[PATCH net-next v5 09/13] net: phylink: Use phy_caps_lookup for fixed-link configuration

[Andrew Lunn]

On Fri, Mar 28, 2025 at 04:26:04PM -0700, Alexander Duyck wrote:
> On Fri, Mar 28, 2025 at 2:45 PM Andrew Lunn <andrew at lunn.ch> wrote:
> >
> > > Also I am not sure it makes sense to say we can't support multiple
> > > modes on a fixed connection. For example in the case of SerDes links
> > > and the like it isn't unusual to see support for CR/KR advertised at
> > > the same speed on the same link and use the exact same configuration
> > > so a fixed config could support both and advertise both at the same
> > > time if I am not mistaken.
> >
> > Traditionally, fixed link has only supported one mode. The combination
> > of speed and duplex fully describes a base-T link. Even more
> > traditionally, it was implemented as an emulated C22 PHY, using the
> > genphy driver, so limited to just 1G. With multigige PHY we needed to
> > be a bit more flexible, so phylink gained its own fixed link
> > implementation which did not emulate a PHY, just the results of
> > talking to a multigige PHY.
> >
> > But i don't think you are actually talking about a PHY. I think you
> > mean the PCS advertises CR/KR, and you want to emulate a fixed-link
> > PCS? That is a different beast.
> >
> >         Andrew
> 
> A serdes PHY is part of it, but not a traditional twisted pair PHY as
> we are talking about 25R, 50R(50GAUI & LAUI), and 100P interfaces. I
> agree it is a different beast, but are we saying that the fixed-link
> is supposed to be a twisted pair PHY only?

With phylink, the PCS enumerates its capabilities, the PHY enumerates
its capabilities, and the MAC enumerates it capabilities. phylink then
finds the subset which all support.

As i said, historically, fixed_link was used in place of a PHY, since
it emulated a PHY. phylinks implementation of fixed_link is however
different. Can it be used in place of both a PCS and a PHY? I don't
know.

You are pushing the envelope here, and maybe we need to take a step
back and consider what is a fixed link, how does it fit into the MAC,
PCS, PHY model of enumeration? Maybe fixed link should only represent
the PHY and we need a second sort of fixed_link object to represent
the PCS? I don't know?

> In addition one advantage is that it makes it possible to support
> speeds that don't yet have a type in the phy_interface_t, so as I was
> enabling things it allowed some backwards compatibility with older
> kernels.

I don't like the sound of that. I would simply not support the older
kernels, rather than do some hacks.

	 Andrew