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Date:   Fri, 22 Oct 2021 10:18:11 -0500
From:   Patrick Williams <>
To:     Greg Kroah-Hartman <>
Cc:     Zev Weiss <>,,
        "Rafael J. Wysocki" <>,
        Kirti Wankhede <>,
        Jeremy Kerr <>,
        Rajat Jain <>,
        Frank Rowand <>,
        Jianxiong Gao <>,
        Dave Jiang <>,
        Saravana Kannan <>,
        Mauro Carvalho Chehab <>,,,
        Konrad Rzeszutek Wilk <>,
        Alex Williamson <>,
        Rob Herring <>,
        Bhaskar Chowdhury <>,
        Thomas Gleixner <>,
        Andy Shevchenko <>,
        Andrew Jeffery <>,
        Cornelia Huck <>,, Vinod Koul <>,
Subject: Re: [PATCH 4/5] driver core: inhibit automatic driver binding on
 reserved devices

Hi Greg,

On Fri, Oct 22, 2021 at 10:57:21AM +0200, Greg Kroah-Hartman wrote:
> On Fri, Oct 22, 2021 at 01:32:32AM -0700, Zev Weiss wrote:
> > On Thu, Oct 21, 2021 at 11:46:56PM PDT, Greg Kroah-Hartman wrote:
> > > On Thu, Oct 21, 2021 at 07:00:31PM -0700, Zev Weiss wrote:

> > So we want the kernel to be aware of the device's existence (so that we
> > *can* bind a driver to it when needed), but we don't want it touching the
> > device unless we really ask for it.
> > 
> > Does that help clarify the motivation for wanting this functionality?
> Sure, then just do this type of thing in the driver itself.  Do not have
> any matching "ids" for this hardware it so that the bus will never call
> the probe function for this hardware _until_ a manual write happens to
> the driver's "bind" sysfs file.

It sounds like you're suggesting a change to one particular driver to satisfy
this one particular case (and maybe I'm just not understanding your suggestion).
For a BMC, this is a pretty regular situation and not just as one-off as Zev's

Another good example is where a system can have optional riser cards with a
whole tree of devices that might be on that riser card (and there might be
different variants of a riser card that could go in the same slot).  Usually
there is an EEPROM of some sort at a well-known address that can be parsed to
identify which kind of riser card it is and then the appropriate sub-devices can
be enumerated.  That EEPROM parsing is something that is currently done in
userspace due to the complexity and often vendor-specific nature of it.

Many of these devices require quite a bit more configuration information than
can be passed along a `bind` call.  I believe it has been suggested previously
that this riser-card scenario could also be solved with dynamic loading of DT
snippets, but that support seems simple pretty far from being merged.

Patrick Williams

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