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

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Date:	Thu, 7 May 2015 10:34:20 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Lu, Baolu" <baolu.lu@...ux.intel.com>
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 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?

How would xhci-hcd use "msg" to determine this?  And why doesn't your
2/3 patch already do it?

Alan Stern

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