linux-kernel - Re: [PATCH v1 2/5] powerpc/32: Don't always pass -mcpu=powerpc to the compiler

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Date:   Thu, 18 Aug 2022 19:46:34 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Arnd Bergmann <arnd@...db.de>,
        Segher Boessenkool <segher@...nel.crashing.org>
Subject: Re: [PATCH v1 2/5] powerpc/32: Don't always pass -mcpu=powerpc to
 the compiler

On Monday 11 July 2022 16:19:30 Christophe Leroy wrote:
> Since commit 4bf4f42a2feb ("powerpc/kbuild: Set default generic
> machine type for 32-bit compile"), when building a 32 bits kernel
> with a bi-arch version of GCC, or when building a book3s/32 kernel,
> the option -mcpu=powerpc is passed to GCC at all time, relying on it
> being eventually overriden by a subsequent -mcpu=xxxx.
> 
> But when building the same kernel with a 32 bits only version of GCC,
> that is not done, relying on gcc being built with the expected default
> CPU.
> 
> This logic has two problems. First, it is a bit fragile to rely on
> whether the GCC version is bi-arch or not, because today we can have
> bi-arch versions of GCC configured with a 32 bits default. Second,
> there are some versions of GCC which don't support -mcpu=powerpc,
> for instance for e500 SPE-only versions.
> 
> So, stop relying on this approximative logic and allow the user to
> decide whether he/she wants to use the toolchain's default CPU or if
> he/she wants to set one, and allow only possible CPUs based on the
> selected target.

Hello! Exactly same issue is still in file arch/powerpc/boot/Makefile:

  ifdef CONFIG_PPC64_BOOT_WRAPPER
  ifdef CONFIG_CPU_LITTLE_ENDIAN
  BOOTCFLAGS	+= -m64 -mcpu=powerpc64le
  else
  BOOTCFLAGS	+= -m64 -mcpu=powerpc64
  endif
  else
  BOOTCFLAGS	+= -m32 -mcpu=powerpc
  endif

It cause compile error:

  make ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnuspe- mpc85xx_smp_defconfig uImage
  ...
    BOOTAS  arch/powerpc/boot/crt0.o
  powerpc-linux-gnuspe-gcc: error: unrecognized argument in option ‘-mcpu=powerpc’
  powerpc-linux-gnuspe-gcc: note: valid arguments to ‘-mcpu=’ are: 8540 8548 native
  make[1]: *** [arch/powerpc/boot/Makefile:231: arch/powerpc/boot/crt0.o] Error 1

> Reported-by: Pali Rohár <pali@...nel.org>
> Tested-by: Pali Rohár <pali@...nel.org>
> Reviewed-by: Arnd Bergmann <arnd@...db.de>
> Cc: Segher Boessenkool <segher@...nel.crashing.org>
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
>  arch/powerpc/Makefile                  | 26 +-------------------------
>  arch/powerpc/platforms/Kconfig.cputype | 21 ++++++++++++++++++---
>  2 files changed, 19 insertions(+), 28 deletions(-)