[RFC v3 PATCH 1/7] ARM: add support for kernel mode NEON in atomic context

[Catalin Marinas]

On Tue, Oct 15, 2013 at 06:30:50PM +0100, Ard Biesheuvel wrote:
> On 15 October 2013 19:26, Catalin Marinas <catalin.marinas at arm.com> wrote:
> > On Sun, Oct 13, 2013 at 01:14:57PM +0100, Ard Biesheuvel wrote:
> >> diff --git a/arch/arm/include/asm/neon.h b/arch/arm/include/asm/neon.h
> >> index 8f730fe..800d85c 100644
> >> --- a/arch/arm/include/asm/neon.h
> >> +++ b/arch/arm/include/asm/neon.h
> >> @@ -8,10 +8,30 @@
> >>   * published by the Free Software Foundation.
> >>   */
> >>
> >> +#include <linux/types.h>
> >> +#include <linux/hardirq.h>
> >> +#include <asm/fpstate.h>
> >>  #include <asm/hwcap.h>
> >>
> >>  #define cpu_has_neon()               (!!(elf_hwcap & HWCAP_NEON))
> >>
> >> +/*
> >> + * Avoid wasting stack space by making the size of the allocated area depend on
> >> + * whether we are currently running in process context. (If this is the case, we
> >> + * will use the normal preserve/restore mechanism, leaving the allocated stack
> >> + * space unused.)
> >> + */
> >> +#define __QREG_SIZE(num)     \
> >> +     ((!in_interrupt()) ? 0 : (num) > 16 ? 256 : 16 * (((num) + 1) & ~1U))
> >> +
> >> +#define DEFINE_NEON_REGSTACK_PARTIAL(v, num)         \
> >> +     struct {                                        \
> >> +             struct vfp_partial_state regs;          \
> >> +             u8 qregs[__QREG_SIZE(num)];             \
> >> +     } v
> >
> > Oh, interesting gcc feature. What does it generate?
> >
> 
> Well, it's not a feature particular to GCC, as far as I am aware. The
> anonymous struct is just runtime variably sized depending on
> in_interrupt() and the requested number of registers.

OK, it looks like it's valid C99. I was worried the compiler may
generate something like an alloca() library call.

-- 
Catalin