[PATCH] ARM: TWD: fix the clock calculation for TWD

On Thu, Jan 20, 2011 at 02:44:09PM +0000, Jamie Iles wrote:

On Thu, Jan 20, 2011 at 02:08:11PM +0000, Russell King - ARM Linux wrote:

quoted

On Thu, Jan 20, 2011 at 02:33:57PM +0800, Chao Xie wrote:

quoted

The original code will do count * (HZ /5). It will make the twd
timer rate decreased if HZ can not be excatly divided. For
example HZ=128.

Signed-off-by: Chao Xie <redacted>
---
 arch/arm/kernel/smp_twd.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/arch/arm/kernel/smp_twd.c b/arch/arm/kernel/smp_twd.c
index 35882fb..ca6405b 100644
--- a/arch/arm/kernel/smp_twd.c
+++ b/arch/arm/kernel/smp_twd.c

@@ -111,7 +111,8 @@ static void __cpuinit twd_calibrate_rate(void)
 
 		count = __raw_readl(twd_base + TWD_TIMER_COUNTER);
 
-		twd_timer_rate = (0xFFFFFFFFU - count) * (HZ / 5);
+		twd_timer_rate = ((unsigned long)(0xFFFFFFFFU - count))
+			* HZ / 5;
 
 		printk("%lu.%02luMHz.\n", twd_timer_rate / 1000000,
 			(twd_timer_rate / 100000) % 100);

I don't think this patch has any effect what so ever, so I just tried:

#define HZ      128
unsigned long twd_timer_rate;
void calc1(unsigned long count)
{
        twd_timer_rate = (0xffffffffU - count) * (HZ / 5);
}
void calc2(unsigned long count)
{
        twd_timer_rate = ((unsigned long)(0xffffffffU - count)) * (HZ / 5);
}

Hi Russell,

In Chao's patch the parentheses around (HZ / 5) have been removed so its 
now dowing the multiplication before the division (if I remember the 
precedences correctly!).  I don't think the cast to unsigned long is 
needed though.

Hmm.  That means the maximum twd timer rate we can support is about
860MHz.  Is that enough?