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    SubjectRe: [PATCH 2/2] net: dsa: qca8k: introduce SGMII configuration options
    On Sat, Jun 06, 2020 at 11:59:09AM +0100, Jonathan McDowell wrote:
    > So the device in question is a 7 port stand alone switch chip. There's a
    > single SGMII port which is configurable between port 0 + 6 (they can
    > also be configure up as RGMII, while the remaining 5 ports have their
    > own phys).
    >
    > It sounds like there's a strong preference to try and auto configure
    > things as much as possible, so I should assume the CPU port is in MAC
    > mode, and anything not tagged as a CPU port is talking to a PHY/BASEX.
    >
    > I assume I can use PHY_INTERFACE_MODE_1000BASEX on the
    > phylink_mac_config call to choose BASEX?

    Yes, but from what you've mentioned above, I think I need to ensure that
    there's a proper understanding here.

    1000BASE-X is the IEEE 802.3 defined 1G single lane Serdes protocol.
    SGMII is different; it's a vendor derivative of 1000BASE-X which has
    become a de-facto standard.

    Both are somewhat compatible with each other; SGMII brings with it
    additional data replication to achieve 100M and 10M speeds, while
    keeping the link running at 1.25Gbaud. In both cases, there is a
    16-bit "configuration" word that is passed between the partners.

    1000BASE-X uses this configuration word to advertise the abilities of
    each end, which is limited to duplex and pause modes only. This you
    get by specifying the phy-mode="1000base-x" and
    managed="in-band-status" in DT.

    SGMII uses this configuration word for the media side to inform the
    system side which mode it wishes to operate the link: the speed and
    duplex. Some vendors extend it to include EEE parameters as well,
    or pause modes. You get this via phy-mode="sgmii" and
    managed="in-band-status" in DT.

    Then there are variants where the configuration word is not present.
    In this case, the link has to be manually configured, and without the
    configuration word, SGMII operating at 1G is compatible with
    1000base-X operating at 1G. Fixed-link can be used for this, although
    fixed-link will always report that the link is up at the moment; that
    may change in the future, it's something that is being looked into at
    the moment.

    --
    RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
    FTTC for 0.8m (est. 1762m) line in suburbia: sync at 13.1Mbps down 424kbps up

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