linux-kernel - Re: [PATCH 4/5] driver core: inhibit automatic driver binding on reserved devices

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Date:   Mon, 25 Oct 2021 06:44:26 -0500
From:   Patrick Williams <patrick@...cx.xyz>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Frank Rowand <frowand.list@...il.com>,
        Zev Weiss <zev@...ilderbeest.net>, kvm@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Jeremy Kerr <jk@...econstruct.com.au>,
        Rajat Jain <rajatja@...gle.com>,
        Jianxiong Gao <jxgao@...gle.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Saravana Kannan <saravanak@...gle.com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        openbmc@...ts.ozlabs.org, devicetree@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Rob Herring <robh+dt@...nel.org>,
        Bhaskar Chowdhury <unixbhaskar@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andrew Jeffery <andrew@...id.au>,
        Cornelia Huck <cohuck@...hat.com>,
        linux-kernel@...r.kernel.org, Vinod Koul <vkoul@...nel.org>,
        dmaengine@...r.kernel.org
Subject: Re: [PATCH 4/5] driver core: inhibit automatic driver binding on
 reserved devices

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.

-- 
Patrick Williams

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