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

[jamie@jamieiles.com (Jamie Iles)]

On Thu, Jan 20, 2011 at 02:08:11PM +0000, Russell King - ARM Linux wrote:
> On Thu, Jan 20, 2011 at 02:33:57PM +0800, Chao Xie wrote:
> > 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 <chao.xie@marvell.com>
> > ---
> >  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.

Jamie