linux-kernel - Re: Enabling pmbus power control

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Message-ID: <20210420033648.GA227111@roeck-us.net>
Date:   Mon, 19 Apr 2021 20:36:48 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Zev Weiss <zev@...ilderbeest.net>
Cc:     Mark Brown <broonie@...nel.org>, Jean Delvare <jdelvare@...e.com>,
        linux-hwmon@...r.kernel.org, Andrew Jeffery <andrew@...id.au>,
        Liam Girdwood <lgirdwood@...il.com>,
        linux-kernel@...r.kernel.org, openbmc@...ts.ozlabs.org
Subject: Re: Enabling pmbus power control

On Mon, Apr 19, 2021 at 08:29:53PM -0500, Zev Weiss wrote:
> On Tue, Mar 30, 2021 at 02:38:10PM CDT, Guenter Roeck wrote:
> > On Tue, Mar 30, 2021 at 07:02:00PM +0100, Mark Brown wrote:
> > > On Tue, Mar 30, 2021 at 12:56:56PM -0500, Zev Weiss wrote:
> > > 
> > > > Okay, to expand a bit on the description in my initial message -- we've
> > > > got a single chassis with multiple server boards and a single manager board
> > > > that handles, among other things, power control for the servers.
> > > > The manager board has one LM25066 for each attached server, which acts as
> > > > the "power switch" for that server.  There thus really isn't any driver to
> > > > speak of for the downstream device.
> > > 
> > > This sounds like you need a driver representing those server boards (or
> > > the slots they plug into perhaps) that represents everything about those
> > > boards to userspace, including power switching.  I don't see why you
> > > wouldn't have a driver for that - it's a thing that physically exists
> > > and clearly has some software control, and you'd presumably also expect
> > > to represent some labelling about the slot as well.
> > 
> > Absolutely agree.
> > 
> > Thanks,
> > Guenter
> 
> Hi Guenter, Mark,
> 
> I wanted to return to this to try to explain why structuring the kernel
> support for this in a way that's specific to the device behind the PMIC
> seems like an awkward fit to me, and ultimately kind of artificial.
> 
> In the system I described, the manager board with the LM25066 devices is its
> own little self-contained BMC system running its own Linux kernel (though
> "BMC" is perhaps a slightly misleading term because there's no host system
> that it manages).  The PMICs are really the only relevant connection it has
> to the servers it controls power for -- they have their own dedicated local
> BMCs on board as well doing all the usual BMC tasks.  A hypothetical
> dedicated driver for this on the manager board wouldn't have any other
> hardware to touch aside from the pmbus interface of the LM25066 itself, so
> calling it a server board driver seems pretty arbitrary -- and in fact the
> same system also has another LM25066 that controls the power supply to the
> chassis cooling fans (a totally different downstream device, but one that
> would be equally well-served by the same software).
> 
> More recently, another system has entered the picture for us that might
> illustrate it more starkly -- the Delta Open19 power shelf [0] supported by
> a recent code release from LinkedIn [1].  This is a rackmount power supply

All I can see is that this code is a mess.

> with fifty outputs, each independently switchable via an LM25066 attached to
> an on-board BMC-style management controller (though again, no host system
> involved).  We (Equinix Metal) are interested in porting a modern OpenBMC to
> it to replace the dated, crufty, pre-Linux-Foundation version of OpenBMC it
> currently runs (as found in the linked repo).  The exact nature of the
> devices powered by its outputs is well outside the scope of the firmware
> running on that controller (it could be any arbitrary thing that runs on
> 12VDC), but we still want to be able to both (a) retrieve per-output
> voltage/current/power readings as provided by the existing LM25066 hwmon
> support, and (b) control the on/off state of those outputs from userspace.
> 
> Given the array of possible use-cases, an approach of adding power-switch
> functionality to the existing LM25066 support seems like the most obvious
> way to support this, so I'm hoping to see if I can get some idea of what an
> acceptable implementation of that might look like.
> 

Sorry, that is simply a no-go for the LM25066 driver. I mentioned it before,
and it is still true: The hwmon subsystem is not in the business of power
control.

Have you looked into enclosures (drivers/misc/enclosure.c) ? Maybe that
or something similar could be used for your purposes.

Thanks,
Guenter