Re: CPU_BIG_ENDIAN in generic code (was: Re: [PATCH v3 3/7] arch/sparc: Define config parameter CPU_BIG_ENDIAN)

[Babu Moger]

On 5/29/2017 9:56 PM, Michael Ellerman wrote:
> Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> writes:
>
> > Hi Michael,
> >
> > On Mon, May 29, 2017 at 2:07 PM, Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
> > > Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> writes:
> > > > On Tue, May 23, 2017 at 11:45 PM, Babu Moger <babu.moger@xxxxxxxxxx> wrote:
> > > > > Found this problem while enabling queued rwlock on SPARC.
> > > > > The parameter CONFIG_CPU_BIG_ENDIAN is used to clear the
> > > > > specific byte in qrwlock structure. Without this parameter,
> > > > > we clear the wrong byte. Here is the code.
> > > > >
> > > > > static inline u8 *__qrwlock_write_byte(struct qrwlock *lock)
> > > > >   {
> > > > >          return (u8 *)lock + 3 * IS_BUILTIN(CONFIG_CPU_BIG_ENDIAN);
> > > > >   }
> > > > >
> > > > > Define CPU_BIG_ENDIAN for SPARC to fix it.
> > > > > --- a/arch/sparc/Kconfig
> > > > > +++ b/arch/sparc/Kconfig
> > > > > @@ -92,6 +92,10 @@ config ARCH_DEFCONFIG
> > > > >   config ARCH_PROC_KCORE_TEXT
> > > > >          def_bool y
> > > > >
> > > > > +config CPU_BIG_ENDIAN
> > > > > +       bool
> > > > > +       default y if SPARC
> > > > Nice catch!
> > > >
> > > > Traditionally, CPU_BIG_ENDIAN and CPU_LITTLE_ENDIAN were defined only on
> > > > architectures that may support both.  And it was checked in platform code
> > > > and drivers only.
> > > > Hence the symbol is lacking from most architectures. Heck, even
> > > > architectures that support both may default to one endiannes, and declare
> > > > only the symbol for the other endianness:
> > > I guess there's a reason we can't use __BIG_ENDIAN__ / __LITTLE_ENDIAN__ ?
> > I (C/asm) code we can, in Kconfig we cannot.
> >
> > So far we tried always doing that, but a few checks for the semi-existing
> > Kconfig symbol crept in in generic code. Those could be replaced by the __*__
> > variants, but consistently having the Kconfig symbols would be useful anyway
> > (e.g. to avoid building the broken-on-big-endian ISDN drivers).
> Ah OK, the original mail was citing C code, but yeah I guess it would be
> handy in Makefiles etc.

Thanks for all the responses.  I see couple of options here.

1. Fix the c code in include/asm-generic/qrwlock.h using ifdef 
__BIG_ENDIAN__
    This will fix the issue for us.

2. Define CPU_BIG_ENDIAN for all the missing fixed endian architectures. 
Because the problem is only for fixed big endian archs.

I prefer the option 1.  What do you guys think ?

> cheers