linux-kernel - Re: [PATCH v2] powerpc: Pass correct CPU reference to assembler

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Date:   Sat, 17 Dec 2022 01:59:57 +0100
From:   Pali Rohár <pali@...nel.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jan-Benedict Glaw <jbglaw@...-owl.de>,
        Joel Stanley <joel@....id.au>
Subject: Re: [PATCH v2] powerpc: Pass correct CPU reference to assembler

On Friday 16 December 2022 12:10:48 Segher Boessenkool wrote:
> On Fri, Dec 16, 2022 at 05:57:46PM +0000, Christophe Leroy wrote:
> > Le 16/12/2022 à 18:18, Segher Boessenkool a écrit :
> > > On Fri, Dec 16, 2022 at 09:35:50AM +0100, Christophe Leroy wrote:
> > >> Today we have CONFIG_TARGET_CPU which provides the identification of the
> > >> expected CPU, it is used for GCC. Use it as well for the assembler.
> > > 
> > > Why do you use -Wa, at all for this?  The compiler should already pass
> > > proper options always!
> > 
> > That's historical I guess. Comes from commit 14cf11af6cf6 ("powerpc: 
> > Merge enough to start building in arch/powerpc.")
> 
> Ah.  The patch moves stuff around, I thought more of it is new than it
> really is.  Sorry.
> 
> It would be good to get rid of all such things that do no good and can
> easily cause problems, of course, but that does not belong to this patch
> of course.

Just a coincident but u-boot has similar problem...
https://patchwork.ozlabs.org/project/uboot/patch/20221211141204.8153-1-pali@kernel.org/

So I agree that removal of -Wa,-mXXX is a good idea. I checked that gcc
pass correct -Wa,-mXXX flag from -mcpu=YYY flag.

> > >> +cpu-as-$(CONFIG_PPC_BOOK3S_64)	+= $(call as-option,-Wa$(comma)-many)
> > > 
> > > What is this for?  Using -many is a huge step back, it hides many
> > > problems :-(
> > 
> > The only thing I did is removed the -Wa,-mpower4 from the line, leaving 
> > the remaining part. Initialy it was:
> > 
> > cpu-as-$(CONFIG_PPC_BOOK3S_64) += $(call as-option,-Wa$(comma)-mpower4) 
> > $(call as-option,-Wa$(comma)-many)
> > 
> > It was added in 2018 by commit 960e30029863 ("powerpc/Makefile: Fix 
> > PPC_BOOK3S_64 ASFLAGS"). There is a long explanation it the commit.
> > 
> > Should we remove it ?
> 
> The commit says it is a workaround for clang problems, so it needs
> testing there.  It also needs testing everywhere else, because as I said
> it hides a lot of problems, so removing it will make a lot of sloppy
> code that has crept in since 2018 scream bloody murder :-(
> 
> 
> Segher