netdev - Re: PoE complex usage of regulator API

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Message-ID: <57a79abd-722c-4907-b0e7-2396392ae675@lunn.ch>
Date: Sat, 27 Apr 2024 00:41:19 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Kory Maincent <kory.maincent@...tlin.com>
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

> 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.

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.

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.

      Andrew