[PATCH net] net: sparx5: fix reconfiguration of PCS on link mode change

[Russell King (Oracle)]

On Fri, Apr 05, 2024 at 11:53:15AM +0200, Daniel Machon wrote:
> It was observed that the PCS would be misconfigured on link mode change,
> if the negotiated link mode went from no-inband capabilities to in-band
> capabilities. This bug appeared after the neg_mode change of phylink [1],
> but is really due to the wrong config being used when reconfiguring the PCS.

I don't see how the change you point to could have changed the
behaviour. Old code:

	conf.inband = phylink_autoneg_inband(mode);
	conf.autoneg = phylink_test(advertising, Autoneg);

New code:

	conf.inband = neg_mode == PHYLINK_PCS_NEG_INBAND_DISABLED ||
		      neg_mode == PHYLINK_PCS_NEG_INBAND_ENABLED;
	conf.autoneg = neg_mode == PHYLINK_PCS_NEG_INBAND_ENABLED;

where, for SGMII or 802.3z modes, neg_mode will be one of
PHYLINK_PCS_NEG_INBAND_DISABLED or PHYLINK_PCS_NEG_INBAND_ENABLED if
phylink_autoneg_inband(mode) is true, or PHYLINK_PCS_NEG_OUTBAND if
not.

It does change conf.autoneg slightly in that this will always be true
for SGMII, but will only be true for Autoneg + 802.3z modes.

As far as your code change goes, it looks correct to me, but I think
it's fixing a bug that goes back long before the commit you have
identified.

However, I think there's another issue here which is more relevant to
the problem you describe in your commit message. If you look at
port_conf_has_changed(), you will notice that it fails to compare
conf.inband, and thus fails to notice any change in the setting of
that configuration item. This will result in sparx5_port_pcs_set()
not even being called if only conf.inband changes state.

Thus, changing from in-band to out-of-band or vice versa won't have
any effect if this is the only change that occurs, and this also
exists prior to my change.

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