspend -----> xhci suspend -----> power off
>> >> >> >> usb controller. After that if the system wants to enter suspend state,
>> >> >> >> then it also will issue usb_dev_suspend(), then the
>> >> >> >> pm_runtime_resume() function (issued in choose_wakeup() function) will
>> >> >> >> return -ESHUTDOWN due to xhci has been suspend and hardware is not
>> >> >> >> accessible.
>> >> >> >
>> >> >> > Why the controller does not be resumed when the root hub has issued
>> >> >> > runtime resume?
>> >> >>
>> >> >> Because now there is no slave attached, we will not power on usb
>> >> >> controller and resume xhci.
>> >> >>
>> >> >
>> >> > I don't know why you set policy like this. Even the controller is
>> >> > resumed, it will still be suspended by system suspend. What's more,
>> >> > if you disallow it, how can you change your wakeup setting?
>> >> > Eg, at runtime suspend, we enable wakeup by default. But for system
>> >> > sleep, we disable wakeup by default.
>> >> >
>> >> > At runtime, if there is no device on the port, even we resumes the
>> >> > controller and roohub, it still will be suspended soon.
>> >>
>> >> For saving power, if there is no slave attached, why we need to power
>> >> on usb controller and resume xhci? Especially for mobile device.
>> >
>> > You need to power-on the USB controller in order to change the wakeup
>> > setting.
>>
>> pm_runtime_resume() function issued in choose_wakeup() can just resume
>> usb device, which can not power-on the USB controller and resume xHCI.
>> Cause the user won't know you need to power-on usb controller now,
>> then how to power-on the USB controller when changing wakeup setting?
>>
>> >
>> >> Moreover the usb device runtime suspend/resume is separate with xhci
>> >> suspend/resume, when xhci entered suspend state, only slave attaching
>> >> can resume xhci.
>> >
>> > That's right.  Suppose the user wants the system to stay asleep when a
>> > slave attaches.  But when you suspended the xHCI controller, it was set
>> > to wake up when a new slave attaches.  So when a slave is attached, the
>> > controller will wake up the system.  That's bad.
>> >
>> > If the wakeup setting needs to be changed when the system suspend
>> > begins, then you have to power-on the controller to make that change.
>> >
>> > Given a choice between using a little power or behaving incorrectly,
>> > you should choose to use the power.
>>
>> OK. But that is a real problem. It will pm_runtime_resume() falied
>> (issued in choose_wakeup()), cause usb controller has powered-off and
>> xHCI controller has suspended and we have no method to notify the user
>> to power-on USB controller. Any good suggestion to solve this, Alan
>> and Peter? Thanks.
>>
>
> Maybe you can show us the call stack why pm_runtime_resume has failed
> at your environment.

OK. I try to explain it at below:

For example: No slave attached----> usb interface runtime suspend
----> usb device runtime suspend (routine is:
usb_suspend_device()--->generic_suspend() --> hcd_bus_suspend()-->
xhci_bus_suspend()) -----> xhci_ suspend() -----> power off usb
controller. After that if the system wants to enter suspend state,
then it also will issue usb_dev_suspend(), then the
pm_runtime_resume() function (issued in choose_wakeup() function) will
return -ESHUTDOWN due to xhci has been suspend and hardware is not accessible.

We issue the pm_runtime_resume() function routine: usb_resume_device()
----> generic_resume() ----> hcd_bus_resume() ---> xhci_bus_resume(),
but now xHCI is not accessible due to xhci_suspend() is issued and USB
controller is power off when no slave attached. That is why
pm_runtime_resume failed.

-- 
Baolin.wang
Best Regards