[PATCH v3] net: ethernet: mtk_eth_soc: implement Clause 45 MDIO access

[Russell King (Oracle)]

On Mon, Dec 27, 2021 at 04:09:22PM +0000, Daniel Golle wrote:
> Implement read and write access to IEEE 802.3 Clause 45 Ethernet
> phy registers.
> Tested on the Ubiquiti UniFi 6 LR access point featuring
> MediaTek MT7622BV WiSoC with Aquantia AQR112C.
> 
> Signed-off-by: Daniel Golle <daniel at makrotopia.org>
> ---
> v3: return -1 instead of 0xffff on error in _mtk_mdio_write

Oh no, not this "-1 disease" again.

        eth->mii_bus->write = mtk_mdio_write;

static int mtk_mdio_write(struct mii_bus *bus, int phy_addr,
                          int phy_reg, u16 val)
{
        struct mtk_eth *eth = bus->priv;

        return _mtk_mdio_write(eth, phy_addr, phy_reg, val);
}

This means if you return -1 from _mtk_mdio_write() (which for some
strange reason returns a u32, not an "int") then you actually end
up returning -EPERM. This is not an appropriate errno code.

As a general rule of thumb, if you're returning an "int" and wish
to return "this failed" then always return an appropriate negative
errno code in the kernel so there isn't any possibility of
accidentially returning -EPERM through using "return -1".

This driver needs fixing _both_ due to returning -1, and also the
return type from both _mtk_mdio_write() and _mtk_mdio_read().

To see why it's important to return a proper error code, see
drivers/net/phy/phy_device.c::get_phy_c22_id() where -ENODEV and
-EIO are specifically checked. Any other negative value here will
stop the bus being scanned and cause the bus to be torn down.

Thanks.

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