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

From: Felipe Balbi <balbi@ti.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Douglas Anderson <dianders@chromium.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	John Youn <johnyoun@synopsys.com>, Felipe Balbi <balbi@ti.com>,
	<linux-usb@vger.kernel.org>, Chris Zhong <zyw@rock-chips.com>,
	Heiko Stuebner <heiko@sntech.de>,
	Julius Werner <jwerner@chromium.org>,
	Andrew Bresticker <abrestic@chromium.org>,
	Alexandru Stan <amstan@chromium.org>, <lyz@rock-chips.com>,
	<linux-rockchip@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>
Subject: Re: [REPOST PATCH 3/3] USB: dwc2: Don't turn off the usbphy in suspend if wakeup is enabled
Date: Mon, 6 Jul 2015 14:02:12 -0500	[thread overview]
Message-ID: <20150706190212.GN20779@saruman.tx.rr.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1507061450190.1860-100000@iolanthe.rowland.org>

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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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