Messages in this thread | | | From | Vineet Gupta <> | Subject | Re: [PATCH] ARC: Fix __udelay parentheses | Date | Thu, 29 Aug 2013 05:55:11 +0000 |
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On 08/29/2013 12:00 AM, Mischa Jonker wrote: > Make sure that usecs is casted to long long, to ensure that the > (usecs * 4295 * HZ) multiplication is 64 bit. > > Initially, the (usecs * 4295 * HZ) part was done as a 32 bit > multiplication, with the result casted to 64 bit. This led to some bits > falling off. > > Signed-off-by: Mischa Jonker <mjonker@synopsys.com> > --- > arch/arc/include/asm/delay.h | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/arc/include/asm/delay.h b/arch/arc/include/asm/delay.h > index 442ce5d..8d35fe1 100644 > --- a/arch/arc/include/asm/delay.h > +++ b/arch/arc/include/asm/delay.h > @@ -56,8 +56,8 @@ static inline void __udelay(unsigned long usecs) > /* (long long) cast ensures 64 bit MPY - real or emulated > * HZ * 4295 is pre-evaluated by gcc - hence only 2 mpy ops > */ > - loops = ((long long)(usecs * 4295 * HZ) * > - (long long)(loops_per_jiffy)) >> 32; > + loops = (((long long) usecs) * 4295 * HZ * > + (long long) loops_per_jiffy) >> 32; > > __delay(loops); > }
The intent of writing orig code was to generate only 1 MPYHU insn (32*32 = high-part-64) for the whole math, at any optimization level whatsoever. If the first MPY is overflowing, u r likely spinning for > 10,000 usec (10ms) which is 1 scheduling tick on ARC - not good - presumably for hardware debug. It would be better to use a tight loop there and throw it out later.
The API abuse would only be caught for const @usecs case. Maybe we need to add a WARN_ON() there.
OTOH, if we really want to fix this, it would be cleaner to rewrite this as loops = ((u64)usecs * 4295 * HZ * loops_per_jiffy) >> 32;
Since one factor is upcasted, all are promoted to 64 bit. And we leave the optimizations to whims of gcc.
@Joern, I would assume that long long vs u64 (or unsigned long long) doesn't matter in this particular case.
-Vineet
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