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Date:   Fri, 2 Sep 2022 09:11:48 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v2 2/2] powerpc/math-emu: Remove -w build flag and fix
 warnings
Message-ID: <YxIrRJz3+fkzogay@dev-arch.thelio-3990X>
References: <a7384eafc6a27aea15bdc9e8f9a12aac593fccb7.1662113301.git.christophe.leroy@csgroup.eu>
 <35c86b7ca823954c6cd593acc3690dc3748da9b1.1662113301.git.christophe.leroy@csgroup.eu>
 <YxIjM/jdLajq4dFk@dev-arch.thelio-3990X>
 <20220902155954.GP25951@gate.crashing.org>
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On Fri, Sep 02, 2022 at 10:59:54AM -0500, Segher Boessenkool wrote:
> On Fri, Sep 02, 2022 at 08:37:23AM -0700, Nathan Chancellor wrote:
> > On Fri, Sep 02, 2022 at 12:08:55PM +0200, Christophe Leroy wrote:
> > > This should have been detected by gcc at build time, but due to
> > > '-w' flag it went undetected.
> > > 
> > > Removing that flag leads to many warnings hence errors.
> 
> > Thanks for figuring out what was going on here! I took this patch for a
> > spin with clang and it has a few more errors around
> > -Wimplicit-fallthrough:
> 
> Maybe add -Wno-implicit-fallthrough?  This code is a copy from outside
> the kernel, no one has ever wanted to maintain it, if nothing else (the
> more politically correct formulation is "we cannot as easily pick up
> improvements from upstream if we modify stuff").

Sure, we could do something like this if you preferred:

diff --git a/arch/powerpc/math-emu/Makefile b/arch/powerpc/math-emu/Makefile
index 26fef2e5672e..ed775747a2a5 100644
--- a/arch/powerpc/math-emu/Makefile
+++ b/arch/powerpc/math-emu/Makefile
@@ -16,3 +16,7 @@ obj-$(CONFIG_SPE)		+= math_efp.o
 
 CFLAGS_fabs.o = -fno-builtin-fabs
 CFLAGS_math.o = -fno-builtin-fabs
+
+ifdef CONFIG_CC_IS_CLANG
+ccflags-remove-y := $(CONFIG_CC_IMPLICIT_FALLTHROUGH)
+endif

At the same time, I see other modifications to these files that appear
to be for the kernel only so I suspect that this is already in the "we
cannot as easily pick up improvements from upstream" category,
regardless of that diff. No strong opinion from me, although I see
Christophe already included my suggestion in the most recent series:

https://lore.kernel.org/2663961738a46073713786d4efeb53100ca156e7.1662134272.git.christophe.leroy@csgroup.eu/

Cheers,
Nathan