netdev - Re: [PATCH net-next v3 6/8] net: phy: add calibration callbacks to phy

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Date: Wed, 15 Nov 2023 16:31:07 +0100 (CET)
From: Romain Gantois <romain.gantois@...tlin.com>
To: Andrew Lunn <andrew@...n.ch>
cc: Romain Gantois <romain.gantois@...tlin.com>, davem@...emloft.net, 
    Rob Herring <robh+dt@...nel.org>, 
    Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, 
    Jakub Kicinski <kuba@...nel.org>, Eric Dumazet <edumazet@...gle.com>, 
    Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
    linux-kernel@...r.kernel.org, devicetree@...r.kernel.org, 
    thomas.petazzoni@...tlin.com, Florian Fainelli <f.fainelli@...il.com>, 
    Heiner Kallweit <hkallweit1@...il.com>, 
    Russell King <linux@...linux.org.uk>, linux-arm-kernel@...ts.infradead.org, 
    Vladimir Oltean <vladimir.oltean@....com>, 
    Luka Perkov <luka.perkov@...tura.hr>, 
    Robert Marko <robert.marko@...tura.hr>, Andy Gross <agross@...nel.org>, 
    Bjorn Andersson <andersson@...nel.org>, 
    Konrad Dybcio <konrad.dybcio@...ainline.org>
Subject: Re: [PATCH net-next v3 6/8] net: phy: add calibration callbacks to
 phy_driver

On Tue, 14 Nov 2023, Andrew Lunn wrote:

> > +static inline
> > +int phy_start_calibration(struct phy_device *phydev)
> > +{
> > +	if (!(phydev->drv &&
> > +	      phydev->drv->calibration_start &&
> > +	      phydev->drv->calibration_stop))
> > +		return -EOPNOTSUPP;
> > +
> > +	return phydev->drv->calibration_start(phydev);
> > +}
> > +
> > +static inline
> > +int phy_stop_calibration(struct phy_device *phydev)
> > +{
> > +	if (!(phydev->drv &&
> > +	      phydev->drv->calibration_stop))
> > +		return -EOPNOTSUPP;
> > +
> > +	return phydev->drv->calibration_stop(phydev);
> > +}
> > +
> 
> What is the locking model?
> 
>      Andrew
> 
This driver currently uses an atomic flag to make sure that the calibration 
doesn't run twice. It doesn't acquire any locks before calling 
phy_start_calibration(), which is a mistake.

I think a good locking model for this would be similar to the one used for 
phy_cable_test. The phy_start_calibration() and phy_stop_calibration() wrappers 
would acquire a lock on the PHY device and then test phydev->state, to check for 
an ongoing calibration. A new enum member such as PHY_CALIB could be defined for 
this purpose. The lock would be released by the phylib wrapper once the 
phy_driver callback returns.

The problem with this is that one calibration run can access multiple 
phy_device instances at the same time, e.g. if a switch is linked to a multiport 
PHY via a PSGMII link.

So acquiring a lock on a single phy device isn't enough. Ideally, these 
calls could somehow acquire one lock on all the hardware resources of a 
multiport PHY simultaneously. From what I've seen, there is no standard kernel 
interface that allows MAC drivers to know about link-sharing between phy 
devices. I'll have to do more research on this but if you know of an existing 
interface that I can use for this, please tell me.

Best,

-- 
Romain Gantois, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com