Re: [PATCH 1/6] dt-bindings: mfd: add binding for Apple Mac System Management Controller

[Rob Herring <robh@xxxxxxxxxx>]

On Tue, Sep 06, 2022 at 08:36:25PM +0900, Hector Martin wrote:
> On 06/09/2022 20.22, Linus Walleij wrote:
> > On Tue, Sep 6, 2022 at 11:31 AM Mark Kettenis <mark.kettenis@xxxxxxxxx> wrote:
> > 
> >> Another argument for having sub-nodes is that the firmware actually
> >> exposes *two* GPIO controllers.  For now we only support the "master"
> >> PMU GPIOs, but there also is a "slave" PMU GPIO controller that uses a
> >> separate set of SMC "keys".  We currently don't need any of the pins
> >> on the "slave", so we don't expose it in the DT yet.
> > 
> > That sounds backward, like we don't expose device X as DT node
> > because $OS doesn't use it yet. DT should just expose (by nodes or
> > other ways) all hardware that exist or at least all hardware we know
> > about no matter what $OS is using.
> 
> How so? The are piles and piles of unused hardware not exposed in the
> DT, and piles and piles of hardware that will be used but we haven't
> figured out how to do it yet, so it's not exposed. For example, we know
> there are like 8 or so UARTs, but we don't define them in the DT because
> they are not connected to anything on any existing device and we don't
> need them. Apple does the same thing in their DTs (only used hardware is
> defined).
> 
> I don't really see the point of exposing a GPIO controller when we don't
> actually do anything with the pins yet, and might never do so. Having
> drivers bind and stay unused just increases the amount of code running
> without any ultimate purpose, so why do it? It's not like any other OS
> would use the hardware either - GPIOs are only useful if they are
> referenced in the DT for something, and we don't have anything that
> would reference these.
> 
> For SMC in particular, there's a huge amount of functionality we don't
> have drivers for yet, and I don't see the point of trying to conjure up
> DT bindings for it until someone writes a driver (and has a reason to do
> so) :)

Exposing in a DT is one thing. Defining in the binding is another. Even 
if it's not complete bindings, but just a fuller description of what 
functionality the MFD contains is. For example, just knowing there 
are 2 instances of GPIO, I'm much more inclined to agree GPIO should be 
a subnode.

Rob