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

Your patch is better. 64bit is enough, I think.

-----Original Message-----
From: Jamie Iles [mailto:jamie at jamieiles.com] 
Sent: Friday, January 21, 2011 2:11 AM
To: Russell King - ARM Linux
Cc: Jamie Iles; Chao Xie; linux-arm-kernel at lists.infradead.org
Subject: Re: [PATCH] ARM: TWD: fix the clock calculation for TWD

On Thu, Jan 20, 2011 at 04:38:36PM +0000, Russell King - ARM Linux wrote:

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

quoted

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?

I'm not familiar with any ARM SMP systems so I wouldn't like to commit 
to that!  If the answer is no, then I guess could we do something like 
the (untested in a kernel) patch below to get a few more bits and we 
should then be able to support rates of up to ~4294MHz.

Jamie

8<----

diff --git a/arch/arm/kernel/smp_twd.c b/arch/arm/kernel/smp_twd.c
index fd91566..334ad95 100644
--- a/arch/arm/kernel/smp_twd.c
+++ b/arch/arm/kernel/smp_twd.c

@@ -89,6 +89,8 @@ static void __cpuinit twd_calibrate_rate(void)
 	 * the timer ticks
 	 */
 	if (twd_timer_rate == 0) {
+		u64 rate64;
+
 		printk(KERN_INFO "Calibrating local timer... ");
 
 		/* Wait for a tick to start */

@@ -111,7 +113,9 @@ static void __cpuinit twd_calibrate_rate(void)
 
 		count = __raw_readl(twd_base + TWD_TIMER_COUNTER);
 
-		twd_timer_rate = (0xFFFFFFFFU - count) * (HZ / 5);
+		rate64 = (0xFFFFFFFFLLU - count) * (u64)HZ;
+		do_div(rate64, 5);
+		twd_timer_rate = rate64;
 
 		printk("%lu.%02luMHz.\n", twd_timer_rate / 1000000,
 			(twd_timer_rate / 1000000) % 100);