On Mon, 14 Apr 2014, Dave Martin wrote:
> On Fri, Apr 11, 2014 at 06:16:24PM +0800, Lu Xiangyu wrote:
> > From: Xiangyu Lu <luxiangyu@xxxxxxxxxx>
> >
> > In big-endian systems, "%1" get the most significant part of the value, cause
> > the instruction to get the wrong result.
> >
> > When viewing ftrace record in big-endian ARM systems, we found that
> > the timestamp errors:
> >
> > swapper-0 [001] 1325.970000: 0:120:R ==> [001] 16:120:R events/1
> > events/1-16 [001] 1325.970000: 16:120:S ==> [001] 0:120:R swapper
> > swapper-0 [000] 1325.1000000: 0:120:R + [000] 15:120:R events/0
> > swapper-0 [000] 1325.1000000: 0:120:R ==> [000] 15:120:R events/0
> > swapper-0 [000] 1326.030000: 0:120:R + [000] 1150:120:R sshd
> > swapper-0 [000] 1326.030000: 0:120:R ==> [000] 1150:120:R sshd
> >
> > When viewed ftrace records, it will call the do_div(n, base) function, which
> > achieved arch/arm/include/asm/div64.h in. When n = 10000000, base = 1000000, in
> > do_div(n, base) will execute "umull %Q0, %R0, %1, %Q2".
> >
> > Cc: <stable@xxxxxxxxxxxxxxx> # 2.6.20+
> > Signed-off-by: Alex Wu <wuquanming@xxxxxxxxxx>
> > Signed-off-by: Xiangyu Lu <luxiangyu@xxxxxxxxxx>
> > ---
> > arch/arm/include/asm/div64.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm/include/asm/div64.h b/arch/arm/include/asm/div64.h
> > index 191ada6..662c7bd 100644
> > --- a/arch/arm/include/asm/div64.h
> > +++ b/arch/arm/include/asm/div64.h
> > @@ -156,7 +156,7 @@
> > /* Select the best insn combination to perform the */ \
> > /* actual __m * __n / (__p << 64) operation. */ \
> > if (!__c) { \
> > - asm ( "umull %Q0, %R0, %1, %Q2\n\t" \
> > + asm ( "umull %Q0, %R0, %Q1, %Q2\n\t" \
>
> This looks plausible: these if() clauses are all concerned with
> multiplying the low parts of __m and __n together, and this seems
> to be the only 64-bit asm operand reference where Q or R is suspiciously
> missing: so it looks likely that "Q" is required here for consistency.
>
> My understanding of the details of this code are limited: do you have
> a simple test case to demonstrate the error and the fix?
No need -- it is indeed wrong on big endian and has been so for the last
7.5 years.
Nicolas
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