linux-kernel - Re: [PATCH net-next v3 08/47] net: phylink: Support differing link speeds and interface speeds

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Date:   Sat, 16 Jul 2022 22:06:01 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Sean Anderson <sean.anderson@...o.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Madalin Bucur <madalin.bucur@....com>, netdev@...r.kernel.org,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        linux-arm-kernel@...ts.infradead.org,
        Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org,
        Alexandru Marginean <alexandru.marginean@....com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Vladimir Oltean <olteanv@...il.com>
Subject: Re: [PATCH net-next v3 08/47] net: phylink: Support differing link
 speeds and interface speeds

> +/**
> + * phy_interface_speed() - get the speed of a phy interface
> + * @interface: phy interface mode defined by &typedef phy_interface_t
> + * @link_speed: the speed of the link
> + *
> + * Some phy interfaces modes adapt to the speed of the underlying link (such as
> + * by duplicating data or changing the clock rate). Others, however, are fixed
> + * at a particular rate. Determine the speed of a phy interface mode for a
> + * particular link speed.
> + *
> + * Return: The speed of @interface
> + */
> +static int phy_interface_speed(phy_interface_t interface, int link_speed)
> +{
> +	switch (interface) {
> +	case PHY_INTERFACE_MODE_100BASEX:
> +		return SPEED_100;
> +
> +	case PHY_INTERFACE_MODE_TBI:
> +	case PHY_INTERFACE_MODE_MOCA:
> +	case PHY_INTERFACE_MODE_RTBI:
> +	case PHY_INTERFACE_MODE_1000BASEX:
> +	case PHY_INTERFACE_MODE_1000BASEKX:
> +	case PHY_INTERFACE_MODE_TRGMII:
> +		return SPEED_1000;
> +
> +	case PHY_INTERFACE_MODE_2500BASEX:
> +		return SPEED_2500;
> +
> +	case PHY_INTERFACE_MODE_5GBASER:
> +		return SPEED_5000;
> +
> +	case PHY_INTERFACE_MODE_XGMII:
> +	case PHY_INTERFACE_MODE_RXAUI:
> +	case PHY_INTERFACE_MODE_XAUI:
> +	case PHY_INTERFACE_MODE_10GBASER:
> +	case PHY_INTERFACE_MODE_10GKR:
> +		return SPEED_10000;
> +
> +	case PHY_INTERFACE_MODE_25GBASER:
> +		return SPEED_25000;
> +
> +	case PHY_INTERFACE_MODE_XLGMII:
> +		return SPEED_40000;
> +
> +	case PHY_INTERFACE_MODE_USXGMII:
> +	case PHY_INTERFACE_MODE_RGMII_TXID:
> +	case PHY_INTERFACE_MODE_RGMII_RXID:
> +	case PHY_INTERFACE_MODE_RGMII_ID:
> +	case PHY_INTERFACE_MODE_RGMII:
> +	case PHY_INTERFACE_MODE_QSGMII:
> +	case PHY_INTERFACE_MODE_SGMII:
> +	case PHY_INTERFACE_MODE_GMII:
> +	case PHY_INTERFACE_MODE_REVRMII:
> +	case PHY_INTERFACE_MODE_RMII:
> +	case PHY_INTERFACE_MODE_SMII:
> +	case PHY_INTERFACE_MODE_REVMII:
> +	case PHY_INTERFACE_MODE_MII:
> +	case PHY_INTERFACE_MODE_INTERNAL:
> +		return link_speed;
> +
> +	case PHY_INTERFACE_MODE_NA:
> +	case PHY_INTERFACE_MODE_MAX:
> +		break;
> +	}
> +
> +	return SPEED_UNKNOWN;

This seem error prone when new PHY_INTERFACE_MODES are added. I would
prefer a WARN_ON_ONCE() in the default: so we get to know about such
problems.

I'm also wondering if we need a sanity check here. I've seen quite a
few boards a Fast Ethernet MAC, but a 1G PHY because they are
cheap. In such cases, the MAC is supposed to call phy_set_max_speed()
to indicate it can only do 100Mbs. PHY_INTERFACE_MODE_MII but a
link_speed of 1G is clearly wrong. Are there other cases where we
could have a link speed faster than what the interface mode allows?

Bike shedding a bit, but would it be better to use host_side_speed and
line_side_speed? When you say link_speed, which link are your
referring to? Since we are talking about the different sides of the
PHY doing different speeds, the naming does need to be clear.

	  Andrew