Scott Wood scottwood@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@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.