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

On Mon, Jul 11, 2022 at 04:19:30PM +0200, 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.

More fundamentally, the *only* thing you should check biarch for is
for determining if you can use both -m32 and -m64 with the same
compiler.  Everything behaves identically in a biarch and a non-biarch
compiler, other than the latter screams bloody murder if you try to
change the architecture from 32 to 64 bit or vice versa.

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@kernel.org>
Tested-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Cc: Segher Boessenkool <redacted>
Signed-off-by: Christophe Leroy <redacted>

Reviewed-by: Segher Boessenkool <redacted>

Looks good to me.  This untangles/demystifies quite some code :-)


Segher