linux-kernel - Re: [PATCH] clocksource: vf_pit_timer: use complement for sched

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Date:	Thu, 6 Mar 2014 09:45:36 +0800
From:	Shawn Guo <shawn.guo@...aro.org>
To:	Stefan Agner <stefan@...er.ch>
CC:	<daniel.lezcano@...aro.org>, <tglx@...utronix.de>,
	<linux@....linux.org.uk>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, Jingchang Lu <b35083@...escale.com>
Subject: Re: [PATCH] clocksource: vf_pit_timer: use complement for
 sched_clock reading

On Wed, Mar 05, 2014 at 11:11:08PM +0100, Stefan Agner wrote:
> Vybrids PIT register is monitonic decreasing. However, sched_clock
> reading needs to be monitonic increasing. Use bitwise not to get
> the complement of the clock register. This fixes the clock going
> backward. Also, the clock now starts at 0 since we load the
> register with the maximum value at start.
> 
> Signed-off-by: Stefan Agner <stefan@...er.ch>

Looks correct to me, so

Acked-by: Shawn Guo <shawn.guo@...aro.org>

But I also copied the driver author Jingchang to see if he has any
comment.

Shawn

> ---
>  drivers/clocksource/vf_pit_timer.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/clocksource/vf_pit_timer.c b/drivers/clocksource/vf_pit_timer.c
> index 02821b0..a918bc4 100644
> --- a/drivers/clocksource/vf_pit_timer.c
> +++ b/drivers/clocksource/vf_pit_timer.c
> @@ -54,7 +54,7 @@ static inline void pit_irq_acknowledge(void)
>  
>  static u64 pit_read_sched_clock(void)
>  {
> -	return __raw_readl(clksrc_base + PITCVAL);
> +	return ~__raw_readl(clksrc_base + PITCVAL);
>  }
>  
>  static int __init pit_clocksource_init(unsigned long rate)
> -- 
> 1.9.0
> 

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