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

[Rob Herring <robh@kernel.org>]

On Mon, Feb 08, 2021 at 08:56:53PM +0900, Hector Martin 'marcan' wrote:
> On 08/02/2021 20.04, Krzysztof Kozlowski wrote:
> > apple
> > 
> > Don't make things different for this one platform (comparing to all
> > other platforms). Apple is not that special. :)
> 
> AAPL is the old vendor prefix used in the PowerPC era. I'm happy to use
> `apple`, as long as we're OK with having two different prefixes for the same
> vendor, one for PPC and one for ARM64. I've seen opinions go both ways on
> this one :)
> 
> > > + * Copyright 2021 Hector Martin <marcan@marcan.st>
> > 
> > A lot here might be difficult to reverse-egineer or figure out by
> > ourself, so usually people rely on vendor sources (the open source
> > compliance package). Didn't you receive such for the iOS (or whatever
> > was on your Mac)?
> 
> Apple source drops are sparse (they don't even include things like the
> irqchip driver, only the very core OS code) and APSL licensed, which is a
> license incompatible with the GPL. Worse, they've moved to a partial-blob
> model with the M1; M1-compatible XNU source code drops now include a .a blob
> with startup and CPU-specific code, for which no source code is provided.
> (to be clear: Apple does not ship Linux for these machines)
> 
> Honestly, beyond what's in this patchset and a few more details about CPU
> registers like performance monitoring stuff that exist in public XNU drops
> but I haven't looked into yet, Apple's source code drops are going to be
> practically useless to us from here on out. It's all binaries after this.
> 
> Apple device trees are not open source at all; those are provided by iBoot
> and ship with device firmware, which is not openly licensed. Those device
> trees are OF-inspired, but otherwise in a different format and structure to
> Linux device trees.
> 
> Since there is zero Apple-published code or data with a license compatible
> with the Linux kernel to be used here, there can be zero copyright lines
> claiming any submissions are copyright Apple from us, because that would
> imply a license violation has occurred. I am treating this as I would any
> other no-source reverse engineering project, that is, ensuring that I only
> look at Apple code (binaries, source, devicetrees, whatever) to understand
> how the hardware functions, build documentation for it (at least in my head,
> but I am also trying to document things on our wiki as I go), and then write
> original code to drive it from Linux, unrelated to whatever Apple was doing.
> 
> We're also trying to avoid looking at any Apple stuff in general as much as
> possible, preferring black-box approaches where feasible, to minimize
> exposure. For example, I only looked at an (outdated, arm32 era) AIC
> register name list in XNU to write the AIC driver; there is no actual AIC
> driver code in the source, and instead of decompiling Apple's binary blob
> AIC driver module, I figured out how the hardware actually worked via
> probing and experimentation. The entire userspace GPU stack is being reverse
> engineered via a black-box approach, without any decompilation. I'm going to
> see what I can do about the kernel driver in the future, and prefer some
> kind of mmio tracing solution if I can get it all to work on macOS.
> 
> As for this file specifically: while I am obviously looking at Apple's DTs
> to figure out things like register offsets and what hardware exists, those
> are facts, and facts are not copyrightable, and thus Apple does not hold any
> copyright interest over this code as I submitted it. Short of verbatim
> copying and pasting of entire nodes with bespoke property names (which would
> never fly here anyway because Apple does things very differently from Linux
> DTs when you get down into the details), it would be extremely hard to argue
> that translating hardware information from decompiled Apple DTs to Linux DTs
> would constitute a copyright violation, since the entire purpose of DTs is
> to describe hardware facts.
> 
> You can read more about our reverse engineering and copyright policy at
> https://alx.sh/re - if you have any suggestions or spot anything
> problematic, please let me know.
> 
> (I'm actually probably going to change that copyright line to "The Asahi
> Linux Contributors" for v2, if that's okay with the kernel folks, to be in
> line with our other projects; I defaulted to my name since so far I'm the
> only contributor to these files, but I expect other people to throw PRs at
> me in the future and the history to end up with more names here)

Does there need to be a legal entity behind 'The Asahi Linux 
Contributors' to be valid?

From a more practical standpoint, if we want to relicense something in 
say 5 years from now, who do we ask for an okay?

> > I guess Rob will comment on the dt-bindings more... but for me a generic
> > "arm-platform" is too generic. What's the point of it? I didn't see any
> > of such generic compatibles in other platforms.
> 
> This is a hack for patches #11/#12 to use, and I expect it will go away once
> we figure out how to properly handle that problem (which needs further
> discussion). Sorry for the noise, this should not be there in the final
> version.

I was going to ask on this. If you have a user of it, I'm okay with it. 
Generally though, 3 or 4 levels of compatible don't really have users.

> > > +		bootargs = "earlycon";
> > 
> > This should not be hard-coded in DTS. Pass it from bootloader.
> 
> My apologies, this was garbage left over from before I had bootargs support
> in the bootloader. Will be gone for v2.
> 
> > > +	clk24: clk24 {
> > 
> > Just "clock". Node names should be generic.
> 
> Really? Almost every other device device tree uses unique clock node names.

It's a WIP to be more consistent around node names. For actual
clock controllers we have 'clock-controller(@.*)?'. There's not really 
something established for 'fixed-clock'. We probably should define 
something, but that goes in the schema first.

> > > +		compatible = "fixed-clock";
> > > +		#clock-cells = <0>;
> > > +		clock-frequency = <24000000>;
> > > +		clock-output-names = "clk24";
> > 
> > What clock is it? Part of board or SoC? Isn't it a work-around for
> > missing clock drivers?
> 
> The clock topology isn't entirely known yet; I'm submitting this as an
> initial bring-up patchset and indeed there should be a clockchip driver in
> the future. The UART driver wants a clock to be able to calculate baud
> rates. I figured we can get away with a fixed-clock for now while that part
> of the SoC gets figured out.

That is normal. It does break compatibility between an old kernel 
and new DT. There's not really a good way to avoid that.

Rob

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