On Thu, Nov 30, 2017 at 04:50:50PM +0100, Andrew Lunn wrote: > On Thu, Nov 30, 2017 at 10:00:35AM +0530, Bhaskar Upadhaya wrote: > > Add 2.5G SGMII interface mode(PHY_INTERFACE_MODE_2500SGMII) > > in existing phy_interface list.As auto-negotiation is not > > supported for 2.5G SGMII > > Hi Bhaskar > > I've been thinking about this some more... > > Is auto negotiation not supported in 2.5G SGMII in general, or just in > the system you are using? Is it the PHY or the MAC which does not > support it. Are we going to get into trouble if we find an 2.5G SGMII > link which does negotiate?
I've been doing some research on this 2.5G SGMII "thing", and what I've found so far (from Xilinx, I haven't been able to look through anything else yet) is that at least Xilinx do the same thing. There's a Xilinx document (pg047-gig-eth-pcs-pma.pdf) which says throughout that the speed bits are not applicable for 2.5G SGMII. Speeds > 1G appear to be defined only for full duplex (since the latency through the PHY is too great for CDMA half-duplex to work.) So, the duplex bits in SGMII would be meaningless for >1G. Many MACs, incidentally, do not support half-duplex at 1G speeds either. That just leaves pause mode in the SGMII word. > My understanding is that one of the main differences between SGMII and > 1000BASE-X is the negotiation. True. 1000BASE-X is an 802.3 standard, with the configuration word defined by IEEE. SGMII is derived from that by Cisco, but with the configuration word replaced by a system whereby the PHY can inform the MAC about the results of negotiation. SGMII has a handshake so the PHY knows that the MAC has configured itself, and some PHYs will not pass data until that handshake has completed. I suspect for 2.5G SGMII, I would expect that the handshake also has to complete, so the PHY knows that the pause modes have been properly received by the MAC. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up According to speedtest.net: 8.21Mbps down 510kbps up
