linux-kernel - Re: [PATCH v2 2/2] clocksource: arm_arch_timer: Correct fault programming of CNTKCTL

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Date:   Thu, 03 Dec 2020 14:57:21 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Keqian Zhu <zhukeqian1@...wei.com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
        Steven Price <steven.price@....com>,
        Andrew Jones <drjones@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        wanghaibin.wang@...wei.com
Subject: Re: [PATCH v2 2/2] clocksource: arm_arch_timer: Correct fault
 programming of CNTKCTL_EL1.EVNTI

On 2020-08-18 04:28, Keqian Zhu wrote:
> ARM virtual counter supports event stream, it can only trigger an event
> when the trigger bit (the value of CNTKCTL_EL1.EVNTI) of CNTVCT_EL0 
> changes,
> so the actual period of event stream is 2^(cntkctl_evnti + 1). For 
> example,
> when the trigger bit is 0, then virtual counter trigger an event for 
> every
> two cycles.
> 
> Fixes: 037f637767a8 ("drivers: clocksource: add support for
>        ARM architected timer event stream")

Fixes: tags should on a single line.

> Suggested-by: Marc Zyngier <maz@...nel.org>
> Signed-off-by: Keqian Zhu <zhukeqian1@...wei.com>
> ---
>  drivers/clocksource/arm_arch_timer.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/clocksource/arm_arch_timer.c
> b/drivers/clocksource/arm_arch_timer.c
> index 777d38c..e3b2ee0 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -824,10 +824,14 @@ static void arch_timer_configure_evtstream(void)
>  {
>  	int evt_stream_div, pos;
> 
> -	/* Find the closest power of two to the divisor */
> -	evt_stream_div = arch_timer_rate / ARCH_TIMER_EVT_STREAM_FREQ;
> +	/*
> +	 * Find the closest power of two to the divisor. As the event
> +	 * stream can at most be generated at half the frequency of the
> +	 * counter, use half the frequency when computing the divider.
> +	 */
> +	evt_stream_div = arch_timer_rate / ARCH_TIMER_EVT_STREAM_FREQ / 2;
>  	pos = fls(evt_stream_div);
> -	if (pos > 1 && !(evt_stream_div & (1 << (pos - 2))))
> +	if ((pos == 1) || (pos > 1 && !(evt_stream_div & (1 << (pos - 2)))))
>  		pos--;

You don't explain why you are special-casing pos == 1.

>  	/* enable event stream */
>  	arch_timer_evtstrm_enable(min(pos, 15));

Also, please Cc the subsystem maintainers:

CLOCKSOURCE, CLOCKEVENT DRIVERS
M:      Daniel Lezcano <daniel.lezcano@...aro.org>
M:      Thomas Gleixner <tglx@...utronix.de>
L:      linux-kernel@...r.kernel.org
S:      Supported
T:      git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git 
timers/core
F:      Documentation/devicetree/bindings/timer/
F:      drivers/clocksource/

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...