netdev - Re: [PATCH RFC net-next] net: phy: marvell10g: add downshift tunable support

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Message-ID: <YPbdfBqjgHzM+6+Z@lunn.ch>
Date:   Tue, 20 Jul 2021 16:28:12 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Russell King <rmk+kernel@...linux.org.uk>
Cc:     Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Marek Beh__n <kabel@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH RFC net-next] net: phy: marvell10g: add downshift tunable
 support

> +static int mv3310_set_downshift(struct phy_device *phydev, u8 ds)
> +{
> +	struct mv3310_priv *priv = dev_get_drvdata(&phydev->mdio.dev);
> +	u16 val;
> +	int err;
> +
> +	/* Fails to downshift with v0.3.5.0 and earlier */
> +	if (priv->firmware_ver < MV_VERSION(0,3,5,0))
> +		return -EOPNOTSUPP;
> +
> +	if (ds == DOWNSHIFT_DEV_DISABLE)
> +		return phy_clear_bits_mmd(phydev, MDIO_MMD_PCS, MV_PCS_DSC1,
> +					  MV_PCS_DSC1_ENABLE);
> +
> +	/* FIXME: The default is disabled, so should we disable? */
> +	if (ds == DOWNSHIFT_DEV_DEFAULT_COUNT)
> +		ds = 2;

Interesting question.

It is a useful feature, so i would enable it by default.

Is it possible to read the actual speed via some vendor register?  The
phy-core might then give a warning, but it is 50/50 since the link
peer might perform the downshift.

   Andrew