Re: [PATCH] net: phylink: Update SFP selected interface on advertising changes

On Thu, Aug 26, 2021 at 07:12:30AM +0000, Nathan Rossi wrote:

From: Nathan Rossi <redacted>

Currently changes to the advertising state via ethtool do not cause any
reselection of the configured interface mode after the SFP is already
inserted and initially configured.

While it is not typical to change the advertised link modes for an
interface using an SFP in certain use cases it is desirable. In the case
of a SFP port that is capable of handling both SFP and SFP+ modules it
will automatically select between 1G and 10G modes depending on the
supported mode of the SFP. However if the SFP module is capable of
working in multiple modes (e.g. a SFP+ DAC that can operate at 1G or
10G), one end of the cable may be attached to a SFP 1000base-x port thus
the SFP+ end must be manually configured to the 1000base-x mode in order
for the link to be established.

This change causes the ethtool setting of advertised mode changes to
reselect the interface mode so that the link can be established.

This may be a better solution than the phylink_helper_basex_speed()
approach used to select between 2500 and 1000 base-X, but the config
needs to be validated after selecting a different interface mode, as
per what phylink_sfp_config() does.

I also suspect that this will allow you to select e.g. 1000base-X but
there will be no way back to 10G modes as they will be masked out of
the advertising mask from that point on, as the 1000base-X interface
mode does not allow them.

So, I think the suggested code is problematical as it stands.

phylink_sfp_config() uses a multi-step approach to selecting the
interface mode for a reason - first step is to discover what the MAC
is capable of in _any_ interface mode using _NA to reduce the supported
and advertised mask down, and then to select the interface from that.
I'm not entirely convinced that is a good idea here yet though.

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