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    SubjectRe: [PATCH net-next] net: phy: at803x: add cable diagnostics support
    Am 2020-05-10 16:52, schrieb Andrew Lunn:
    > More of a note to self:
    >
    > Now we have three implementations, we start to see bits of common code
    > which could be pulled out and shared.
    >
    >> +static bool at803x_cdt_fault_length_valid(u16 status)
    >> +{
    >> + switch (FIELD_GET(AT803X_CDT_STATUS_STAT_MASK, status)) {
    >> + case AT803X_CDT_STATUS_STAT_OPEN:
    >> + case AT803X_CDT_STATUS_STAT_SHORT:
    >> + return true;
    >> + }
    >> + return false;
    >> +}
    >
    > If we uses the netlink attribute values, not the PHY specific values,
    > this could be put in the core.
    >
    >> +
    >> +static int at803x_cdt_fault_length(u16 status)
    >> +{
    >> + int dt;
    >> +
    >> + /* According to the datasheet the distance to the fault is
    >> + * DELTA_TIME * 0.824 meters.
    >> + *
    >> + * The author suspect the correct formula is:
    >> + *
    >> + * fault_distance = DELTA_TIME * (c * VF) / 125MHz / 2
    >> + *
    >> + * where c is the speed of light, VF is the velocity factor of
    >> + * the twisted pair cable, 125MHz the counter frequency and
    >> + * we need to divide by 2 because the hardware will measure the
    >> + * round trip time to the fault and back to the PHY.
    >> + *
    >> + * With a VF of 0.69 we get the factor 0.824 mentioned in the
    >> + * datasheet.
    >> + */
    >> + dt = FIELD_GET(AT803X_CDT_STATUS_DELTA_TIME_MASK, status);
    >> +
    >> + return (dt * 824) / 10;
    >> +}
    >
    > There seems to be a general consensus of 0.824 meters. So we could
    > have helpers to convert back and forth in the core.

    Yeah, but I don't know why VF is 0.69 (and it differs per cable, so
    its just some kind of sensible default value). According to
    https://en.wikipedia.org/wiki/Velocity_factor it should be something
    around 0.64 or 0.65.

    -michael

    >
    >> +static int at803x_cable_test_start(struct phy_device *phydev)
    >> +{
    >> + /* Enable auto-negotiation, but advertise no capabilities, no link
    >> + * will be established. A restart of the auto-negotiation is not
    >> + * required, because the cable test will automatically break the
    >> link.
    >> + */
    >> + phy_write(phydev, MII_BMCR, BMCR_ANENABLE);
    >> + phy_write(phydev, MII_ADVERTISE, ADVERTISE_CSMA);
    >> + phy_write(phydev, MII_CTRL1000, 0);
    >
    > Could be a genphy_ helper.
    >
    > Lets get the code merged, when we can come back and do some
    > refactoring.
    >
    > Andrew

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