linux-kernel - Re: GPIO registration for external Ethernet PHY oscillator enable/disable

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Message-ID: <54247372.40803@gmail.com>
Date:	Thu, 25 Sep 2014 12:56:34 -0700
From:	Florian Fainelli <f.fainelli@...il.com>
To:	Michael Welling <mwelling@...cinc.com>,
	linux-kernel@...r.kernel.org
CC:	grant.likely@...aro.org, linus.walleij@...aro.org,
	rjw@...ysocki.net, gregkh@...uxfoundation.org,
	Nicolas Ferre <nicolas.ferre@...el.com>
Subject: Re: GPIO registration for external Ethernet PHY oscillator enable/disable

On 09/25/2014 12:17 PM, Michael Welling wrote:
> Looks like my original message got buried in the mailing list.
> 
> Lets try this again with a few key developers CC'd.
> 
> Original Message:
> I have some questions that span multiple subsystems including
> gpio/pinctrl, apm, and net subsystems.
> 
> On some of our system on module designs, we use a GPIO to toggle the
> enable pin on external oscillators. In particular, we are using a 50Mhz
> oscillator to drive a clock on a RMII Ethernet PHY.
> 
> Though I can configure the pin such that the Ethernet interface works we
> are looking to disable the oscillator during APM sleep but after the PHY
> is put into a low power mode.
> 
> How do I register a GPIO for use in the PHY suspend and resume code?

So, PHY drivers are allowed to provide specialized implementations for
suspend/resume operations that are called by phy_suspend() and
phy_resume(), the current Micrel PHY driver uses the generic
suspend/resume implementation and it is best if we can keep doing that.

> Can it be handled outside of the PHY driver?

I see a few possible options:

- hook a pm_runtime callbacks for your platform, check the device
pointer to make sure this is the PHY device, and when that is the case,
toggle the GPIO accordingly

- add an additional "osc_gpio" configuration parameter passed to the
Ethernet MAC driver (presumably drivers/net/ethernet/cadence/macb.c?)
and toggle the GPIO before and after the calls to the PHY state machine
(phy_suspend, phy_resume, phy_start, phy_stop), that might be simpler

- last but not least, make the PHY driver aware of that optional GPIO,
create customized PHY suspend/resume/config_aneg callbacks


> If so how do ensure the appropriate suspend and resume sequencing?
> 
> For reference, we are using a Micrel KSZ8081 PHY connected to a
> AT91SAMA5D35 processor.
> 
> Addendum:
> I ran into another situation where a GPIO enabled oscillator was used.
> The oscillator in this case drives the master clock for a audio codec.
> In the old days (before device tree), I could initialize the GPIO in the
> platform board file. Now with device tree I can setup the pin multipler
> but the initial state of the GPIO I am not sure how to set.
> 
> Is there a way to directly change the state of a GPIO pin from a
> devicetree entry?
> 

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