Re: [PATCH v11 3/3] arm64: dts: rockchip: rk3568-photonicat: enable SGMII LAN port

[Andrew Lunn <andrew@lunn.ch>]

On Thu, Oct 22, 2026 at 07:13:06PM +0200, David Heidelberg wrote:
> On 22/10/2026 18:58, Andrew Lunn wrote:
> > > +	sgmii_phy: ethernet-phy@3 {
> > > +		compatible = "ethernet-phy-ieee802.3-c22";
> > > +		reg = <0x3>;
> > > +		max-speed = <1000>;
> > 
> > Pointless. SGMII cannot do faster, so there is no need to limit it.
> 
> device-tree is a description. whatever the device can do should be there,
> 
> "Maximum PHY supported speed in Mbits / seconds." thus 1000 is correct value.
> 
> thus max-speed <1000>; LGTM

I did not say it was wrong. I said it was pointless. The PHY will
report what it can support, which will be speeds up to 1G for an SGMII
PHY. The max-speed property will then be applied, and does nothing,
making it pointless.

max-speed is interesting when you have an odd design, like only 2 of
the 4 pairs are actually connect to the RJ-45 socket, limiting the
media to 100Mbps. Using max-speed in this situation will remove the 1G
speeds from the supported list, and so it will not try to negotiate
such a link, which will probably be successful, but then data transfer
will fail. If the PHY supports downshift, it will then drop to
100Mbps, but it can be confusing because everything says it should be
doing 1G, but in practice it does 100MBs, or nothing.

      Andrew