Re: [RFC] arm: use built-in byte swap function

[Kim Phillips]

On Thu, 31 Jan 2013 09:28:01 +0000
Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> wrote:

> On Wed, Jan 30, 2013 at 08:09:00PM -0600, Kim Phillips wrote:
> > The savings come mostly from device-tree related code, and some
> > from drivers.
> 
> You forget that IP networking is all big endian, so these will be using
> the byte swapping too (search it for htons/ntohs/htonl/ntohl).

As David mentioned, there isn't much gain from net/ code.

> > v2:
> > - at91 and lpd270 builds fixed by limiting to ARMv6 and above
> >   (i.e., ARM cores that have support for the 'rev' instruction).
> >   Otherwise, the compiler emits calls to libgcc's __bswapsi2 on
> >   these ARMv4/v5 builds (and arch ARM doesn't link with libgcc).
> 
> Which compiler version?  gcc 4.5.4 doesn't do this, except for the 16-bit
> swap, so I doubt that any later compiler does.

I've tried both gcc 4.6.3 [1] and 4.6.4 [2].  If you can point me to
a 4.5.x, I'll try that, too, but as it stands now, if one moves the
code added to swab.h below outside of its armv6 protection,
gcc adds calls to __bswapsi2.

> > --- a/arch/arm/include/uapi/asm/swab.h
> > +++ b/arch/arm/include/uapi/asm/swab.h
> > @@ -50,4 +50,14 @@ static inline __attribute_const__ __u32 __arch_swab32(__u32 x)
> >  
> >  #endif
> >  
> > +#if defined(__KERNEL__) && __LINUX_ARM_ARCH__ >= 6
> > +#if GCC_VERSION >= 40600
> > +#define __HAVE_BUILTIN_BSWAP32__
> > +#define __HAVE_BUILTIN_BSWAP64__
> > +#endif
> > +#if GCC_VERSION >= 40800
> > +#define __HAVE_BUILTIN_BSWAP16__
> > +#endif
> > +#endif
> 
> If this is __KERNEL__ only, it should not be in a uapi header.  UAPI
> headers get exported to userland, this is not userland interface code.
> IT should be in arch/arm/include/asm/swab.h

right, I've fixed this and Boris' remove the help text comment, and
made a v3: