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

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Message-ID: <YUSs+efLowuhL09Q@shell.armlinux.org.uk>
Date:   Fri, 17 Sep 2021 15:58:01 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Andrew Lunn <andrew@...n.ch>
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 net-next] net: phy: marvell10g: add downshift tunable
 support

On Fri, Sep 17, 2021 at 04:45:03PM +0200, Andrew Lunn wrote:
> > +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;
> > +
> > +	if (!priv->has_downshift)
> > +		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;
> 
> Hi Russell
> 
> Rather than a FIXME, maybe just document that the hardware default is
> disabled, which does not make too much sense, so default to 2 attempts?

Sadly, the downshift parameters aren't documented at all in the kernel,
and one has to dig into the ethtool source to find out what they mean:

DOWNSHIFT_DEV_DEFAULT_COUNT -
	ethtool --set-phy-tunable ethN downshift on
DOWNSHIFT_DEV_DISABLE -
	ethtool --set-phy-tunable ethN downshift off
otherwise:
	ethtool --set-phy-tunable ethN downshift count N

This really needs to be documented somewhere in the kernel.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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