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Date: Wed, 6 May 2015 10:30:55 -0400 (EDT)
From: Alan Stern <stern@rowland.harvard.edu>
X-X-Sender: stern@iolanthe.rowland.org
To: Lu Baolu <baolu.lu@linux.intel.com>
cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Mathias Nyman <mathias.nyman@intel.com>, <linux-usb@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 2/3] usb: xhci: implement device_suspend/device_resume
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On Wed, 6 May 2015, Lu Baolu wrote:

> This patch implements device_suspend/device_resume entries for xHC driver.
> device_suspend will be called when a USB device is about to suspend. It
> will issue a stop endpoint command for each endpoint in this device. The
> Suspend(SP) bit in the command TRB will set which will give xHC a hint
> about the suspend. device_resume will be called when a USB device is just
> resumed. It will ring doorbells of all endpoint unconditionally. XHC may
> use these suspend/resume hints to optimize its operation.
> 
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>

...

>  #else /* CONFIG_PM */
>  
>  int xhci_set_usb2_hardware_lpm(struct usb_hcd *hcd,
> @@ -4704,6 +4728,16 @@ int xhci_disable_usb3_lpm_timeout(struct usb_hcd *hcd,
>  {
>  	return 0;
>  }
> +
> +void xhci_device_suspend(struct usb_hcd *hcd,
> +			struct usb_device *udev, pm_message_t msg)
> +{
> +}
> +
> +void xhci_device_resume(struct usb_hcd *hcd,
> +			struct usb_device *udev, pm_message_t msg)
> +{
> +}

You don't need to have empty functions.  Just do this:

#define xhci_device_suspend	NULL
#define xhci_device_resume	NULL

in the appropriate place, when CONFIG_PM is not enabled.

Alan Stern