linux-kernel - Re: [PATCH v2 RESEND 1/3] clocksource/vt8500: Use MIN_OSCR

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Date:	Fri, 1 Jan 2016 10:58:06 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Roman Volkov <v1ron@...l.ru>
cc:	arm@...nel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	Alexey Charkov <alchark@...il.com>,
	Roman Volkov <rvolkov@...os.org>,
	Tony Prisk <linux@...sktech.co.nz>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v2 RESEND 1/3] clocksource/vt8500: Use MIN_OSCR_DELTA
 from PXA

On Thu, 31 Dec 2015, Roman Volkov wrote:

> From: Roman Volkov <rvolkov@...os.org>
> 
> Since vt8500 and PXA timers are identical, use MIN_OSCR_DELTA from PXA,
> which is bigger than existing value. It is required to determine the
> minimum delay which hardware can generate.

This changelog makes no sense at all.
 
> This commit fixes vt8500 breakage in Linux 4.2 introduced by
> c6eb3f7 ('hrtimer: Get rid of hrtimer softirq')

> Signed-off-by: Roman Volkov <rvolkov@...os.org>
> ---
>  drivers/clocksource/vt8500_timer.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/clocksource/vt8500_timer.c b/drivers/clocksource/vt8500_timer.c
> index a92e94b..dfc3bb4 100644
> --- a/drivers/clocksource/vt8500_timer.c
> +++ b/drivers/clocksource/vt8500_timer.c
> @@ -50,6 +50,8 @@
>  
>  #define msecs_to_loops(t) (loops_per_jiffy / 1000 * HZ * t)
>  
> +#define MIN_OSCR_DELTA		16
> +
>  static void __iomem *regbase;
>  
>  static cycle_t vt8500_timer_read(struct clocksource *cs)
> @@ -80,7 +82,7 @@ static int vt8500_timer_set_next_event(unsigned long cycles,
>  		cpu_relax();
>  	writel((unsigned long)alarm, regbase + TIMER_MATCH_VAL);
>  
> -	if ((signed)(alarm - clocksource.read(&clocksource)) <= 16)
> +	if ((signed)(alarm - clocksource.read(&clocksource)) <= MIN_OSCR_DELTA)

So how is that value bigger? MIN_OSCR_DELTA is still 16

>  		return -ETIME;
>  
>  	writel(1, regbase + TIMER_IER_VAL);
> @@ -151,7 +153,7 @@ static void __init vt8500_timer_init(struct device_node *np)
>  		pr_err("%s: setup_irq failed for %s\n", __func__,
>  							clockevent.name);
>  	clockevents_config_and_register(&clockevent, VT8500_TIMER_HZ,
> -					4, 0xf0000000);
> +					MIN_OSCR_DELTA * 2, 0xf0000000);

Now here is the real change. You use a larger minimum tick value so that the
above check in vt8500_timer_set_next_event() actually works. That's what you
want to explain in the changelog. The blurb about reusing MIN_OSCR_DELTA is
just useless.

A proper changelog describes:

  1) the problem and the resulting wreckage

  2) the solution

Thanks,

	tglx

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