[PATCH] ARM: fix do_div() bug in big-endian systems

[Dave Martin]

On Fri, Apr 11, 2014 at 06:16:24PM +0800, Lu Xiangyu wrote:
> From: Xiangyu Lu <luxiangyu at huawei.com>
> 
> 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 at vger.kernel.org> # 2.6.20+
> Signed-off-by: Alex Wu <wuquanming at huawei.com>
> Signed-off-by: Xiangyu Lu <luxiangyu at huawei.com>
> ---
>  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?

It should be sufficient to find two values a and b, where do_div(a,b)
demonstrates the bug.

Cheers
---Dave