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

Romain Gantois
2023-11-15T15:30:58Z

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