Re: [PATCH net-next 1/2] net: phylink: explicitly configure in-band autoneg for PHYs that support it

[Vladimir Oltean <olteanv@gmail.com>]

On Sun, Feb 14, 2021 at 10:35:29AM +0000, Russell King - ARM Linux admin wrote:
> > +	if (ret && ret != -EOPNOTSUPP) {
> > +		phylink_warn(pl, "failed to configure PHY in-band autoneg: %d\n",
> > +			     ret);
> 
> Please use %pe and ERR_PTR(ret) so we can get a symbolic errno value.

I didn't know that was possible, thanks for the hint.

> As mentioned in this thread, we have at least one PHY which is unable
> to provide the inband signalling in any mode (BCM84881). Currently,
> phylink detects this PHY on a SFP (in phylink_phy_no_inband()) and
> adjusts not to use inband mode. This would need to be addressed if we
> are creating an alterative way to discover whether the PHY supports
> inband mode or not.

So I haven't studied the SFP code path too deeply, but I think part of
the issue is the order in which things are done. It's almost as if there
should be a validation phase for PHY inband abilities too.

phylink_sfp_connect_phy
-> phylink_sfp_config:
   -> first this checks if phylink_phy_no_inband
   -> then this changes pl->link_config.interface and pl->cur_link_an_mode
-> phylink_bringup_phy:
   -> this is where I'm adding the new phy_config_inband_aneg function

If we were to use only my phy_config_inband_aneg function, it would need
to be moved upwards in the code path, to be precise where phylink_phy_no_inband
currently is. Then we'd have to try MLO_AN_INBAND first, with a fallback
to MLO_AN_PHY if that fails. I think this would unnecessarily complicate
the code.

Alternatively, I could create a second PHY driver method, simply for
validation of supported inband modes. The validation can be done in
place of the current phylink_phy_noinband(), and I can still have the
phy_config_inband_aneg() where I put it now, since we shouldn't have any
surprises w.r.t. supported operating mode, and there should be no reason
to repeat the attempt as there would be with a single PHY driver method.
Thoughts?

> Also, there needs to be consideration of PHYs that dynamically change
> their interface type, and whether they support inband signalling.
> For example, a PHY may support a mode where it dynamically selects
> between 10GBASE-R, 5GBASE-R, 2500BASE-X and SGMII, where the SGMII
> mode may have inband signalling enabled or disabled. This is not a
> theoretical case; we have a PHY like that supported in the kernel and
> boards use it. What would the semantics of your new call be for a PHY
> that performs this?
> 
> Should we also have a phydev->inband tristate, taking values "unknown,
> enabled, disabled" which the PHY driver is required to update in their
> read_status callback if they dynamically change their interface type?
> (Although then phylink will need to figure out how to deal with that.)

I don't have such PHY to test with, but I think the easiest way would be
to just call the validation method again, after we change the
phydev->interface value. The PHY driver can easily take phydev->interface
into consideration when answering the question "is inband aneg supported
or not". I don't think that making phydev->inband a stateful value is
going to be as useful as making it a function, since as you say, we will
be required to keep it up to date from generic PHY driver methods, but
only phylink will use it.