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Date:   Mon, 25 Oct 2021 20:36:47 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Patrick Williams <patrick@stwcx.xyz>
Cc:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Frank Rowand <frowand.list@gmail.com>,
        Zev Weiss <zev@bewilderbeest.net>, kvm@vger.kernel.org,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Kirti Wankhede <kwankhede@nvidia.com>,
        Jeremy Kerr <jk@codeconstruct.com.au>,
        Rajat Jain <rajatja@google.com>,
        Jianxiong Gao <jxgao@google.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Saravana Kannan <saravanak@google.com>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        openbmc@lists.ozlabs.org, devicetree@vger.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Rob Herring <robh+dt@kernel.org>,
        Bhaskar Chowdhury <unixbhaskar@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Jeffery <andrew@aj.id.au>,
        Cornelia Huck <cohuck@redhat.com>,
        linux-kernel@vger.kernel.org, Vinod Koul <vkoul@kernel.org>,
        dmaengine@vger.kernel.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 10:54:21AM -0500, Patrick Williams wrote:
> 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?
> > > 
> > > Ah, ok.
> > > 
> > > Those usually end up being system / device specific.  In the case of the 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.
> > > 
> > > 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 involves
> > > 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-processor 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 JTAG to
> > > figure out if the processors are "making progress".
> > 
> > But which processor is "in control" here over the hardware?  
> 
> The BMC.  It owns the GPIO that controls the SPI mux.  
> 
> 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?  
> 
> 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" implies
> "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 moving
> 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).
> 

Something has to be here doing the mediation between the two processors
and keeping things straight as to what processor is handling the
hardware when.  I suggest you focus on that first...

Good luck!

greg k-h