Re: [PATCH v4 net-next 3/8] net: phy: bcm84881: move the in-band capability check where it belongs

[Vladimir Oltean <vladimir.oltean@nxp.com>]

On Tue, Nov 22, 2022 at 09:38:43AM +0000, Russell King (Oracle) wrote:
> Also, if we get the Marvell driver implementing validate_an_inband()
> then I believe we can get rid of other parts of this patch - 88E1111 is
> the commonly used accessible PHY on gigabit SFPs, as this PHY implements
> I2C access natively. As I mentioned, Marvell PHYs can be set to no
> inband, requiring inband, or inband with bypass mode enabled. So we
> need to decide how we deal with that - especially if we're going to be
> changing the mode from 1000base-X to SGMII (which we do on some SFP
> modules so they work at 10/100/1000.)

Not clear which parts of this patch we could ged rid of, if we
implemented in-band AN reporting/configuration for the 88E1111.
Based on your previous description, it sounds like it would be just more
functionality for software rather than less.

> In that regard, I'm not entirely convinced that validate_an_inband()
> covers the functionality we need - as reading the config register on
> Marvell hardware doesn't guarantee that we're reading the right mode -
> the PHY may be in 1000base-X, and we might change it to
> SGMII-with-bypass - I'd need to go through the PHY datasheets to check
> what we actually do.
> 
> Changing what the PHY driver does would be a recipe for regressions,
> especially for drivers that do not use phylink.

I believe that currently, the "interface" passed to phy_validate_an_inband()
and phy_config_an_inband() is always also the phydev->interface.
We could strive to keep that being the case, and put a phydev_warn() in
the Marvell PHY driver if we get a query for interface != phydev->interface,
and report PHY_AN_INBAND_UNKNOWN.

It's also one of the reasons why I didn't yet want to jump right into
figuring out what should be done with PHYs that change SERDES protocols,
and when exactly we query the in-band capability for the new one. Right
now, phylink will assume that the in-band capability reported for the
first SERDES protocol will continue to be the same for all subsequent
protocols. Obviously this might not be the case.