linux-kernel - Re: [REPOST PATCH 3/3] USB: dwc2: Don't turn off the usbphy in suspend if wakeup is enabled

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Message-ID: <20150706190212.GN20779@saruman.tx.rr.com>
Date:	Mon, 6 Jul 2015 14:02:12 -0500
From:	Felipe Balbi <balbi@...com>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	Douglas Anderson <dianders@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	John Youn <johnyoun@...opsys.com>, Felipe Balbi <balbi@...com>,
	<linux-usb@...r.kernel.org>, Chris Zhong <zyw@...k-chips.com>,
	Heiko Stuebner <heiko@...ech.de>,
	Julius Werner <jwerner@...omium.org>,
	Andrew Bresticker <abrestic@...omium.org>,
	Alexandru Stan <amstan@...omium.org>, <lyz@...k-chips.com>,
	<linux-rockchip@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [REPOST PATCH 3/3] USB: dwc2: Don't turn off the usbphy in
 suspend if wakeup is enabled

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

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