linux-kernel - Re: [PATCH net-next v3 0/5] net: pse-pd: pd692x0: Add permanent configuration management support

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Message-ID: <20250923110451.6d402a79@kmaincent-XPS-13-7390>
Date: Tue, 23 Sep 2025 11:04:51 +0200
From: Kory Maincent <kory.maincent@...tlin.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Oleksij Rempel <o.rempel@...gutronix.de>, Andrew Lunn
 <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Jiri Pirko
 <jiri@...nulli.us>, Simon Horman <horms@...nel.org>, Jonathan Corbet
 <corbet@....net>, Donald Hunter <donald.hunter@...il.com>,
 kernel@...gutronix.de, Dent Project <dentproject@...uxfoundation.org>,
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Maxime Chevallier
 <maxime.chevallier@...tlin.com>, linux-doc@...r.kernel.org, Kyle Swenson
 <kyle.swenson@....tech>, Luka Perkov <luka.perkov@...tura.hr>, Robert Marko
 <robert.marko@...tura.hr>, Sridhar Rao <srao@...uxfoundation.org>
Subject: Re: [PATCH net-next v3 0/5] net: pse-pd: pd692x0: Add permanent
 configuration management support

On Mon, 22 Sep 2025 11:02:20 -0700
Jakub Kicinski <kuba@...nel.org> wrote:

> On Mon, 22 Sep 2025 18:20:02 +0200 Kory Maincent wrote:
>  [...]  
> > > 
> > > Right, subjectively I focused on the last sentence of Oleksij's reply.
> > > I vote we leave it out for now.    
> > 
> > I would like to restart the discussion as I have one more argument besides
> > the boot time optimization coming from Luka Perkov in CC.
> > 
> > According to him, not having this feature supported also brings an issue
> > across reboot:
> > "When a network switch reboots, any devices receiving Power over
> > Ethernet (PoE) from that switch will lose power unless the PoE
> > configuration is persisted across the reboot cycle. This creates a
> > significant operational impact: WiFi access points and other
> > PoE-powered devices will experience an unplanned hard power loss,
> > forcing them offline without any opportunity for graceful shutdown.
> > 
> > The critical issue is not the impact on the switch itself, but rather
> > the cascading effect on all dependent infrastructure. Without
> > kernel-level persistence of PoE settings, a simple switch reboot
> > (whether for maintenance, updates, or recovery) forces all connected
> > PoE devices into an abrupt power cycle. This results in extended
> > downtime as these devices must complete their full boot sequence once
> > power is restored, rather than remaining operational throughout the
> > switch's reboot process."  
> 
> Any sort of hot reset that maintains the pre-existing configuration 
> and doesn't issue resets is orthogonal to storing the configuration
> into the flash.

Indeed if the switch reboot and the PSE lose its power supply, the devices will
in any cases face a power loss. While if the PSE does not lose its power the
configuration won't be reset whether there is a permanent configuration or
not. We just need to detect during the boot if the port matrix has already
been flashed to not reconfigure all the ports.  
This argument is indeed not relevant.

Luka any other arguments in favor of permanent configuration support?

Regards,
-- 
Köry Maincent, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com