netdev - Re: [PATCH net-next v2 0/9] Allow isolating PHY devices

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Message-ID: <20241007122513.4ab8e77b@device-21.home>
Date: Mon, 7 Oct 2024 12:25:13 +0200
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: davem@...emloft.net, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, thomas.petazzoni@...tlin.com, Andrew Lunn
 <andrew@...n.ch>, Jakub Kicinski <kuba@...nel.org>, Eric Dumazet
 <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 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>, Marek
 Behún <kabel@...nel.org>, Köry Maincent
 <kory.maincent@...tlin.com>, Oleksij Rempel <o.rempel@...gutronix.de>
Subject: Re: [PATCH net-next v2 0/9] Allow isolating PHY devices

Hello Russell

On Fri, 4 Oct 2024 18:02:25 +0100
"Russell King (Oracle)" <linux@...linux.org.uk> wrote:

> I'm going to ask a very basic question concerning this.
> 
> Isolation was present in PHYs early on when speeds were low, and thus
> electrical reflections weren't too much of a problem, and thus star
> topologies didn't have too much of an effect. A star topology is
> multi-drop. Even if the PCB tracks go from MAC to PHY1 and then onto
> PHY2, if PHY2 is isolated, there are two paths that the signal will
> take, one to MAC and the other to PHY2. If there's no impediance match
> at PHY2 (e.g. because it's in high-impedance mode) then that
> transmission line is unterminated, and thus will reflect back towards
> the MAC.
> 
> As speeds get faster, then reflections from unterminated ends become
> more of an issue.
> 
> I suspect the reason why e.g. 88x3310, 88E1111 etc do not support
> isolate mode is because of this - especially when being used in
> serdes mode, the topology is essentially point-to-point and any
> side branches can end up causing data corruption.

I suspect indeed that this won't work on serdes interfaces. I didn't
find any reliable information on that, but so far the few PHYs I've
seen seem to work that way.

The 88e1512 supports that, but I was testing in RGMII.

> 
> So my questions would be, is adding support for isolation mode in
> PHYs given todays network speeds something that is realistic, and
> do we have actual hardware out there where there is more than one
> PHY in the bus. If there is, it may be useful to include details
> of that (such as PHY interface type) in the patch series description.

I do have some hardware with this configuration (I'd like to support
that upstream, the topology work was preliminary work for that, and the
next move would be to send an RFC for these topolopgies exactly).

I am working with 3 different HW platforms with this layout :

      /--- PHY
      |
MAC  -|  phy_interface_mode == MII so, 100Mbps Max.
      |
      \--- PHY

and another that is similar but with RMII. I finally have one last case
with MII interface, same layout, but the PHYs can't isolate so we need
to make sure all but one PHYs are powered-down at any given time.

I will include that in the cover.

Could we consider limiting the isolation to non-serdes interfaces ?
that would be :

 - MII
 - RMII
 - GMII
 - RGMII and its -[TX|RX] ID flavours
 - TBI and RTBI ?? (I'm not sure about these)

Trying to isolate a PHY that doesn't have any of the interfaces above
would result in -EOPNOTSUPP ?

Thanks,

Maxime