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

[Jonathan Cameron]

On Wed, 30 Jul 2025 14:12:26 +0530
"Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxx> wrote:

> Jason Gunthorpe <jgg@xxxxxxxx> 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 ;)

> >
> > 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
>