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

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Date:   Sun, 10 Jul 2022 20:56:02 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Pali Rohár <pali@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Segher Boessenkool <segher@...nel.crashing.org>
Subject: Re: [PATCH] powerpc/32: Don't always pass -mcpu=powerpc to the compiler

On Sun, Jul 10, 2022 at 7:36 PM Christophe Leroy
<christophe.leroy@...roup.eu> 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.
>
> Reported-by: Pali Rohár <pali@...nel.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Segher Boessenkool <segher@...nel.crashing.org>
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>

Looks good to me,

Reviewed-by: Arnd Bergmann <arnd@...db.de>