netdev - Re: [PATCH net] net: dsa: qca8k: forbid management frames access to internal PHYs if another device is on the MDIO bus

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Message-ID: <6zkm5l7wscrba2kkbpp5lhr5qeohj4vvd4rbnwrcvzyx6j7ydh@lw5rb22jheme>
Date: Fri, 25 Apr 2025 19:25:04 +0200
From: Marek Behún <kabel@...nel.org>
To: Andrew Lunn <andrew@...n.ch>
Cc: Marek Behún <kabel@...nel.org>, netdev@...r.kernel.org, 
	Vladimir Oltean <olteanv@...il.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>, 
	"Rob Herring (Arm)" <robh@...nel.org>, Javier Carrasco <javier.carrasco.cruz@...il.com>, 
	Wojciech Drewek <wojciech.drewek@...el.com>, Matthias Schiffer <mschiffer@...verse-factory.net>, 
	Christian Marangi <ansuelsmth@...il.com>, Marek Mojík <marek.mojik@....cz>
Subject: Re: [PATCH net] net: dsa: qca8k: forbid management frames access to
 internal PHYs if another device is on the MDIO bus

On Fri, Apr 25, 2025 at 06:53:21PM +0200, Andrew Lunn wrote:
> > >From the ASCII art in commit 526c8ee04bdbd4d8 you can clearly see that
> > I just assumed there is just one MDIO bus, because I saw activity on
> > an oscilloscope. I didn't test it the way you now suggested. I also
> > didn't think of the consequences for the driver design and
> > device-trees. If we prove that there is just one MDIO bus with two
> > masters instead of two separate buses that leak some noise in some
> > situations, the situation will become more complicated...
> 
> It is not really that more complex. Some of the mv88e6xxx devices have
> a similar architecture.
> 
> You need to throw away MDIO over Ethernet and stop using the switches
> master. Because it is async, it cannot be used. The switch MDIO bus
> driver then just issues bus transactions on the host MDIO bus, using
> mdiobus_read_nested().

But this is just in case when there is a separate device on the external
bus besides the switch, right?

The access over ethernet frames needs to stay because it speeds up
DSA operations. The problem is only on Turris 1.x because it breaks
WAN PHY.

Marek