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

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Thu, Aug 26, 2021 at 07:12:30AM +0000, Nathan Rossi wrote:
> From: Nathan Rossi <nathan.rossi@digi.com>
> 
> 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.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!