[U-Boot] merge arm64 to arm

[Måns Rullgård]

Scott Wood <scottwood at freescale.com> writes:

> On Mon, 2013-08-19 at 09:10 -0400, Tom Rini wrote:
>> On 08/19/2013 09:01 AM, Måns Rullgård wrote:
>> > Tom Rini <trini at ti.com> writes:
>> > 
>> >> On 08/19/2013 08:32 AM, Måns Rullgård wrote:
>> >>> If there's a lot of code shared between these architectures,
>> >>> why is it in an architecture-specific directory in the first
>> >>> place?  Maybe the proper solution is to move it out of arch/arm
>> >>> rather than moving code for an entirely different architecture
>> >>> in there.
>> >> 
>> >> We are working in that direction (and one of the requests was to
>> >> hook into that code, rather than duplicate things).  Think of it
>> >> as "all ARM Ltd licensed cores" not "all 32bit-only ARM cores".
>> > 
>> > Why does it matter which company designed it?  By that reasoning,
>> > you'd put i960 (were it supported) under arch/x86 because it's from
>> > Intel.
>> 
>> Probably because I didn't get the "it's a whole new unrelated to
>> everything before world over there!" memo.
>
> Probably because there is still quite a bit of similarity to older ARM.
> There's more to it than just the ISA, and even that isn't *that* much
> more different than x86 versus x86_64.

It is quite a bit more different.  32-bit x86 is a subset of x86_64
where as arm64 is entirely new, if somewhat inspired by 32-bit arm.

> i960 is a bad analogy.  It's often possible to turn arm32 asm into
> arm64 asm with some search and replace and minor manual fixups.

Only if the original uses none of the distinguishing features of ARM
like predicated instructions or variably shifted operands.  Once you
limit yourself to the remaining basic operations, every (RISC)
architecture looks the same.

>>   Seriously tho, our
>> directory structure is different from the kernel and it seems like
>> things might look cleaner this way.  If it doesn't, well, I'll admit
>> to being wrong and we'll go back to a split arch directory.
>
> As I noted before, in Linux a bunch of other architectures started with
> a separate arch for 64-bit (x86, sparc, ppc...), and all of them
> eventually merged.

In those cases, the 64-bitness is merely an extension to the 32-bit
instruction set.  Most of them don't even have a notion of 32-bit vs
64-bit mode of execution.

AArch64 of course shares certain non-ISA aspects with AArch32.  Page
table formats and other architecturally defined system control features
are the same, and code for managing these things should of course be
shared.  Some other features, e.g. exception handling, are different
enough that sharing code is probably difficult.

There is a tendency to see arm64/aarch64 as yet another 64-bit extension
of a 32-bit architecture, which it is not.  Assuming that software
support will or can follow the model used by the others mentioned is
thus a mistake.

-- 
Måns Rullgård
mans at mansr.com