linux-kernel - Re: [PATCH 6/6] USB / PM: Allow USB devices to remain runtime-suspended when sleeping

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Date:	Tue, 31 Mar 2015 13:09:32 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
cc:	Linux-pm mailing list <linux-pm@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Scot Doyle <lkml14@...tdoyle.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Julius Werner <jwerner@...omium.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Pratyush Anand <pratyush.anand@...com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	USB list <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] USB / PM: Allow USB devices to remain runtime-suspended
 when sleeping

On Tue, 31 Mar 2015, Tomeu Vizoso wrote:

> Have dev_pm_ops.prepare return 1 for USB devices, interfaces, endpoints
> and ports so that USB devices can remain runtime-suspended when the
> system goes to a sleep state.
> 
> Also enable runtime PM for endpoints, which is another requirement for
> the above to work.
> 
> Signed-off-by: Tomeu Vizoso <tomeu.vizoso@...labora.com>

...

> diff --git a/drivers/usb/core/usb.c b/drivers/usb/core/usb.c
> index b1fb9ae..1498faa 100644
> --- a/drivers/usb/core/usb.c
> +++ b/drivers/usb/core/usb.c
> @@ -300,7 +300,7 @@ static int usb_dev_uevent(struct device *dev, struct kobj_uevent_env *env)
>  
>  static int usb_dev_prepare(struct device *dev)
>  {
> -	return 0;		/* Implement eventually? */
> +	return 1;		/* Implement eventually? */
>  }

The rest of the patch is okay, but this part is wrong.  The documented
requirement is that the prepare callback should return 1 if the device
is already in the appropriate state.  This means it has to have the
right wakeup setting.

In other words, if the device is currently in runtime suspend with 
remote wakeup enabled, but device_may_wakeup() returns 0 (so that the 
device should be disabled for wakeup when the system goes into 
suspend), then the prepare callback has to return 0.

Therefore what you need to do here is something like this:

	struct usb_device	*udev = to_usb_device(dev);

	/* Return 0 if the current wakeup setting is wrong, otherwise 1 */
	if (udev->do_remote_wakeup && !device_may_wakeup(dev))
		return 0;
	return 1;

Aside from this issue, I like the patch set.  Do you think you can do 
something similar for drivers/scsi/scsi_pm.c?  I'm not aware of any 
wakeup-capable SCSI devices -- not disk or CD/DVD drives, anyway.

Alan Stern

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