linux-kernel - Re: [PATCH v2 1/3] usb: notify hcd when USB device suspend or resume

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Date:	Sat, 09 May 2015 08:42:00 +0800
From:	"Lu, Baolu" <baolu.lu@...ux.intel.com>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mathias Nyman <mathias.nyman@...el.com>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] usb: notify hcd when USB device suspend or resume

On 05/08/2015 10:21 PM, Alan Stern wrote:
> On Fri, 8 May 2015, Lu, Baolu wrote:
>
>> On 05/07/2015 10:34 PM, Alan Stern wrote:
>>> On Thu, 7 May 2015, Lu, Baolu wrote:
>>>
>>>>>> +	void	(*device_suspend)(struct usb_hcd *, struct usb_device *udev,
>>>>>> +			pm_message_t msg);
>>>>>> +	void	(*device_resume)(struct usb_hcd *, struct usb_device *udev,
>>>>>> +			pm_message_t msg);
>>>>>>     };
>>>>> Your callbacks don't use the msg argument.  What makes you think it is
>>>>> needed?
>>>> This msg argument is valuable. XHCI spec defines a capability named FSC
>>>> (Force Save context Capability). When this capability is implemented, the
>>>> Save State operation (do during host suspend) shall save any cached Slot,
>>>> Endpoint, Stream or other Context information to memory. xHCI hcd could
>>>> use this "msg" to determine whether it needs to issue stop endpoint with
>>>> SP (suspend) bit set.
>>> I don't understand.  What is the advantage of using FSC?
>> I'm sorry, I didn't make it clear.
>>
>> As part of host suspend, controller save state will be issued to save
>> host internal state in xhci_suspend():
> ...
>
>> If FSC is supported,  the cached Slot, Endpoint, Stream, or other
>> Context information are also saved.
>>
>> Hence, when FSC is supported, software does not have to issue Stop
>> Endpoint Command to push public and private endpoint state into
>> memory as part of system suspend process.
> Why do you have to push this state into memory at all?  Does the
> controller hardware lose the cached state information when it is in low
> power?

I don't think controller hardware will lose the cached state information
when it is in low power. But since cache in controller consumes power
and resources, by pushing state into memory, hardware can power
off the cache logic during suspend. Hence more power saving gains.

>
>> The logic in xhci_device_suspend() will look like:
>>
>> if xhci_device_suspend() callback was called due to system suspend,
>> (mesg.event & PM_EVENT_SUSPEND is true) and FSC is supported by
>> the xHC implementation, xhci_device_suspend() could ignore stop
>> endpoint command, since CSS will be done in xhc_suspend() later and
>> where all the endpoint caches will be pushed to memory.
> I still don't understand this.  You said earlier that according
> to section 4.15.1.1 of the xHCI spec, the endpoint rings should
> _always_ be stopped with SP set when a device is suspended.  Now you're

The intention of stop endpoint with SP set is to tell hardware that
"a device is going to suspend, hardware don't need to contain the
endpoint state in internal cache anymore". Hardware _could_ use
this hint to push endpoint state into memory to reduce power
consumption.

> saying that they don't need to be stopped during a system suspend if
> the controller supports FSC.  (Or maybe you're saying they need to be
> stopped but SP doesn't need to be set -- it's hard to tell.)

Even FSC is supported, controller hardware still need to push cached
endpoint state into memory when a USB device is suspended. The
difference is when FSC is enforced, CSS command will push any
cached endpoint state into memory unconditionally.

So, when xhci_device_suspend() knows that CSS command will be
executed later and CSS command will push cached endpoint state
into memory (a.k.a. FSC is enforced), it could skip issuing stop
endpoint command with SP bit set to avoid duplication and reduce
the suspend time.

This is the case for system suspend since CSS command is part of
xhci_suspend() and xhci_suspend() will be executed after all USB
devices have been suspended. But it's not case for run-time suspend
(auto-pm) since USB device suspend and host controller suspend
are independent for run-time case.

That's the reason why I wanted to keep 'msg' parameter. But just as
Greg said, we don't need to keep a parameter when it's not used
and can add it later when it is required.

>
> So which is it?  Do you need to stop the endpoint rings?  Is it okay
> not to set SP?

"stop endpoint" and "stop endpoint with SP set" serve different purposes
in Linux xhci driver as my understanding. "stop endpoint" command is
used to stop a active ring when upper layer want to cancel a URB.
"stop endpoint with SP set" is used to hint hardware that a USB is going
to suspend. Hence "stop endpoint with SP set" must be executed in case
that the transfer ring is empty.

>
> Alan Stern

Thank you,
Baolu
>
>
>

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