Message-ID: <Zi--4ko_vAtFSxyn@finisterre.sirena.org.uk>
Date: Tue, 30 Apr 2024 00:38:10 +0900
From: Mark Brown <broonie@...nel.org>
To: Kory Maincent <kory.maincent@...tlin.com>
Cc: 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 Fri, Apr 26, 2024 at 12:42:53PM +0200, Kory Maincent 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?

Why would these be different to the existing support for doing current
limiting?  If the voltage for the supply is known then the power is a
simple function of the current and the voltage.  I suppose you could try
to do a convenience functions for a fixed voltage, but there'd be issues
there if the voltage isn't configured to an exact voltage and might
vary.

> Port priority, more complex subject:
> Indeed a PSE controller managing several ports may be able to turn off ports
> with low priority if the total power consumption exceed a certain level.
> - There are controller like PD692x0 that can managed this on the hardware side.
>   In that case we would have a regulator_get/set_power_limit() callbacks from
>   the regulator parent (the PSE contoller) and a regulator_get/set_priory()
>   callbacks for the regulator children (PSE ports).

All this priority stuff feels very PSE specific but possibly doable.
You'd have to define the domains in which priorities apply as well as
the priorities themselves.

> - There are controller like TPS23881 or LTC4266 that can set two priorities
>   levels on their ports and a level change in one of their input pin can
>   shutdown all the low priority ports. In that case the same callbacks could be
>   used. regulator_get/set_power_limit() from the parent will be only at software
>   level. regulator_get/set_priority() will set the priorities of the ports on
>   hardware level. A polling function have to read frequently the total power
>   used and compare it to the power budget, then it has to call something like
>   regulator_shutdown_consumer() in case of power overflow.  

I would expect the regulators can generate notifications when they go
out of regulation?  Having to poll feels very crude for something with
configurable power limits.

> https://lore.kernel.org/netdev/20240417-feature_poe-v9-10-242293fd1900@bootlin.com/
> But in case the port is enabled from Linux then shutdown from the PSE controller
> for any reason, I have to run disable and enable command to enable it again. Not
> really efficient :/

If that is a hot path something has gone very wrong with the system,
especially if it's such a hot path that the cost of a disable is making
a difference.  Note that hardware may have multiple error handling
strategies, some hardware will turn off outputs when there's a problem
while other implementations will try to provide as good an output as
they can.  Sometimes the strategy will depend on the specific error
condition, and there may be timeouts involved.  This all makes it very
difficult to assume any particular state after an error has occurred, or
that the state configured in the control registers reflects the physical
state of the hardware so you probably want some explicit handling for
any new state you're looking for.

> I am thinking of disabling the usage of counters in case of a
> regulator_get_exclusive(). What do you think? Could it break other usage?

Yes, that seems likely to break other users and in general a sharp edge
for people working with the API.

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