Re: [PATCH net-next] net: phy: introduce phydev->port

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Wed, Feb 10, 2021 at 02:51:34AM +0100, Andrew Lunn wrote:
> This is a general comment, not a problem specific to this patch.
> 
> There is some interesting race conditions here. The marvell driver
> first checks the fibre page and gets the status of the fiber port. As
> you can see from the hunk above, it clears out pause, duplex, speed,
> sets port to PORT_FIBRE, and then reads the PHY registers to set these
> values. If link is not detected on the fibre, it swaps page and does
> it all again, but for the copper port. So once per second,
> phydev->port is going to flip flop PORT_FIBER->PORT_TP, if copper has
> link.
> 
> Now, the read_status() call into the driver should be performed while
> holding the phydev->lock. So to the PHY state machine, this flip/flop
> does not matter, it is atomic with respect to the lock. But
> phy_ethtool_ksettings_get() is not talking the lock. It could see
> speed, duplex, and port while they have _UNKNOWN values, or port is
> part way through a flip flop. I think we need to take the lock here.
> phy_ethtool_ksettings_set() should also probably take the lock.

phy_ethtool_ksettings_get() needs to take the lock, otherwise it could
read the phy_device members in the middle of an update. This is likely
a long-standing phylib bug.

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