Re: Question about linux non-standard powerpc SPE FPU

On Sat, Feb 15, 2025 at 01:20:23PM +0100, Martina Hřebcová wrote:
> Hi all,
> 
> I have one question about kernel / debian operation.
> 
> I have computer A1222+ with CPU P1022 ( two e500v2 cores ). It is NG Amiga
> with AmigaOS4, but linux runs here nice too.
> 
> P1022 CPU is 32-bit powerpc processor, it has no powerpc FPU but have
> embedded SPE FPU instead.  Also it has SPE SIMD unit. It has no FP or vector
> registers, instead it has GPRs extended for 64bit for FPU and SIMD
> operations. So it has different ABI than standard powerpc for floating-point
> parameter calls.
> 
> There was in past powerpcspe port of Debian. It was dropped somewhere in 
> 2016, due to excluding of SPE extension from gcc.
> 
> Debian 8+ powerpcspe snapshot from these times runs very well, with full 3D
> desktop support. There is customized kernel for A1222+, which was maintained
> by xeno74. Last version I found is 5.4. All this is normal and expected.
> 
> There are no other native powerpcspe distro ( or at least I am not hear 
> about it).
> 
> But now I tried standart powerpc Ubuntu 16.04 and also last version of
> powerpc 32-bit Void linux.
> 
> If I used custom kernel from xeno74 and standart powerpc distro, it works on
> my powerpcspe machine with different ABI.
> 
> ( So now I have in plan to install more modern Debian from snapshots, or 
> Fienix distro, which is based on modern Debian )
> 
> Please, how does it works? How is possible that powerpc code with fpu not 
> crashes?
> 
> I can imagine, that in custom kernel there are soft-float calls. But there
> shoud be some powerpc FPU emulator or what...
> 
> Please, do you have any idea?

Any software that doesn't use FPU at all should work I believe.  Any that
uses FPU would not, unless the kernel happens to be configured to emulate
an FPU.  I can't remember if powerpc ever had an option to do that.
I know some architectures in the past did at least.

Looking at the kernel config, it appears CONFIG_MATH_EMULATION exists
and may in fact do just that.

In fact I found some discussions from 8 years ago that said tha A1222 can
in fact run powerpc code with the MATH_EMULATION kernel option enabled.
It's not recommended, since the FPU performance is terrible.

I will continue to despise the A1222 even existing for the fact they
put that garbage CPU on it that is not a proper powerpc and broke all
existing compatibility of AmigaOS 4 FPU using code.