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Subject: Re: [PATCH v3 2/2] clocksource: arm_arch_timer: Correct fault
 programming of CNTKCTL_EL1.EVNTI
To:     Keqian Zhu <zhukeqian1@huawei.com>, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        kvm@vger.kernel.org
Cc:     Marc Zyngier <maz@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        James Morse <james.morse@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Julien Thierry <julien.thierry.kdev@gmail.com>,
        Mark Brown <broonie@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexios Zavras <alexios.zavras@intel.com>,
        wanghaibin.wang@huawei.com
References: <20201204073126.6920-1-zhukeqian1@huawei.com>
 <20201204073126.6920-3-zhukeqian1@huawei.com>
From:   Daniel Lezcano <daniel.lezcano@linaro.org>
Message-ID: <a82cf9ff-f18d-ce0a-f7a2-82a56cbbec40@linaro.org>
Date:   Sat, 5 Dec 2020 12:15:01 +0100
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Hi Marc,

are you fine with this patch ?


On 04/12/2020 08:31, 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")
> Suggested-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
> ---
>  drivers/clocksource/arm_arch_timer.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index 777d38cb39b0..d0177824c518 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -822,15 +822,24 @@ static void arch_timer_evtstrm_enable(int divider)
>  
>  static void arch_timer_configure_evtstream(void)
>  {
> -	int evt_stream_div, pos;
> +	int evt_stream_div, lsb;
> +
> +	/*
> +	 * 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;
> +
> +	/*
> +	 * Find the closest power of two to the divisor. If the adjacent bit
> +	 * of lsb (last set bit, starts from 0) is set, then we use (lsb + 1).
> +	 */
> +	lsb = fls(evt_stream_div) - 1;
> +	if (lsb > 0 && (evt_stream_div & BIT(lsb - 1)))
> +		lsb++;
>  
> -	/* Find the closest power of two to the divisor */
> -	evt_stream_div = arch_timer_rate / ARCH_TIMER_EVT_STREAM_FREQ;
> -	pos = fls(evt_stream_div);
> -	if (pos > 1 && !(evt_stream_div & (1 << (pos - 2))))
> -		pos--;
>  	/* enable event stream */
> -	arch_timer_evtstrm_enable(min(pos, 15));
> +	arch_timer_evtstrm_enable(max(0, min(lsb, 15)));
>  }
>  
>  static void arch_counter_set_user_access(void)
> 


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