On Mon, Jul 06, 2015 at 02:58:16PM -0400, Alan Stern wrote:
> On Mon, 6 Jul 2015, Douglas Anderson wrote:
> 
> > If the 'snps,need-phy-for-wake' is set in the device tree then:
> > 
> > - We know that we can wakeup, so call device_set_wakeup_capable().
> >   The USB core will use this knowledge to enable wakeup by default.
> > - We know that we should keep the PHY on during suspend if something
> >   on our root hub needs remote wakeup.  This requires the patch (USB:
> >   Export usb_wakeup_enabled_descendants()).  Note that we don't keep
> >   the PHY on at suspend time if it's not needed because it would be a
> >   power draw.
> 
> You know, this is the first time I've run across this optimization.
> 
> In principle it applies to any USB host controller, not just to PHYs.  
> There's no reason to enable wakeup for a controller if none of the 
> attached devices can issue a wakeup request.
> 
> I don't know if implementing this in other HCDs would save any power.  
> Any ideas?

most likely it would. Enabling wakeup usually boils down to keeping a
tiny part of the controller (or PHY) powered up. Sometimes that lies in
an always-on power domain, so there would be no difference.

cheers

-- 
balbi