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Date:   Mon, 25 Oct 2021 15:34:05 +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 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:
> > >  
> > > > We have the bind/unbind ability today, from userspace, that can control
> > > > 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 "unbind" in
> > > > sysfs, and then when all is done, it writes to the "bind" file and then
> > > > Linux takes back over.
> > > > 
> > > > Unless for some reason Linux should _not_ grab the device when booting,
> > > > then things get messier, as we have seen in this thread.
> > > 
> > > 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 would rip away
> > > the BIOS chip from the running host.
> > > 
> > > The BMC almost always needs to come up in a "I don't know what could possibly be
> > > going on in the system" state and re-discover where the system was left off.
> > 
> > Isn't it an architectural issue then?
> 
> I'm not sure what "it" you are referring to here.
> 
> 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.

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?

thanks,

greg k-h