[RFC PATCH 1/3] ARM: iwmmxt: Fix Makefile rules for building iwmmxt for Thumb-2

[Nicolas Pitre]

On Wed, 7 Sep 2011, Eric Miao wrote:

> On Wed, Sep 7, 2011 at 8:59 AM, Dave Martin <dave.martin at linaro.org> wrote:
> > Because gcc/gas have no sane way to turn on individual CPU
> > extensions from the command-line, iwmmxt.S was previously built
> > with -mcpu=iwmmxt.  Unfortunately, this also downgrades the CPU to
> > v5, with the result that this file fails to build for a Thumb-2
> > kernel.
> >
> > New versions of the tools support -march=<base arch>+iwmmxt, and it
> > seems reasonable to require up-to-date tools when building in
> > Thumb-2.  So, this patch uses -march=armv7-a+iwmmxt for
> > CONFIG_THUMB2_KERNEL=y.
> >
> > Signed-off-by: Dave Martin <dave.martin at linaro.org>
> > ---
> >  arch/arm/kernel/Makefile |    7 +++++++
> >  1 files changed, 7 insertions(+), 0 deletions(-)
> >
> > diff --git a/arch/arm/kernel/Makefile b/arch/arm/kernel/Makefile
> > index f7887dc..8a58339 100644
> > --- a/arch/arm/kernel/Makefile
> > +++ b/arch/arm/kernel/Makefile
> > @@ -65,7 +65,14 @@ obj-$(CONFIG_CPU_PJ4)                += pj4-cp0.o
> >  obj-$(CONFIG_IWMMXT)           += iwmmxt.o
> >  obj-$(CONFIG_CPU_HAS_PMU)      += pmu.o
> >  obj-$(CONFIG_HW_PERF_EVENTS)   += perf_event.o
> > +
> > +# When enough people have binutils which support -march=...+iwmmxt, this
> > +# should change to something like if __LINUX_ARM_ARCH__ < 7.
> > +ifdef CONFIG_THUMB2_KERNEL
> > +AFLAGS_iwmmxt.o                        := -Wa,-march=armv7-a+iwmmxt
> > +else
> >  AFLAGS_iwmmxt.o                        := -Wa,-mcpu=iwmmxt
> > +endif
> 
> It looks more like the switch should depend on the compiler version.
> Unless there is a clear way to decide if gcc supports this switch, I
> think it's reasonable to have the change like above.

Normally the way to go with gcc version dependent alternatives is to use 
something like:

AFLAGS_foo.o := $(call cc-option,<the_new_flag>,<the_fallback_flag>)

This will test if <the_new_flag> is supported by the used gcc, and use 
the fallback otherwise.


Nicolas