linux-kernel - Re: [PATCH v3 1/3] clocksource/vt8500: Increase the minimum delta

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Date:	Tue, 5 Jan 2016 10:01:07 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Roman Volkov <v1ron@...l.ru>, arm@...nel.org,
	linux+armsoc@....linux.org.uk
Cc:	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>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 1/3] clocksource/vt8500: Increase the minimum delta

On 01/01/2016 02:24 PM, Roman Volkov wrote:
> From: Roman Volkov <rvolkov@...os.org>
>
> The vt8500 clocksource driver declares itself as capable to handle the
> minimum delay of 4 cycles by passing the value into
> clockevents_config_and_register(). The vt8500_timer_set_next_event()
> requires the passed cycles value to be at least 16. The impact is that
> userspace hangs in nanosleep() calls with small delay intervals.
>
> This problem is reproducible in Linux 4.2 starting from:
> c6eb3f70d448 ('hrtimer: Get rid of hrtimer softirq')
>
> Signed-off-by: Roman Volkov <rvolkov@...os.org>
> Acked-by: Alexey Charkov <alchark@...il.com>

Hi Roman,

I looked at the email thread, and IIUC if set_next_event fails, the 
system freeze. Your patch fixes the issue for your driver but not the 
real issue because if set_next_event fails, at least a warning should 
appear in the log or better nanosleep should fail gracefully.

BTW why min delta is MIN_OSCR_DELTA * 2 in clockevents_config_and_register ?

> ---
>   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)
>   		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);
>   }
>
>   CLOCKSOURCE_OF_DECLARE(vt8500, "via,vt8500-timer", vt8500_timer_init);
>


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