Problem with 64-bit registers on i.MX53

[Dave Martin]

On Mon, Oct 08, 2012 at 06:10:11PM +0100, Russell King - ARM Linux wrote:
> On Mon, Oct 08, 2012 at 06:01:24PM +0100, Russell King - ARM Linux wrote:
> > On Mon, Oct 08, 2012 at 06:08:41PM +0200, Michael Olbrich wrote:
> > > 
> > > Hi,
> > > 
> > > I have a problem that looks like that 64-bit registers (I think) are not
> > > saved/restored correctly on a context switch. I've reduced it to the
> > > following test case:
> > > 
> > > - Latest Linux mainline kernel (v3.6-8559-ge9eca4d)
> > >   v3.5 is also affected
> > > - imx_v6_v7_defconfig
> > > - arch/arm/boot/dts/imx53-evk.dts
> > > 
> > > The following test program is compiled with "-mcpu=cortex-a8 -mfpu=neon
> > > -O2".
> > > ------------------------>8--------------------------------
> > > #include <inttypes.h>
> > > #include <assert.h>
> > > 
> > > volatile int x = 2;
> > > volatile int64_t y = 2;
> > > 
> > > int main() {
> > > 	volatile int a = 0;
> > > 	volatile int64_t b = 0;
> > > 	while (1) {
> > > 		a = (a + x) % (1 << 30);
> > > 		b = (b + y) % (1 << 30);
> > > 		assert(a == b);
> > > 	}
> > > }
> > > ------------------------>8--------------------------------
> > > The ".. (b + y) .." should result in "vadd.i64 d19, d18, d16" or
> > > something like that.

Just for my curiosity, can you let me know what compiler version you're
using and the disassembly?  I'm actually a little surprised to see
NEON code being generated here, though the patch below fixes what
definitely looks like a context switch bug for combined v6+v7 kernels...

Cheers
---Dave

> > 
> > Hmm.
> > 
> > Can you send me the output of 'grep ^Features /proc/cpuinfo' please?
> 
> You may also like to try the patch below... it will probably fix your
> problem.
> 
> diff --git a/arch/arm/include/asm/vfpmacros.h b/arch/arm/include/asm/vfpmacros.h
> index a7aadbd..6a6f1e4 100644
> --- a/arch/arm/include/asm/vfpmacros.h
> +++ b/arch/arm/include/asm/vfpmacros.h
> @@ -28,7 +28,7 @@
>  	ldr	\tmp, =elf_hwcap		    @ may not have MVFR regs
>  	ldr	\tmp, [\tmp, #0]
>  	tst	\tmp, #HWCAP_VFPv3D16
> -	ldceq	p11, cr0, [\base],#32*4		    @ FLDMIAD \base!, {d16-d31}
> +	ldceql	p11, cr0, [\base],#32*4		    @ FLDMIAD \base!, {d16-d31}
>  	addne	\base, \base, #32*4		    @ step over unused register space
>  #else
>  	VFPFMRX	\tmp, MVFR0			    @ Media and VFP Feature Register 0
> @@ -52,7 +52,7 @@
>  	ldr	\tmp, =elf_hwcap		    @ may not have MVFR regs
>  	ldr	\tmp, [\tmp, #0]
>  	tst	\tmp, #HWCAP_VFPv3D16
> -	stceq	p11, cr0, [\base],#32*4		    @ FSTMIAD \base!, {d16-d31}
> +	stceql	p11, cr0, [\base],#32*4		    @ FSTMIAD \base!, {d16-d31}
>  	addne	\base, \base, #32*4		    @ step over unused register space
>  #else
>  	VFPFMRX	\tmp, MVFR0			    @ Media and VFP Feature Register 0
> 
> 
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