netdev - Re: [PATCH v2] net: phy: micrel: ksz9031/ksz9131: add cabletest support

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Date:   Thu, 7 Apr 2022 12:47:54 +0200
From:   Marek Vasut <marex@...x.de>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     netdev@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        "David S . Miller" <davem@...emloft.net>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Paolo Abeni <pabeni@...hat.com>, kernel@...gutronix.de
Subject: Re: [PATCH v2] net: phy: micrel: ksz9031/ksz9131: add cabletest
 support

On 4/7/22 07:06, Oleksij Rempel wrote:
> On Thu, Apr 07, 2022 at 04:08:12AM +0200, Marek Vasut wrote:
> ...
> 
>> +static int ksz9x31_cable_test_result_trans(u16 status)
>> +{
>> +	switch (FIELD_GET(KSZ9x31_LMD_VCT_ST_MASK, status)) {
>> +	case KSZ9x31_LMD_VCT_ST_NORMAL:
>> +		return ETHTOOL_A_CABLE_RESULT_CODE_OK;
>> +	case KSZ9x31_LMD_VCT_ST_OPEN:
>> +		return ETHTOOL_A_CABLE_RESULT_CODE_SAME_SHORT;
>> +	case KSZ9x31_LMD_VCT_ST_SHORT:
>> +		return ETHTOOL_A_CABLE_RESULT_CODE_OPEN;
> 
> This conversation looks twisted:
> VCT_ST_OPEN -> CODE_SAME_SHORT
> VCT_ST_SHORT -> CODE_OPEN

Doh ... fixed

>> +	case KSZ9x31_LMD_VCT_ST_FAIL:
>> +		fallthrough;
>> +	default:
>> +		return ETHTOOL_A_CABLE_RESULT_CODE_UNSPEC;
>> +	}
>> +}
>> +
> ...
>> +static int ksz9x31_cable_test_get_status(struct phy_device *phydev,
>> +					 bool *finished)
>> +{
>> +	struct kszphy_priv *priv = phydev->priv;
>> +	unsigned long pair_mask = 0xf;
>> +	int retries = 20;
>> +	int pair, ret, rv;
>> +
>> +	*finished = false;
>> +
>> +	/* Try harder if link partner is active */
>> +	while (pair_mask && retries--) {
>> +		for_each_set_bit(pair, &pair_mask, 4) {
>> +			ret = ksz9x31_cable_test_one_pair(phydev, pair);
>> +			if (ret == -EAGAIN)
>> +				continue;
>> +			if (ret < 0)
>> +				return ret;
>> +			clear_bit(pair, &pair_mask);
>> +		}
>> +		/* If link partner is in autonegotiation mode it will send 2ms
>> +		 * of FLPs with at least 6ms of silence.
>> +		 * Add 2ms sleep to have better chances to hit this silence.
>> +		 */
>> +		if (pair_mask)
>> +			usleep_range(2000, 3000);
> 
> Not a blocker, just some ideas:
> In my experience, active link partner may affect test result in way,
> that the PHY wont report it as failed test. Especially, if the cable is
> on the edge of specification (for example too long cable).
> At same time 6ms of silence is PHY specific implementation. For example
> KSZ PHYs tend to send burst of FLPs and the switch to other MDI-X pair
> (if auto MDI-X is enabled). Some RTL PHYs tend to send random firework of pulses
> on different pairs.
> 
> May be we should not fight against autoneg and misuse it a bit? For
> example:
> 0. force MDI configuration
> 1. limit autoneg to 10mbit

We can't do that in this case, can we ?

KSZ9131RNX, DS00002841B-page 38, 4.14 LinkMD (R) Cable Diagnostic
"
Prior to running the cable diagnostics, Auto-negotiation should
be disabled, full duplex set and the link speed set to 1000Mbps
via the Basic Control Register.
"

> 2. allow the link
> 3. switch to test and run on unused pair
> 4. force MDI-X configuration and goto 1.
> 
> Other options can be:
> - implement autoneg next page tx/rx and pass linux-vendor specific
>    information over it (some thing like, please stop autoneg for X amount
>    of sec)
> - advertise remote fault

Wouldn't that only work if the link partner is also up to date linux 
machine ?