Messages in this thread | | | From | Arnd Bergmann <> | Date | Mon, 4 Jul 2022 14:07:10 +0200 | Subject | Re: [PATCH] powerpc: e500: Fix compilation with gcc e500 compiler |
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On Mon, Jul 4, 2022 at 12:39 PM Pali Rohár <pali@kernel.org> wrote: > On Monday 04 July 2022 20:23:29 Michael Ellerman wrote: > > On 2 July 2022 7:44:05 pm AEST, "Pali Rohár" <pali@kernel.org> wrote: > > >On Tuesday 24 May 2022 11:39:39 Pali Rohár wrote: > > >> gcc e500 compiler does not support -mcpu=powerpc option. When it is > > >> specified then gcc throws compile error: > > >> > > >> gcc: error: unrecognized argument in option ‘-mcpu=powerpc’ > > >> gcc: note: valid arguments to ‘-mcpu=’ are: 8540 8548 native > > >> > > >> So do not set -mcpu=powerpc option when CONFIG_E500 is set. Correct option > > >> -mcpu=8540 for CONFIG_E500 is set few lines below in that Makefile. > > >> > > >> Signed-off-by: Pali Rohár <pali@kernel.org> > > >> Cc: stable@vger.kernel.org > > > > > >Michael, do you have any objections about this patch? > > > > I don't particularly like it :) > > > > From the discussion with Segher, it sounds like this is a problem with a specific build of gcc that you're using, not a general problem with gcc built with e500 support. > > Well, the "full" build of gcc for e500 cores with SPE does not support > -mcpu=powerpc option. So I think this is a general problem. I do not > think that this is "specific build" as this is the correct build of gcc > for these processors with e500 cores. > > "stripped". build of gcc without SPE support for e500 cores does not > have this problem...
I can see a couple of problems with the CPU selection, but I don't think this is a major one, as nobody should be using those SPE compilers for building the kernel. Just use a modern powerpc-gcc build.
> > Keying it off CONFIG_E500 means it will fix your problem, but not anyone else who has a different non-e500 compiler that also doesn't support -mcpu=powerpc (for whatever reason). > > > > So I wonder if a better fix is to use cc-option when setting -mcpu=powerpc. > > > > Comment for that code which adds -mpcu=powerpc says: > > they are needed to set a sane 32-bit cpu target for the 64-bit cross > compiler which may default to the wrong ISA. > > So I'm not sure how to handle this in other way. GCC uses -mpcu=8540 > option for specifying to compile code for e500 cores and seems that > -mcpu=8540 is supported by all e500 compilers... > > Few lines below is code > > CFLAGS-$(CONFIG_E500) += $(call cc-option,-mcpu=8540 -msoft-float,-mcpu=powerpc) > > which for e500 kernel builds user either -mcpu=8540 or -mcpu=powerpc > (probably as a fallback if -mcpu=8540 is not supported).
The -mcpu=powerpc fallback can probably be skipped here, that must have been for compilers predating the addition of -mcpu=8540, and even the oldest ones support that now.
> So for me it looks like that problematic code > > KBUILD_CFLAGS += -mcpu=powerpc > KBUILD_AFLAGS += -mcpu=powerpc > > needs to be somehow skipped when compiling for CONFIG_E500. >> My change which skips that code base on ifndef CONFIG_E500 should be > fine as when CONFIG_E500 is disabled it does nothing and when it is > enabled then code > > CFLAGS-$(CONFIG_E500) += $(call cc-option,-mcpu=8540 -msoft-float,-mcpu=powerpc) > > is called which sets -mcpu option suitable for e500.
I think this part is indeed fishy, but adding another special case for E500 seems to take it in the wrong direction.
Nick added this in 4bf4f42a2feb ("powerpc/kbuild: Set default generic machine type for 32-bit compile") as a compile-time fix to prevent the default target from getting used when the compiler supports both 64-bit and 32-bit. This is the right idea, but it's inconsistent to pass different flags depending on the type of toolchain, and it loses the more specific options.
Another problem I see is that a kernel that is built for both E500 and E500MC uses -mcpu=e500mc and may not actually work on the older ones either (even with your patch).
I think what you actually want is to set one option for each of the possible CPU types:
CFLAGS_CPU-$(CONFIG_PPC_BOOK3S_32) := -mcpu=powerpc CFLAGS_CPU-$(CONFIG_PPC_85xx) := -mcpu=8540 CFLAGS_CPU-$(CONFIG_PPC8xx) := -mcpu=860 CFLAGS_CPU-$(CONFIG_PPC44x) := -mcpu=440 CFLAGS_CPU-$(CONFIG_PPC40x) := -mcpu=405 ifdef CONFIG_CPU_LITTLE_ENDIAN CFLAGS_CPU-$(CONFIG_BOOK3S_64) := -mcpu=power8 else CFLAGS_CPU-$(CONFIG_BOOK3S_64) := -mcpu=power5 endif CFLAGS_CPU-$(CONFIG_BOOK3E_64) := -mcpu=powerpc64
For the non-generic CPU types, there is also CONFIG_TARGET_CPU, and the list above could just get folded into that instead.
Arnd
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