netdev - Re: [PATCH net-next v5 07/13] net: ethtool: Introduce a command to list PHYs on an interface

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Date: Thu, 25 Jan 2024 09:22:25 +0100
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 thomas.petazzoni@...tlin.com, Eric Dumazet <edumazet@...gle.com>, Paolo
 Abeni <pabeni@...hat.com>, Russell King <linux@...linux.org.uk>,
 linux-arm-kernel@...ts.infradead.org, Christophe Leroy
 <christophe.leroy@...roup.eu>, Herve Codina <herve.codina@...tlin.com>,
 Florian Fainelli <f.fainelli@...il.com>, Heiner Kallweit
 <hkallweit1@...il.com>, Vladimir Oltean <vladimir.oltean@....com>,
 Köry Maincent <kory.maincent@...tlin.com>, Jesse Brandeburg
 <jesse.brandeburg@...el.com>, Jonathan Corbet <corbet@....net>, Marek
 Behún <kabel@...nel.org>, Piergiorgio Beruto
 <piergiorgio.beruto@...il.com>, Oleksij Rempel <o.rempel@...gutronix.de>,
 Nicolò Veronese <nicveronese@...il.com>, Simon Horman
 <horms@...nel.org>
Subject: Re: [PATCH net-next v5 07/13] net: ethtool: Introduce a command to
 list PHYs on an interface

On Wed, 24 Jan 2024 17:54:53 +0100
Andrew Lunn <andrew@...n.ch> wrote:

> > > Another option might be to add PHY support to netdevsim. Add a debugfs
> > > interface to allow you to create arbitrary PHY topologies? You can
> > > then even add a test script.  
> > 
> > Sorry for the delayed answer, I just took a few hours to give it a try,
> > and I was able to spin some very basic PHY support for the netdevsim,
> > allowing to attach arbitrary instances of fixed_phy devices. I can
> > therefore use that as a mean of testing the dump operation, I'll try to
> > include that in the next iteration, that should pave the way for some
> > testability of more PHY stuff hopefully.  
> 
> Great that you looked at this.
> 
> FYI: Jakub would like to see changes to netdevsim accompanied with
> self tests making you of the features you add. There is also now a
> build bot running the self tests against net-next, so these tests
> should get run quite frequently.

No problem, I'll include these as well.

I do face a problem with fixed_phy though now that I've played around
with it. As fixed_phys share the same global MDIO bus, what can happen
is that netdevsim-registered PHYs can starve the dummy MDIO bus by
exhausting all 32 mdio addresses, preventing real interfaces from
getting their own fixed-phy instance.

I'll probably register a dedicated mdio bus per netdevsim (or even
per-phy, so that we can imagine controling the returned register
values), let's see how it goes.

Thanks,

Maxime