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Date: Mon, 25 Oct 2021 10:54:21 -0500
From: Patrick Williams <patrick@stwcx.xyz>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH 4/5] driver core: inhibit automatic driver binding on
 reserved devices
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On Mon, Oct 25, 2021 at 04:09:59PM +0200, Greg Kroah-Hartman wrote:
> On Mon, Oct 25, 2021 at 09:02:40AM -0500, Patrick Williams wrote:
> > On Mon, Oct 25, 2021 at 03:34:05PM +0200, Greg Kroah-Hartman wrote:
> > > On Mon, Oct 25, 2021 at 08:20:05AM -0500, Patrick Williams wrote:
> > > I think "it" is "something needs to be the moderator between the two
> > > operating systems".  What is the external entity that handles the
> > > switching between the two?
> >=20
> > Ah, ok.
> >=20
> > Those usually end up being system / device specific.  In the case of th=
e BIOS
> > flash, most designs I've seen use a SPI mux between the BMC and the host
> > processor or IO hub (PCH on Xeons).  The BMC has a GPIO to control the =
mux.
> >=20
> > As far as state, the BMC on start-up will go through a set of discovery=
 code to
> > figure out where it left the system prior to getting reset.  That invol=
ves
> > looking at the power subsystem and usually doing some kind of query to =
the host
> > to see if it is alive.  These queries are mostly system / host-processo=
r design
> > specific.  I've seen anything from an IPMI/IPMB message alert from the =
BMC to
> > the BIOS to ask "are you alive" to reading host processor state over JT=
AG to
> > figure out if the processors are "making progress".
>=20
> But which processor is "in control" here over the hardware? =20

The BMC.  It owns the GPIO that controls the SPI mux. =20

But, the BMC is responsible for doing all operations in a way that doesn't =
mess
up the running host processor(s).  Pulling away the SPI flash containing the
BIOS code at an incorrect time might do that.

> What method
> is used to pass the device from one CPU to another from a logical point
> of view? =20

The state of the server as a whole is determined and maintained by the BMC.=
  I'm
simplifying here a bit but the operation "turn on the host processors" impl=
ies
"the host processors will access the BIOS" so the BMC must ensure "SPI mux =
is
switched towards the host" before "turn on the host processors".

> Sounds like it is another driver that needs to handle all of
> this, so why not have that be the one that adds/removes the devices
> under control here?

If what you're describing is moving all of the state control logic into the
kernel, I don't think that is feasible.  For some systems it would mean mov=
ing
yet another entire IPMI stack into the kernel tree.  On others it might be
somewhat simpler, but it is still a good amount of code.  We could probably
write up more details on the scope of this.

If what you're describing is a small driver, similar to the board support
drivers that were used before the device tree, that instantiates subordinate
devices it doesn't seem like an unreasonable alternative to DT overlays to =
me
(for whatever my limited kernel contribution experience counts for).

--=20
Patrick Williams

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