2013-08-30 09:56:34

by Mischa Jonker

Subject: [PATCH] ARC: Fix __udelay calculation

Cast usecs to u64, 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, causing a "DMA initialization error" in the stmmac Ethernet
driver, due to a premature timeout.

Signed-off-by: Mischa Jonker <[email protected]>
---
arch/arc/include/asm/delay.h | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/arc/include/asm/delay.h b/arch/arc/include/asm/delay.h
index 442ce5d..43de302 100644
--- a/arch/arc/include/asm/delay.h
+++ b/arch/arc/include/asm/delay.h
@@ -53,11 +53,10 @@ static inline void __udelay(unsigned long usecs)
{
unsigned long loops;

- /* (long long) cast ensures 64 bit MPY - real or emulated
+ /* (u64) 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 = ((u64) usecs * 4295 * HZ * loops_per_jiffy) >> 32;

__delay(loops);
}
--
1.7.9.5

2013-08-30 11:13:36

by Vineet Gupta

Subject: Re: [PATCH] ARC: Fix __udelay calculation

On 08/30/2013 03:26 PM, Mischa Jonker wrote:
> Cast usecs to u64, 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, causing a "DMA initialization error" in the stmmac Ethernet
> driver, due to a premature timeout.
>
> Signed-off-by: Mischa Jonker <[email protected]>
>

Applied.

Thx,
-Vineet