Re: [PATCH net-next] net: phylink: Support disabling autonegotiation for PCS

On Wed, Jun 30, 2021 at 11:49:27AM -0600, Robert Hancock wrote:

The auto-negotiation state in the PCS as set by
phylink_mii_c22_pcs_config was previously always enabled when the driver is
configured for in-band autonegotiation, even if autonegotiation was
disabled on the interface with ethtool. Update the code to set the
BMCR_ANENABLE bit based on the interface's autonegotiation enabled
state.

Update phylink_mii_c22_pcs_get_state to not check
autonegotiation-related fields when autonegotiation is disabled.

Update phylink_mac_pcs_get_state to initialize the state based on the
interface's configured speed, duplex and pause parameters rather than to
unknown when autonegotiation is disabled, before calling the driver's
pcs_get_state functions, as they are not likely to provide meaningful data
for these fields when autonegotiation is disabled. In this case the
driver is really just filling in the link state field.

Note that in cases where there is a downstream PHY connected, such as
with SGMII and a copper PHY, the configuration set by ethtool is handled by
phy_ethtool_ksettings_set and not propagated to the PCS. This is correct
since SGMII or 1000Base-X autonegotiation with the PCS should normally
still be used even if the copper side has disabled it.

In theory, this seems to be correct, but...

We do have some cases where, if a port is in 1000Base-X mode, the
documentation explicitly states that AN must be enabled. So, I think
if we are introducing the possibility to disable the negotiation in
1000Base-X mode, we need to give an option to explicitly reject that
configuration attempt.

We also need this to be consistently applied over all the existing
phylink-using drivers that support 1000Base-X without AN - we shouldn't
end up in the situation where we have different behaviours with
different network drivers.

So, we need mvneta and mvpp2 to reject such a configuration - with
these ports in 1000Base-X mode, the documentation states:

"Bit 2 Field InBandAnEn In-band Auto-Negotiation enable. ...
When <PortType> = 1 (1000BASE-X) this field must be set to 1."

We should be aware that there may be other hardware out there which
may not support 1000BASE-X without inband.

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