Re: [PATCH v1] mfd: core: Support auxiliary device

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Apr 03, 2025 at 04:30:53PM +0530, Raag Jadav wrote:
> Extend MFD subsystem to support auxiliary child device. This is useful for
> MFD usecases where parent device is on a discoverable bus and doesn't fit
> into the platform device criteria. Current support is limited to just PCI
> devices, but this can be further extended to support other types like USB
> in the future.
> 
> Signed-off-by: Raag Jadav <raag.jadav@intel.com>
> ---
> 
> I've been cooking this on my spare time during merge window. I'm not
> very confident about this but thought I'd share it. It might be
> controversial since I stole quite a bit from platform infrastructure,
> so please consider this an RFC and let's discuss how to approach this.
> 
> More discussion at [*].
> [*] https://lore.kernel.org/r/2025032609-query-limit-491b@gregkh
> 
> A few things that are still open,
> 
> 1. Since we're doing it for PCI devices (Greg's recommendation), how do
>    we force the existing ones to use their original platform path?

That's up to you all, but I wouldn't recommend forcing everything to
change for existing drivers.  So make it "opt-in" somehow?  I would love
to make it change for existing drivers, but I don't know what that might
break (it _should_ not break anything, but we've already found breakages
when moving some platform devices over to other busses like faux due to
bugs in userspace scripts.)

> 2. Should we allow auxiliary drivers to manage their own resources
>    (MEM, IO, IRQ etc)?

The resources are all shared by the "parent" device, that's what makes
aux drivers work, they need to handle this as there is no unique way to
carve up the resources here.

So I don't know how you would do this, sorry.

thanks,

greg k-h