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

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Date:   Sat, 23 Oct 2021 10:56:41 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Patrick Williams <patrick@...cx.xyz>
Cc:     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>,
        Frank Rowand <frowand.list@...il.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 Fri, Oct 22, 2021 at 10:18:11AM -0500, Patrick Williams wrote:
> 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
> example.
> 
> 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.

Then work to get the DT code merged!  Do not try to create
yet-another-way of doing things here if DT overlays is the correct
solution here (and it seems like it is.)

thanks,

greg k-h