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

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Date:   Mon, 17 Aug 2020 13:56:48 +0100
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 2/2] clocksource: arm_arch_timer: Correct fault
 programming of CNTKCTL_EL1.EVNTI

On 2020-08-17 13:24, 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.
> 
> Signed-off-by: Marc Zyngier <maz@...nel.org>

I have never given you this tag, you are making it up. Please read
Documentation/process/submitting-patches.rst to understand what
tag you can put by yourself.

At best, put "Suggested-by" tag, as this is different from what
I posted anyway.

Thanks,

         M.

> 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 6e11c60..4140a37 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -794,10 +794,14 @@ 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--;
>  	/* enable event stream */
>  	arch_timer_evtstrm_enable(min(pos, 15));

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