Re: [PATCH net-next v1 2/4] net: phy: micrel: lan8841: set default PTP latency values

[Andrew Lunn]

On Wed, Apr 17, 2024 at 06:43:14PM +0200, Oleksij Rempel wrote:
> Set default PTP latency values to provide realistic path delay
> measurements and reflecting internal PHY latency asymetry.
> 
> This values are based on ptp4l measurements for the path delay against
> identical PHY as link partner and latency asymmetry extracted from
> documented SOF Latency values of this PHY.
> 
> Documented SOF Latency values are:
> TX 138ns/RX 430ns @ 1000Mbps
> TX 140ns/RX 615ns @ 100Mbps (fixed latency mode)
> TX 140ns/RX 488-524ns @ 100Mbps (variable latency mode)
> TX 654ns/227-2577ns @ 10Mbps

Does Half Duplex vs Full Duplex make a difference here?

> +static int lan8841_ptp_latency_init(struct phy_device *phydev)
> +{
> +	int ret;
> +
> +	ret = phy_write_mmd(phydev, KSZ9131RN_MMD_COMMON_CTRL_REG,
> +			    LAN8841_PTP_RX_LATENCY_10M,
> +			    LAN8841_PTP_RX_LATENCY_10M_VAL);
> +	if (ret)
> +		return ret;
> +
> +	ret = phy_write_mmd(phydev, KSZ9131RN_MMD_COMMON_CTRL_REG,
> +			    LAN8841_PTP_TX_LATENCY_10M,
> +			    LAN8841_PTP_TX_LATENCY_10M_VAL);
> +	if (ret)
> +		return ret;
> +
> +	ret = phy_write_mmd(phydev, KSZ9131RN_MMD_COMMON_CTRL_REG,
> +			    LAN8841_PTP_RX_LATENCY_100M,
> +			    LAN8841_PTP_RX_LATENCY_100M_VAL);
> +	if (ret)
> +		return ret;
> +
> +	ret = phy_write_mmd(phydev, KSZ9131RN_MMD_COMMON_CTRL_REG,
> +			    LAN8841_PTP_TX_LATENCY_100M,
> +			    LAN8841_PTP_TX_LATENCY_100M_VAL);
> +	if (ret)
> +		return ret;
> +
> +	ret = phy_write_mmd(phydev, KSZ9131RN_MMD_COMMON_CTRL_REG,
> +			    LAN8841_PTP_RX_LATENCY_1000M,
> +			    LAN8841_PTP_RX_LATENCY_1000M_VAL);
> +	if (ret)
> +		return ret;
> +
> +	return phy_write_mmd(phydev, KSZ9131RN_MMD_COMMON_CTRL_REG,
> +			     LAN8841_PTP_TX_LATENCY_1000M,
> +			     LAN8841_PTP_TX_LATENCY_1000M_VAL);
> +}

What affect does this have on systems which have already applied
adjustments in user space to correct for this? Will this cause
regressions for such systems?

I know Richard has rejected changes like this in the past.

	Andrew