Re: [PATCH net-next] net: dsa: mv88e6xxx: support in-band signalling with external PHY for 1000BaseX/2500BaseX

[Andrew Lunn <andrew@lunn.ch>]

> I briefly looked at the SFP connector and there don't seem to be pins for
> out-of-band signalling. So there must be some inband signalling.

Hi Heiner

Optical SFPs have an i2c bus with an 'EEPROM' on it. The EEPROM
contains information about the SFP, including its maximum
bit-rate. The Linux SFP driver will look at this bitrate, and pick the
link mode appropriate to it. It then calls mac_config with that link
mode, and the link speed.

The MAC then needs to configure the SERDES to that mode/speed. After a
while, it will get sync and trigger an interrupt. At is then reported
via phylink_mac_change() at which point the carrier is indicated as
up. If one end is using 1000Base-X and the other 2500Base-X, they will
fail to sync. There is no negotiation down to 1000Base-X. 

Optical SFPs are pretty passive devices, they just do electrical to
optical, and not a lot more. All the 'intelligence' is in the SERDES
layers, and they talk to each other end-to-end, unlike copper, where
the MAC SERDES is talking to the PHY SERDES.

Copper SFPs are different. They use SGMII with inband signalling.
There is also a mechanism to encapsulate MDIO over i2c.

       Andrew