Re: [PATCH v3 1/3] net: dsa: microchip: implement KSZ87xx Module 3 low-loss cable errata

[Andrew Lunn <andrew@lunn.ch>]

> Yes, I think a reasonable compromise could be to expose three tunables:
> 
> - a boolean "short-cable" tunable, which applies the known good settings
>   (LPF 62 MHz BW, DSP EQ initial value 0).
> 
> - an integer LPF bandwidth tunable, for advanced use cases where further
>   tuning is needed;
> 
> - an integer DSP EQ initial value tunable, for the same advanced cases.
> 
> The boolean tunable would follow the KISS principle and cover the common
> scenario, while the more granular controls would remain optional.

How do the three interact? Do you need to first enable short-cable
before you set LPG bandwidth or DSP EQ? If it is not enabled, do you
get -EINVAL?

It seems like having extack would be useful to return informative
error messages to user space, however, that requires netlink
ethtool. And ETHTOOL_PHY_STUNABLE has not been added to netlink
ethtool yet :-(

	Andrew