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

[Aneesh Kumar K . V]

Jason Gunthorpe <jgg@xxxxxxxx> writes:

> On Fri, Oct 10, 2025 at 07:10:58AM -0500, Jeremy Linton wrote:
>> > 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.
>
> Faux devices are not intended to be bound, it says so right on the label:
>
>  * A "simple" faux bus that allows devices to be created and added
>  * automatically to it.  This is to be used whenever you need to create a
>  * device that is not associated with any "real" system resources, and do
>  * not want to have to deal with a bus/driver binding logic.  It is
>                         ^^^^^^^^^^^^^^^^^^^^^^^^^^
>  * intended to be very simple, with only a create and a destroy function
>  * available.
>
> auxiliary_device is quite similar to faux except it is intended to be
> bound to drivers, supports module autoloading and so on.
>
> What you have here is the platform firmware provides the ARM SMC
> (Secure Monitor Call Calling Convention) interface which is a generic
> function call multiplexer between the OS and ARM firmware.
>
> Then we have things like the TSM subsystem that want to load a driver
> to use calls over SMC if the underlying platform firmware supports the
> RSI group of SMC APIs. You'd have a TSM subsystem driver that uses the
> RSI call group over SMC that autobinds when the RSI call group is
> detected when the SMC is first discovered.
>
> So you could use auxiliary_device, you'd consider SMC itself to be the
> shared HW block and all the auxiliary drivers are per-subsystem
> aspects of that shared SMC interface. It is not a terrible fit for
> what it was intended for at least.
>

IIUC, auxiliary_device needs a parent device, and the documentation
explains that it’s intended for cases where a large driver is split into
multiple dependent smaller ones.

If we want to use auxiliary_device for this case, what would serve as
the parent device?

-aneesh