netdev - Re: [PATCH net-next] net: phy: at803x: add cable diagnostics support

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Message-ID: <20200510145250.GK362499@lunn.ch>
Date:   Sun, 10 May 2020 16:52:50 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Michael Walle <michael@...le.cc>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next] net: phy: at803x: add cable diagnostics support

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.

> +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