linux-kernel - Re: [PATCH v3 01/11] usb: phy: Add APIs for runtime power management

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Date:	Tue, 23 Apr 2013 12:53:04 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Vivek Gautam <gautamvivek1987@...il.com>
cc:	Felipe Balbi <balbi@...com>,
	Kishon Vijay Abraham I <kishon@...com>,
	Vivek Gautam <gautam.vivek@...sung.com>,
	<linux-usb@...r.kernel.org>, <linux-samsung-soc@...r.kernel.org>,
	<linux-omap@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<gregkh@...uxfoundation.org>, <sarah.a.sharp@...ux.intel.com>,
	<rob.herring@...xeda.com>, <kgene.kim@...sung.com>,
	<dianders@...omium.org>, <t.figa@...sung.com>,
	<p.paneri@...sung.com>
Subject: Re: [PATCH v3 01/11] usb: phy: Add APIs for runtime power management

On Tue, 23 Apr 2013, Vivek Gautam wrote:

> >> Alright, so here's my understanding:
> >>
> >> I suggested letting e.g. DWC3 enable the PHY's runtime_pm; Alan said
> >> that it could be done before that so that DWC3 sees an enabled PHY
> >> during probe.
> >
> > Basically right.  Help me to understand the overall situation a little
> > better:
> >
> >         What code registers the PHY initially?
>            PHY is added to global list by PHY drivers (like
> phy-samsung-usb2.c/phy-omap-usb2.c)
>            by usb_add_phy() API

Then this routine should initialize the PHY.  The initialized state 
could be either active or suspended, your choice.  Suspended would be 
best, in case the PHY never gets used.

> >         What routine does the DWC3 driver call to register itself
> >         as a consumer of the PHY?
>            I think DWC3 doesn't registers itself as consumer of PHY,
> rather it gets that PHY from
>            the list using devm_usb_get_phy()/devm_usb_get_phy_by_phandle() API.
>            DWC3 can now call PHY's initialization sequence using usb_phy_init().
>            So, before DWC3 initializes the PHY, PHYs should be in active state.

Then usb_phy_init should make sure the PHY is in the active state.  If 
usb_add_phy() left the PHY suspended, then this routine should call 
pm_runtime_get_sync().

After DWC3 (or any other driver) has acquired the PHY, it can call
pm_runtime_put/get() however it likes, so long as the calls balance
properly.  If the driver isn't runtime-PM aware then it won't use any
of these calls, and the PHY will remain active the entire time.

> >         Likewise, what routine does it call to unregister itself?
>            Once DWC3's remove() is called PHYs are put.

Is there a routine analogous to usb_phy_init() that gets called when
PHY is released?  That routine would do the opposite of usb_phy_init(),
putting the PHY back into its initialized state.

Alan Stern

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