linux-kernel - Re: [PATCH net-next v2 1/1] phy: micrel: add Signal Quality Indicator (SQI) support for KSZ9477 switch PHYs

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Message-ID: <aFwlc6Iwko8UFwa5@pengutronix.de>
Date: Wed, 25 Jun 2025 18:36:03 +0200
From: Oleksij Rempel <o.rempel@...gutronix.de>
To: Maxime Chevallier <maxime.chevallier@...tlin.com>
Cc: Andrew Lunn <andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	kernel@...gutronix.de, linux-kernel@...r.kernel.org,
	Russell King <linux@...linux.org.uk>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2 1/1] phy: micrel: add Signal Quality
 Indicator (SQI) support for KSZ9477 switch PHYs

On Wed, Jun 25, 2025 at 05:33:23PM +0200, Maxime Chevallier wrote:
> Hi Oleksij,
> 
> On Wed, 25 Jun 2025 14:41:26 +0200
> Oleksij Rempel <o.rempel@...gutronix.de> wrote:
> 
> > Add support for the Signal Quality Index (SQI) feature on KSZ9477 family
> > switches. This feature provides a relative measure of receive signal
> > quality.
> > 
> > The KSZ9477 PHY provides four separate SQI values for a 1000BASE-T link,
> > one for each differential pair (Channel A-D). Since the current get_sqi
> > UAPI only supports returning a single value per port, this
> > implementation reads the SQI from Channel A as a representative metric.
> > This can be extended to provide per-channel readings once the UAPI is
> > enhanced for multi-channel support.
> > 
> > The hardware provides a raw 7-bit SQI value (0-127), where lower is
> > better. This raw value is converted to the standard 0-7 scale to provide
> > a usable, interoperable metric for userspace tools, abstracting away
> > hardware-specifics. The mapping to the standard 0-7 SQI scale was
> > determined empirically by injecting a 30MHz sine wave into the receive
> > pair with a signal generator. It was observed that the link becomes
> > unstable and drops when the raw SQI value reaches 8. This
> > implementation is based on these test results.
> 
> [...]
> 
> > +/**
> > + * kszphy_get_sqi - Read, average, and map Signal Quality Index (SQI)
> > + * @phydev: the PHY device
> > + *
> > + * This function reads and processes the raw Signal Quality Index from the
> > + * PHY. Based on empirical testing, a raw value of 8 or higher indicates a
> > + * pre-failure state and is mapped to SQI 0. Raw values from 0-7 are
> > + * mapped to the standard 0-7 SQI scale via a lookup table.
> > + *
> > + * Return: SQI value (0–7), or a negative errno on failure.
> > + */
> > +static int kszphy_get_sqi(struct phy_device *phydev)
> > +{
> > +	int sum = 0;
> > +	int i, val, raw_sqi, avg_raw_sqi;
> > +	u8 channels;
> > +
> > +	/* Determine applicable channels based on link speed */
> > +	if (phydev->speed == SPEED_1000)
> > +		/* TODO: current SQI API only supports 1 channel. */
> > +		channels = 1;
> > +	else if (phydev->speed == SPEED_100)
> > +		channels = 1;
> 
> I understand the placeholder logic waiting for some improved uAPI, but
> this triggers an unused variable warning :( I think the commit log and
> the comment below are enough to explain that this can be improved later
> on.

Grr.. sorry.. 

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