linux-kernel - Re: [PATCH 18/18] arm64: apple: Add initial Mac Mini 2020 (M1) devicetree

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Date:   Wed, 10 Feb 2021 21:24:02 +0900
From:   Daniel Palmer <daniel@...f.com>
To:     Hector Martin <marcan@...can.st>
Cc:     Tony Lindgren <tony@...mide.com>, Arnd Bergmann <arnd@...nel.org>,
        DTML <devicetree@...r.kernel.org>, Marc Zyngier <maz@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        SoC Team <soc@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Olof Johansson <olof@...om.net>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 18/18] arm64: apple: Add initial Mac Mini 2020 (M1) devicetree

Hi Hector,

On Wed, 10 Feb 2021 at 20:49, Hector Martin <marcan@...can.st> wrote:

> > Yeah, just don't use an imaginary dummy index for the reg. Use a real
> > register offset from a clock controller instance base, and a register
> > bit offset too if needed.
>
> I mean for fixed input clocks without any particular numbering, or for
> temporary fake clocks while we figure out the clock controller. Once a
> real clock controller is involved, if there are hardware indexes
> involved that are consistent then of course I'll use those in some way
> that makes sense.

This exact problem exists for MStar/SigmaStar too.
As it stands there is no documentation to show what the actual clock
tree looks like so everything is guess and I need to come up with numbers.
I'm interested to see what the solution to this is as it will come up again
when mainlining chips without documentation.

> The purpose of the clock in this particular case is just to make the
> uart driver work, since it wants to know its reference clock; there is
> work to be done here to figure out the real clock tree

FWIW arm/boot/dts/mstar-v7.dtsi has the same issue: Needs uart,
has no uart clock. In that instance the uart clock setup by u-boot
is passed to the uart driver as a property instead of creating a fake
clock.

Cheers,

Daniel