netdev - Re: [PATCH net] r8169: fix rtl8168h wol fail

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Date:   Wed, 11 Jan 2023 22:40:02 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Heiner Kallweit <hkallweit1@...il.com>
Cc:     Hau <hau@...ltek.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        nic_swsd <nic_swsd@...ltek.com>
Subject: Re: [PATCH net] r8169: fix rtl8168h wol fail

> >>> In this application(rtl8168h + rtl8211fs) it also supports 100Mbps fiber
> >> module.
> >>
> >> Does RTL8211FS advertise 100Mbps and 1Gbps on the UTP/MDI side in case
> >> of a 100Mbps fiber module?
> > Yes.
> > 
> I think in this case internal PHY and RTL8211FS would negotiate 1Gbps,
> not matching the speed of the 100Mbps fiber module.
> How does this work?

Fibre line side has no autoneg. Both ends need to be using the same
speed, or the SERDES does not synchronise and does not establish link.

You can ask the SFP module what baud rate it supports, and then use
anything up to that baud rate. I've got systems where the SFP is fast
enough to support a 2.5Gbps link, so the MAC indicates both 2.5G and
1G, defaults to 2.5G, and fails to connect to a 1G link peer. You need
to use ethtool to force it to the lower speed before the link works.

But from what i understand, you cannot use a 1000Base-X SFP, set the
MAC to 100Mbps, and expect it to connect to a 100Base-FX SFP. So for
me, the RTL8211FS should not be advertise 100Mbps and 1Gbps, it needs
to talk to the SFP figure out exactly what it is, and only advertise
the one mode which is supported.

    Andrew