linux-kernel - Re: [RFC net-next 1/4] net: phy: add MediaTek PHY driver

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Date:   Tue, 6 Apr 2021 17:21:23 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     DENG Qingfang <dqfext@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Landen Chao <Landen.Chao@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Sean Wang <sean.wang@...iatek.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sergio Paracuellos <sergio.paracuellos@...il.com>,
        linux-kernel@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        linux-staging@...ts.linux.dev, devicetree@...r.kernel.org,
        netdev@...r.kernel.org, Weijie Gao <weijie.gao@...iatek.com>,
        Chuanhong Guo <gch981213@...il.com>,
        René van Dorst <opensource@...rst.com>
Subject: Re: [RFC net-next 1/4] net: phy: add MediaTek PHY driver

On Tue, Apr 06, 2021 at 10:18:16PM +0800, DENG Qingfang wrote:
> Add support for MediaTek PHYs found in MT7530 and MT7531 switches.

Do you know if this PHY is available standalone?

> +static int mt7531_phy_config_init(struct phy_device *phydev)
> +{
> +	mtk_phy_config_init(phydev);
> +
> +	/* PHY link down power saving enable */
> +	phy_set_bits(phydev, 0x17, BIT(4));
> +	phy_clear_bits_mmd(phydev, MDIO_MMD_VEND1, 0xc6, 0x300);
> +
> +	/* Set TX Pair delay selection */
> +	phy_write_mmd(phydev, MDIO_MMD_VEND1, 0x13, 0x404);
> +	phy_write_mmd(phydev, MDIO_MMD_VEND1, 0x14, 0x404);

This gets me worried about RGMII delays. We have had bad backwards
compatibility problems with PHY drivers which get RGMII delays wrong.

Since this is an internal PHY, i suggest you add a test to the
beginning of mt7531_phy_config_init():

	if (phydev->interface != PHY_INTERFACE_MODE_INTERNAL)
		return -EINVAL;

We can then solve RGMII problems when somebody actually needs RGMII.

   Andrew