linux-kernel - Re: [PATCH RFT 1/2] phylib: add device reset GPIO support

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Date:	Fri, 13 May 2016 22:36:31 +0300
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Roger Quadros <rogerq@...com>,
	Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:	grant.likely@...aro.org, robh+dt@...nel.org,
	devicetree@...r.kernel.org, f.fainelli@...il.com,
	netdev@...r.kernel.org, frowand.list@...il.com, pawel.moll@....com,
	mark.rutland@....com, ijc+devicetree@...lion.org.uk,
	galak@...eaurora.org, linux-kernel@...r.kernel.org,
	Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH RFT 1/2] phylib: add device reset GPIO support

Hello.

On 05/13/2016 12:07 PM, Roger Quadros wrote:

[...]

>>> +}
>>> +EXPORT_SYMBOL(mdio_device_reset);
>>> +
>>>  /**
>>>   * mdio_probe - probe an MDIO device
>>>   * @dev: device to probe
>>> @@ -117,9 +126,16 @@ static int mdio_probe(struct device *dev
>>>  	struct mdio_driver *mdiodrv = to_mdio_driver(drv);
>>>  	int err = 0;
>>>
>>> -	if (mdiodrv->probe)
>>> +	if (mdiodrv->probe) {
>>> +		/* Deassert the reset signal */
>>> +		mdio_device_reset(mdiodev, 0);
>>> +
>>>  		err = mdiodrv->probe(mdiodev);
>>>
>>> +		/* Assert the reset signal */
>>> +		mdio_device_reset(mdiodev, 1);
>>
>> I wonder if it's safe to do this in general. What if ->probe does
>> something with the phy that is lost by resetting but that is relied on
>> later?
>
> mdio_probe is called for non PHY devices only, right?

    Yes, those also can have a reset signal.

> I'm a bit lost as to why we're de-asserting reset at multiple places. i.e.
> mdio_probe(), phy_device_register(), phy_init_hw(), phy_probe(), of_mdiobus_register_phy().

> Isn't it simpler to just de-assert it once at the topmost level?

    I wasn't after simplicity here. The intent was to save some power putting 
the device at reset when it's not needed. Florian Fainelly (the phylib 
maintainer) actually wanted me to go even further with that and assert reset 
in phy_suspend() but it was too much for me.

> i.e. of_mdiobus_register_device() f and of_mdiobus_register_phy()?
>
> Also, how about issuing a reset pulse instead of just de-asserting it.

    If it's already held at reset, my assumption is that it's enough time 
passed already.

> This would tackle the case where PHY is in invalid state with reset already
> de-asserted.

     Good question. I haven't yet met such cases though.

> Another issue is that on some boards we have one reset line tied to
> multiple PHYs.How do we prevent multiple resets being taking place when each of
> the PHYs are registered?

    My patch just doesn't address this case -- it's about the individual 
resets only.

> Do we just specify the reset line only for one PHY in
> the DT or can we have the reset gpio in the mdio_bus node for such case?

    I think there's mii_bus::reset() method for that case. Some Ethernet 
drivers even use it
(mostly instead of the code being suggested here).

> cheers,
> -roger

MBR, Sergei