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

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Date:	Thu, 11 Aug 2016 11:08:52 +0800
From:	Baolin Wang <baolin.wang@...aro.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Peter Chen <hzpeterchen@...il.com>,
	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 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.

-- 
Baolin.wang
Best Regards