linux-kernel - RE: [PATCH 4/5] Clocksource: Flextimer: Fix counter clock prescaler calculation.

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Date:	Thu, 4 Sep 2014 02:18:28 +0000
From:	"Li.Xiubo@...escale.com" <Li.Xiubo@...escale.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"Dongsheng.Wang@...escale.com" <Dongsheng.Wang@...escale.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 4/5] Clocksource: Flextimer: Fix counter clock prescaler
 calculation.

> Subject: Re: [PATCH 4/5] Clocksource: Flextimer: Fix counter clock prescaler
> calculation.
> 
> On Tue, 26 Aug 2014, Xiubo Li wrote:
> 
> > We should minus one after calculating the counter input clock's
> > prescaler.
> >
> > Signed-off-by: Xiubo Li <Li.Xiubo@...escale.com>
> > Signed-off-by: Jingchang Lu <jingchang.lu@...escale.com>
> > ---
> >  drivers/clocksource/fsl_ftm_timer.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/clocksource/fsl_ftm_timer.c
> b/drivers/clocksource/fsl_ftm_timer.c
> > index f70fcf2..974890e 100644
> > --- a/drivers/clocksource/fsl_ftm_timer.c
> > +++ b/drivers/clocksource/fsl_ftm_timer.c
> > @@ -311,7 +311,7 @@ static int __init ftm_calc_closest_round_cyc(unsigned
> long freq)
> >  						HZ * (1 << priv->ps++));
> >  	} while (priv->periodic_cyc > 0xFFFF);
> >
> > -	if (priv->ps > FTM_PS_MAX) {
> > +	if (--priv->ps > FTM_PS_MAX) {
> 
> Looking at this makes me run away screaming. Just because you
> increment priv->ps unconditionally in the loop above, you decrement it
> again here. Why not fix the calculation proper in the first place?
> 
>       for (cyc = ~0UL, ps = 0, div = HZ; cyc > 0xffff; ps++, div *= 2)
>       	       cyc = DIV_ROUND_CLOSEST(freq, div);
> 

Yes, this looks nice.

Thanks,

BRs
Xiubo


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