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

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Date:	Wed, 6 May 2015 10:35:06 -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 Wed, 6 May 2015, Lu Baolu wrote:

> This patch adds two new entries in hc_driver. With these new entries,
> USB core can notify host driver when a USB device is about to suspend
> or just resumed.
> 
> The xHCI spec is designed to allow an xHC implementation to cache the
> endpoint state. Caching endpoint state allows an xHC to reduce latency
> when handling ERDYs and other USB asynchronous events. However holding
> this state in xHC consumes resources and power. The xHCI spec designs
> some methods through which host controller driver can hint xHC about
> how to optimize its operation, e.g. to determine when it holds state
> internally or pushes it out to memory, when to power down logic, etc.
> 
> When a USB device is going to suspend, states of all endpoints cached
> in the xHC should be pushed out to memory to save power and resources.
> Vice versa, when a USB device resumes, those states should be brought
> back to the cache. USB core suspends or resumes a USB device by sending
> set/clear port feature requests to the parent hub where the USB device
> is connected. Unfortunately, these operations are transparent to xHCI
> driver unless the USB device is plugged in a root port. This patch
> utilizes the callback entries to notify xHCI driver whenever a USB
> device suspends or resumes.
> 
> It is harmless if a USB devices under USB 3.0 host controller suspends
> or resumes without a notification to hcd driver. However there may be
> less opportunities for power savings and there may be increased latency
> for restarting an endpoint. The precise impact will be different for
> each xHC implementation. It all depends on what an implementation does
> with the hints.
> 
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>


> --- a/drivers/usb/core/hub.c
> +++ b/drivers/usb/core/hub.c
> @@ -3144,6 +3144,14 @@ int usb_port_suspend(struct usb_device *udev, pm_message_t msg)
>  			goto err_lpm3;
>  	}
>  
> +	/*
> +	 * Call back to hcd if it expects. xHCI compatible host controller
> +	 * driver expects to be notified prior to selectively suspending a
> +	 * device. xHCI hcd could optimize the endpoint cache for power
> +	 * saving purpose. Refer to 4.15.1.1 of xHCI 1.1 for more information.
> +	 */

Doesn't this comment belong in the xhci-hcd source code rather than the 
hub driver source code?

> +	hcd_suspend_notify(udev, msg);
> +
>  	/* see 7.1.7.6 */
>  	if (hub_is_superspeed(hub->hdev))
>  		status = hub_set_port_link_state(hub, port1, USB_SS_PORT_LS_U3);
> @@ -3169,6 +3177,8 @@ int usb_port_suspend(struct usb_device *udev, pm_message_t msg)
>  	if (status) {
>  		dev_dbg(&port_dev->dev, "can't suspend, status %d\n", status);
>  
> +		hcd_resume_notify(udev, msg);
> +
>  		/* Try to enable USB3 LPM and LTM again */
>  		usb_unlocked_enable_lpm(udev);
>   err_lpm3:
> @@ -3422,6 +3432,12 @@ int usb_port_resume(struct usb_device *udev, pm_message_t msg)
>  	}
>  
>   SuspendCleared:
> +	/* Call back to hcd if it expects. xHCI compatible host controller
> +	 * driver expects to be notified after a device is resumed. xHCI
> +	 * hcd could optimize the endpoint cache for latency reducing
> +	 * purpose. Refer to 4.15.1.1 of xHCI 1.1 for more information.
> +	 */

Same for this comment.

> +	hcd_resume_notify(udev, msg);
>  	if (status == 0) {
>  		udev->port_is_suspended = 0;
>  		if (hub_is_superspeed(hub->hdev)) {
> diff --git a/include/linux/usb/hcd.h b/include/linux/usb/hcd.h
> index 68b1e83..621d9b7 100644
> --- a/include/linux/usb/hcd.h
> +++ b/include/linux/usb/hcd.h
> @@ -383,7 +383,13 @@ struct hc_driver {
>  	int	(*find_raw_port_number)(struct usb_hcd *, int);
>  	/* Call for power on/off the port if necessary */
>  	int	(*port_power)(struct usb_hcd *hcd, int portnum, bool enable);
> -
> +	/* Call back to hcd when a USB device is going to suspend or just
> +	 * resumed.
> +	 */
> +	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?

Alan Stern

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