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

From: Patrick Williams <patrick@stwcx.xyz>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: 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>,
	Frank Rowand <frowand.list@gmail.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>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	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
Date: Fri, 22 Oct 2021 10:18:11 -0500	[thread overview]
Message-ID: <YXLWMyleiTFDDZgm@heinlein> (raw)
In-Reply-To: <YXJ88eARBE3vU1aA@kroah.com>

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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.

-- 
Patrick Williams

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