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

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Date:	Wed, 3 Sep 2014 12:57:38 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Xiubo Li <Li.Xiubo@...escale.com>
cc:	daniel.lezcano@...aro.org, dongsheng.wang@...escale.com,
	linux-kernel@...r.kernel.org
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);

Hmm?

	tglx

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