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    SubjectRe: [PATCH 2/2] net: phy: dp83640: Read strapped configuration settings
    Date
    Florian Fainelli <f.fainelli@gmail.com> writes:

    > On 04/05/2018 04:44 AM, esben.haabendal@gmail.com wrote:
    >> From: Esben Haabendal <eha@deif.com>
    >>
    >> Read configration settings, to allow automatic forced speed/duplex setup
    >> by hardware strapping.
    >
    > OK but why? What problem is this solving for you?

    It is ensuring that the PHY is configured according to the HW design.
    If the HW design has set the strap configuration to fx. fixed 100 Mbit
    full-duplex, this avoids Linux configuring it for auto-negotiation.

    > In general, we do not really want to preserve too much of what the PHY
    > has been previously configured with,

    Even when this "previous" configuration is the configuration selected by
    the board configuration?

    > provided that the PHY driver can re-instate these configuration
    > values.

    This is sort of what I try to do here. The PHY driver needs to check
    the BMCR register to figure out what the strapped configuration was.
    Without that, it is necessary for software configuration to duplicate
    the exact same configuration as is encoded in the strap configuration in
    order for the PHY to be configured as it is supposed to.

    > I just wonder how this can be robust when you connect this PHY with
    > auto-negotiation disabled to a peer that expects a set of link
    > parameters not covered by the default advertisement values?

    I assume it will fail just as it will if you use ethtool to configure
    the PHY wrongly.

    > This really looks like a recipe for disaster when you could just
    > disable auto-negotiation with ethtool.

    The current PHY driver approach to always default to enable
    auto-negotation, and then allow user-space to configure it differently
    with ethtool.

    With this patch, the default configuration is chosen based on the strap
    configuration, but user-space can still change the configuration with
    ethtool if needed / desired.

    I don't think it is such a big change.

    Bringing up the PHY with a default configuration according to HW
    strapping instead of an in-kernel SW hard-coded value sounds like a good
    idea to me.

    /Esben

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