[OpenBIOS] [PATCH] ppc: force target CPU

Segher Boessenkool segher at kernel.crashing.org
Mon Jan 18 07:40:38 CET 2016

On Tue, Dec 29, 2015 at 04:43:09PM +0000, Mark Cave-Ayland wrote:
> After some further research, there are currently 2 reasons that make me
> feel we should be targeting earlier CPUs than the G3 for 32-bit OpenBIOS
> builds: firstly there are some people trying to emulate PPC 604 CPUs
> with QEMU/KVM which currently hangs in OpenBIOS, and secondly there has
> been talk of switching over QEMU's PReP machine from OHW to OpenBIOS
> instead.
> To that end the minimum supported CPU for PPC needs to be 602 and so
> with this in mind, I've lightly tested the attached patch which seems to
> work here - Cole, does this work on your compiler setup? If so, I think
> this is the option I would like to apply upstream.

I seriously doubt you have a 602 anywhere, or an emulator for it even.
You want 603/604, which were both called "G2".

It is probably best if you make code that runs on 603 but is tuned
for (say) 750, as in -mcpu=603 -mtune=750.

>          ppc64)
>          select_prefix powerpc64
> -        CFLAGS="-Wa,-a64 -m64 -msoft-float -fno-builtin"
> +        CFLAGS="-Wa,-a64 -m64 -mcpu=G5 -msoft-float -fno-builtin"
>          AS_FLAGS="-Wa,-a64"
>          ;;

G5 is 970.  It includes VMX (AltiVec) vector support; you may want to
aim lower here too, say, power4.

While I'm at it...  You probably want -ffreestanding (and then you don't
need to disable builtins separately).  You should not need -Wa,-a64
in CFLAGS unless your toolchain install is broken; and for AS_FLAGS
you want to include all of CFLAGS.



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