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Date:   Mon, 25 Oct 2021 09:02:40 -0500
From:   Patrick Williams <patrick@stwcx.xyz>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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 03:34:05PM +0200, Greg Kroah-Hartman wrote:
> On Mon, Oct 25, 2021 at 08:20:05AM -0500, Patrick Williams wrote:
> > On Mon, Oct 25, 2021 at 03:58:25PM +0300, Andy Shevchenko wrote:
> > > On Mon, Oct 25, 2021 at 06:44:26AM -0500, Patrick Williams wrote:
> > > > On Mon, Oct 25, 2021 at 08:15:41AM +0200, Greg Kroah-Hartman wrote:
> > > > > On Mon, Oct 25, 2021 at 12:38:08AM -0500, Frank Rowand wrote:
> > > > > > On 10/23/21 3:56 AM, Greg Kroah-Hartman wrote:
> > > > =20
> > > > > We have the bind/unbind ability today, from userspace, that can c=
ontrol
> > > > > this.  Why not just have Linux grab the device when it boots, and=
 then
> > > > > when userspace wants to "give the device up", it writes to "unbin=
d" in
> > > > > sysfs, and then when all is done, it writes to the "bind" file an=
d then
> > > > > Linux takes back over.
> > > > >=20
> > > > > Unless for some reason Linux should _not_ grab the device when bo=
oting,
> > > > > then things get messier, as we have seen in this thread.
> > > >=20
> > > > This is probably more typical on a BMC than atypical.  The systems =
often require
> > > > the BMC (running Linux) to be able to reboot independently from the=
 managed host
> > > > (running anything).  In the example Zev gave, the BMC rebooting wou=
ld rip away
> > > > the BIOS chip from the running host.
> > > >=20
> > > > The BMC almost always needs to come up in a "I don't know what coul=
d possibly be
> > > > going on in the system" state and re-discover where the system was =
left off.
> > >=20
> > > Isn't it an architectural issue then?
> >=20
> > I'm not sure what "it" you are referring to here.
> >=20
> > I was trying to explain why starting in "bind" state is not a good idea=
 for a
> > BMC in most of these cases where we want to be able to dynamically add =
a device.
>=20
> 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 BI=
OS
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 cod=
e 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 de=
sign
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".

--=20
Patrick Williams

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