netdev - Re: PoE complex usage of regulator API

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Message-ID: <Zi_BKFa4Sk3IAFFU@finisterre.sirena.org.uk>
Date: Tue, 30 Apr 2024 00:47:52 +0900
From: Mark Brown <broonie@...nel.org>
To: Andrew Lunn <andrew@...n.ch>
Cc: Kory Maincent <kory.maincent@...tlin.com>,
	Oleksij Rempel <o.rempel@...gutronix.de>,
	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, Apr 27, 2024 at 12:41:19AM +0200, Andrew Lunn wrote:

> 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.

The current limits for regulators are generally imposed in hardware as a
safety measure, this also happens for example with USB where there's
regulators in the PHYs.  Whatever is providing the power is very likely
to have reasonable headroom for robustness.

> We are probably not so much interested in what the actual current
> power draw is, assuming there is no wish to over provision?

One of the goals is to protect the system in the case that something
malfunctions and tries to draw more current than can be sustained.  A
system that is overprovisioned might choose to allow excessive draw,
especially transiently to cover bootsrapping issues, though there's
tradeoffs with system protection vs interoperability with poor quality
implementations there.

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