Re: [PATCH net-next 0/2] net: dsa: realtek: fix PHY register read corruption

[Alvin Šipraga]

Andrew Lunn <andrew@xxxxxxx> writes:
>> Hmm OK. Actually I'm a bit confused about the mdio_lock: can you explain
>> what it's guarding against, for someone unfamiliar with MDIO?
>
> The more normal use case for MDIO is for PHYs, not switches. There can
> be multiple PHYs on one MDIO bus. And these PHYs each have there own
> state machine in phylib. At any point in time, that state machine can
> request the driver to do something, like poll the PHY status, does it
> have link? To prevent two PHY drivers trying to use the MDIO bus at
> the same time, there is an MDIO lock. At the beginning of an MDIO
> transaction, the lock is taken. And the end of the transaction,
> reading or writing one register of a device on the bus, the lock is
> released.
>
> So the MDIO lock simply ensures there is only one user of the MDIO bus
> at one time, for a single read or write.
>
> For PHYs this is sufficient. For switches, sometimes you need
> additional protection. The granularity of an access might not be a
> single register read or a write. It could be you need to read or write
> a few registers in an atomic way. If that is the case, you need a lock
> at a higher level.

Thank you Andrew for the clear explanation.

Somewhat unrelated to this series, but are you able to explain to me the
difference between:

	mutex_lock(&bus->mdio_lock);
and
	mutex_lock_nested(&bus->mdio_lock, MDIO_MUTEX_NESTED);

While looking at other driver examples I noticed the latter form quite a
few times too.

Kind regards,
Alvin