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

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Date:	Fri, 3 Apr 2015 15:05:55 +0200
From:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
To:	Alan Stern <stern@...land.harvard.edu>
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 31 March 2015 at 19:09, Alan Stern <stern@...land.harvard.edu> wrote:
> 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;

Thanks, in v2 I have also covered the other case, as suggested by Oliver.

> 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.

I think I will be able to allocate some time on monday to look at this.

Thanks,

Tomeu

> Alan Stern
>
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