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

[Linus Walleij <linus.walleij@linaro.org>]

On Tue, Sep 6, 2022 at 3:28 PM Hector Martin <marcan@marcan.st> wrote:
> On 06/09/2022 20.57, Linus Walleij wrote:
> > On Tue, Sep 6, 2022 at 1:36 PM Hector Martin <marcan@marcan.st> wrote:

> > This comes from the FDT background in OpenFirmware, and there is
> > definitely a timeline perspective (also called "waterfall model") in that
> > line of thinking: a DT is written that describes the hardware and flashed
> > into the BIOS and never changed, then the operating system is
> > implemented at a later point. This is how e.g. the PC ACPI BIOS tables
> > are also thinking about themselves.
>
> Yes, but again, that only makes sense from the point of view of
> describing hardware that exists, is useful, and could be used by the OS.
>
> For any given platform X, if platform X does not use the secondary GPIO
> controller for any service describable in the DT, then there is no point
> in describing that GPIO controller. Same way ACPI tables do not describe
> every single physical GPIO available on a platform, just whatever is
> used by the AML.

Good point. I don't know what ambition DT should have here.
If there is a discrete component on I2C for example, we tend to
describe it fully, this kind of stuff with misc "dark silicon" didn't
exist when DT was first conceived.

> If some day we find a use for those GPIOs, that would require a DT
> change *anyway*, to describe that usage, and the controller could be
> described then (we did something like that, using a GPIO that Apple
> doesn't, for the interim display-backlight power control support, though
> that is a temporary hack that will go away). Heck, we don't even know
> what these GPIOs are connected to right now!
>
> Ultimately, we're working with a reverse engineered platform here, and
> DTs will inevitaby be incremental.
(...)

That's OK, I think the patch series is good enough as it is and should
be merged, so I have added my Reviewed-by. I think the world
is a better place with support for Apple silicon being developed
in-tree.

> I'm a lot more
> interested in getting bindings upstreamed ASAP (so we can start
> guaranteeing no backwards-compat breaks, which is important to avoid
> outright breakage) than I am in trying to be exhaustive up front with
> device instances. It's perfectly fine to say that users have to upgrade
> both their DTs and kernels to get newer hardware support, on these
> platforms.

I agree.

Yours,
Linus Walleij