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

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Message-ID: <20250730132333.00006fbf@huawei.com>
Date: Wed, 30 Jul 2025 13:23:33 +0100
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
CC: 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>, <gregkh@...uxfoundation.org>
Subject: Re: [RFC PATCH v1 11/38] KVM: arm64: CCA: register host tsm
 platform device

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.

> 
> > >
> > > But alternatively, why do we need a dummy "hw" struct device at all?
> > > Typically a subsystem like TSM should be structured to create its own
> > > struct devices..
> > >
> > > I would expect this to just call 'register tsm' ?
> > >    
> > 
> > The goal is to have tsm class device to be parented by the platform
> > device.  
> 
> > 
> > # ls -al
> > total 0
> > drwxr-xr-x    2 root     root             0 Jan 13 06:07 .
> > drwxr-xr-x   23 root     root             0 Jan  1 00:00 ..
> > lrwxrwxrwx    1 root     root             0 Jan 13 06:07 tsm0 -> ../../devices/platform/arm-rmi-dev/tsm/tsm0
> > # pwd
> > /sys/class/tsm
> > 
> > -aneesh
> >   
> 
>