Re: [PATCH net-next] net: phy: vitesse: add inband caps and configuration

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

On Wed, Mar 11, 2026 at 03:01:35PM +0100, Geert Uytterhoeven wrote:
> 	Hi Russell,
> 
> On Wed, 11 Mar 2026 at 01:07, Russell King <rmk@armlinux.org.uk> wrote:
> > Add support for VSC8662 reporting its inband capabilities, and also
> > hook to configure the PHY's inband mode.
> >
> > This fixes a regression in the macb driver caused by commit
> > 1338cfef1ff1 ("net: macb: fix SGMII with inband aneg disabled")
> 
> Fixes: 1338cfef1ff1b958 ("net: macb: fix SGMII with inband aneg disabled")
> 
> > Reported-by: Conor Dooley <conor@kernel.org>
> > Link: https://lore.kernel.org/r/20260304-nebulizer-rounding-40fbc81a2ba1@spud
> 
> s/Link/Closes/

I avoided it because any other PHY used with macb that is also connected
via SGMII will run into this same issue. It just happens that adding
this support fixes the above commit.

Conversely, we have stmmac, which unconditionally enables SGMII inband
at the MAC end no matter what phylink says to do, and should this
PHY be used with this patch in a stmmac system, it will cause that to
break, because we end up with either end misconfigured.

Also, if this commit is applied without the macb change, you'll
probably also find that macb breaks (please test that.) 

So, while this is a solution and a step forward, I'd rather not suggest
that it is an official fix to the macb issue - it's merely filling in
a missing piece of the jigsaw.

Sadly, this is a fundamental issue with SGMII implementations today:
whether SGMII inband is used/required is completely random both at the
MAC/PCS end but also at the PHY end. If one ends up with a mismatch,
then things stop working.

As I say, the problem with fixing the PHY end of the link is that
it could cause a different network driver to regress.

So yes, this is a solution to the macb problem, but I regard it as
high risk.

That said, given the SGMII mess, there is no easy way forward.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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