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

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Date:	Thu, 6 Mar 2014 03:18:14 +0000
From:	Jingchang Lu <jingchang.lu@...escale.com>
To:	Shawn Guo <shawn.guo@...aro.org>, Stefan Agner <stefan@...er.ch>
CC:	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] clocksource: vf_pit_timer: use complement for
 sched_clock reading



> -----Original Message-----
> From: Shawn Guo [mailto:shawn.guo@...aro.org]
> Sent: Thursday, March 06, 2014 9:46 AM
> To: Stefan Agner
> Cc: daniel.lezcano@...aro.org; tglx@...utronix.de; linux@....linux.org.uk;
> linux-arm-kernel@...ts.infradead.org; linux-kernel@...r.kernel.org; Lu
> Jingchang-B35083
> 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
> 
The fix is correct, thanks.

Best Regards,
Jingchang

> > ---
> >  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
> >