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

[Russell King - ARM Linux]

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:
> > 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 at 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.

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