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

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Message-ID: <20250731121133.GP26511@ziepe.ca>
Date: Thu, 31 Jul 2025 09:11:33 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
	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>, gregkh@...uxfounation.org
Subject: Re: [RFC PATCH v1 11/38] KVM: arm64: CCA: register host tsm platform
 device

On Wed, Jul 30, 2025 at 11:38:27AM +0100, Jonathan Cameron 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. 

Yeah, exactly. Can a TSM driver even be modular? If it has to be built
in then there is no reason to do this:

> > The goal is to have tsm class device to be parented by the platform
> > device.

IMHO the only real point of that is to trigger module autoloading.

Otherwise the tsm core should accept NULL as the parent pointer during
registration, it probably already does..

Jason