netdev - Re: PoE complex usage of regulator API

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Message-ID: <20240429145203.219bee06@kmaincent-XPS-13-7390>
Date: Mon, 29 Apr 2024 14:52:03 +0200
From: Kory Maincent <kory.maincent@...tlin.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: Oleksij Rempel <o.rempel@...gutronix.de>, Mark Brown
 <broonie@...nel.org>, Kyle Swenson <kyle.swenson@....tech>, Liam Girdwood
 <lgirdwood@...il.com>, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org, Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: PoE complex usage of regulator API

On Sat, 27 Apr 2024 00:41:19 +0200
Andrew Lunn <andrew@...n.ch> wrote:

> > Let's begin simple, in PSE world we are more talking about power.
> > Would it be ok to add a regulator_get/set_power_limit() and
> > regulator_get_power() callback to regulator API. Would regulator API have
> > interest to such callbacks?  
> 
> Could you define this API in more details.

The first new PoE features targeted by this API was to read the consumed power
and get set the power limit for each ports. Yes mainly book keeping.
Few drivers callbacks that will be called by ethtool and maybe the read of power
limit and consumed power could be add to read-only sysfs regulator.

> I'm assuming this is mostly about book keeping? When a regulator is
> created, we want to say is can deliver up to X Kilowatts. We then want
> to allocate power to ports. So there needs to be a call asking it to
> allocate part of X to a consumer, which could fail if there is not
> sufficient power budget left. And there needs to be a call to release
> such an allocation.

This is more the aim of the second point I have raised, power priority and
parent power budget. And how the core can manage it.

> We are probably not so much interested in what the actual current
> power draw is, assuming there is no wish to over provision?
> 
> There is in theory a potential second user of this. Intel have been
> looking at power control for SFPs. Typically they are guaranteed a
> minimum of 1.5W. However, they can operate at higher power
> classes. You can have boards with multiple SFPs, with a theoretical
> maximum power draw more than what the supply can supply. So you need
> similar sort of power budget book keeping to allocate power to an SFP
> cage before telling the SFP module it can swap to a higher power
> class. I say this is theoretical, because the device Intel is working
> on has this hidden away in firmware. But maybe sometime in the future
> somebody will want Linux doing this.

So there is a potential second user, that's great to hear it! Could the
priority stuff be also interesting? Like to allow only high priority SFP to use
higher power class in case of a limiting power budget.

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