linux-kernel - Re: [PATCH v2] powerpc/boot: Don't always pass -mcpu=powerpc when building 32-bit uImage

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Date:   Sat, 27 Aug 2022 19:36:10 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Joel Stanley <joel@....id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] powerpc/boot: Don't always pass -mcpu=powerpc when
 building 32-bit uImage

On Saturday 27 August 2022 17:31:10 Christophe Leroy wrote:
> Le 27/08/2022 à 15:39, Pali Rohár a écrit :
> > For 32-bit uImage try to use CONFIG_TARGET_CPU option for -mcpu. This fixes
> > following compiler error when building kernel with powerpc e500 SPE capable
> > cross compilers:
> > 
> >      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
> > 
> > For 64-bit uImage and 64-bit kernels with 32-bit uImage wrapper there is no
> > change.
> > 
> > Similar change was already introduced for the main powerpc Makefile in
> > commit 446cda1b21d9 ("powerpc/32: Don't always pass -mcpu=powerpc to the
> > compiler").
> > 
> > Fixes: 40a75584e526 ("powerpc/boot: Build wrapper for an appropriate CPU")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Pali Rohár <pali@...nel.org>
> > ---
> >   arch/powerpc/boot/Makefile | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> > 
> > diff --git a/arch/powerpc/boot/Makefile b/arch/powerpc/boot/Makefile
> > index a9cd2ea4a861..f56a5f90a5d8 100644
> > --- a/arch/powerpc/boot/Makefile
> > +++ b/arch/powerpc/boot/Makefile
> > @@ -44,8 +44,16 @@ else
> >   BOOTCFLAGS	+= -m64 -mcpu=powerpc64
> >   endif
> >   else
> > +ifdef CONFIG_PPC32
> > +ifdef CONFIG_TARGET_CPU_BOOL
> > +BOOTCFLAGS	+= -m32 -mcpu=$(CONFIG_TARGET_CPU)
> > +else
> > +BOOTCFLAGS	+= -m32 -mcpu=powerpc
> 
> You can't do that. You get here only if user has selected 
> TOOLCHAIN_DEFAULT_CPU, in which case you don't want to for -mcpu=powerpc.

So do I understand it correctly that in this branch I should omit -mcpu=powerpc?

> -mcpu=powerpc is set when user selects CONFIG_POWERPC_CPU, in which case 
> CONFIG_TARGET_CPU_BOOL is set as well.
> 
> > +endif
> > +else
> >   BOOTCFLAGS	+= -m32 -mcpu=powerpc
> 
> Same, for PPC64 I think you don't want that either, unless the 
> CONFIG_GENERIC_CPU has been selected in which case 
> CONFIG_TARGET_CPU_BOOL is not set.
> 
> When CONFIG_TARGET_CPU_BOOL is set for PPC64 you also want 
> -mcpu=$(CONFIG_TARGET_CPU)

I understand that this branch is called for PPC64 build with 32-bit
uImage wrapper. So in this case should not be used TARGET_CPU as it
would be 64-bit and not 32-bit as requited for 32-bit uImage wrapper.

Anyway, in this change I'm touching only PPC32 build, so all PPC64 stay
as it was before.

> >   endif
> > +endif
> >   
> >   BOOTCFLAGS	+= -isystem $(shell $(BOOTCC) -print-file-name=include)
> >