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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 16 Feb 2022 19:26:43 +0000
From:   Alvin Šipraga <ALSI@...g-olufsen.dk>
To:     Andrew Lunn <andrew@...n.ch>
CC:     Luiz Angelo Daros de Luca <luizluca@...il.com>,
        Alvin Šipraga <alvin@...s.dk>,
        Linus Walleij <linus.walleij@...aro.org>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Michael Rasmussen <MIR@...g-olufsen.dk>,
        Arınç ÜNAL <arinc.unal@...nc9.com>,
        "open list:NETWORKING DRIVERS" <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 0/2] net: dsa: realtek: fix PHY register read
 corruption

Andrew Lunn <andrew@...n.ch> 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