linux-kernel - Re: [RFC PATCH v1 11/38] KVM: arm64: CCA: register host tsm platform device

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Message-ID: <2025101020-tiara-procreate-e56f@gregkh>
Date: Fri, 10 Oct 2025 14:38:50 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Jeremy Linton <jeremy.linton@....com>
Cc: Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
	Jason Gunthorpe <jgg@...pe.ca>,
	Dan Williams <dan.j.williams@...el.com>, linux-coco@...ts.linux.dev,
	kvmarm@...ts.linux.dev, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, aik@....com, lukas@...ner.de,
	Samuel Ortiz <sameo@...osinc.com>,
	Xu Yilun <yilun.xu@...ux.intel.com>,
	Suzuki K Poulose <Suzuki.Poulose@....com>,
	Steven Price <steven.price@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>
Subject: Re: [RFC PATCH v1 11/38] KVM: arm64: CCA: register host tsm platform
 device

On Fri, Oct 10, 2025 at 07:10:58AM -0500, Jeremy Linton wrote:
> Hi,
> 
> 
> On 7/30/25 8:07 AM, Greg KH wrote:
> > On Wed, Jul 30, 2025 at 01:23:33PM +0100, Jonathan Cameron wrote:
> > > On Wed, 30 Jul 2025 11:38:27 +0100
> > > Jonathan Cameron <Jonathan.Cameron@...wei.com> wrote:
> > > 
> > > > On Wed, 30 Jul 2025 14:12:26 +0530
> > > > "Aneesh Kumar K.V" <aneesh.kumar@...nel.org> wrote:
> > > > 
> > > > > Jason Gunthorpe <jgg@...pe.ca> writes:
> > > > > > On Tue, Jul 29, 2025 at 06:10:45PM +0100, Jonathan Cameron wrote:
> > > > > > > > +static struct platform_device cca_host_dev = {
> > > > > > > Hmm. Greg is getting increasingly (and correctly in my view) grumpy with
> > > > > > > platform devices being registered with no underlying resources etc as glue
> > > > > > > layers.  Maybe some of that will come later.
> > > > > > 
> > > > > > Is faux_device a better choice? I admit to not knowing entirely what
> > > > > > it is for..
> > > > 
> > > > I'll go with a cautious yes to faux_device. This case of a glue device
> > > > with no resources and no reason to be on a particular bus was definitely
> > > > the intent but I'm not 100% sure without trying it that we don't run
> > > > into any problems.
> > > > 
> > > > Not that many examples yet, but cpuidle-psci.c looks like a vaguely similar
> > > > case to this one.
> > > > 
> > > > All it really does is move the location of the device and
> > > > smash together the device registration with probe/remove.
> > > > That means the device disappears if probe() fails, which is cleaner
> > > > in many ways than leaving a pointless stub behind.
> > > > 
> > > > Maybe it isn't appropriate it if is actually useful to rmmod/modprobe the
> > > > driver.
> > > > 
> > > > +CC Greg on basis I may have wrong end of the stick ;)
> > > This time with at least one less typo in Greg's email address.
> > 
> > Yes, use faux_device if you need/want a struct device to represent
> > something in the tree and it does NOT have any real platform resources
> > behind it.  That's explicitly what it was designed for.
> 
> Right, but this code is intended to trigger the kmod/userspace module
> loader.

Why?

> AFAIK, the faux device is currently missing a faux_device_id in
> mod_devicetable, alias matching logic in file2alias, and probably a few
> other things which keeps it from performing this function.

How would a faux device ever expect to get auto-loaded?  That's not what
is supposed to be happening here at all.

If you have real hardware backing something, then use the real driver
type.  that is NOT a faux driver, which is, as the name says, for "fake"
devices that you wish to add to the device/driver tree.

thanks,

greg k-h