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

[Andrew Lunn]

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