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Subject: Re: [PATCH] clocksource/drivers/fttmr010: fix set_next_event handler
To:     Tao Ren <taoren@fb.com>, Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org
Cc:     openbmc@lists.ozlabs.org, cov@fb.com, tfang@fb.com,
        Linus Walleij <linus.walleij@linaro.org>
References: <20180919221331.2511558-1-taoren@fb.com>
From:   Daniel Lezcano <daniel.lezcano@linaro.org>
Message-ID: <bb452948-0a9e-140d-57d5-85c1732fca86@linaro.org>
Date:   Thu, 20 Sep 2018 01:16:32 +0200
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On 20/09/2018 00:13, Tao Ren wrote:
> Currently, the aspeed MATCH1 register is updated to <current_count -
> cycles> in set_next_event handler, with the assumption that COUNT
> register value is preserved when the timer is disabled and it continues
> decrementing after the timer is enabled. But the assumption is wrong:
> RELOAD register is loaded into COUNT register when the aspeed timer is
> enabled, which means the next event may be delayed because timer
> interrupt won't be generated until <0xFFFFFFFF - current_count +
> cycles>.
> 
> The problem can be fixed by updating RELOAD register to <cycles>, and
> COUNT register will be re-loaded when the timer is enabled and interrupt
> is generated when COUNT register overflows.
> 
> The test result on Facebook Backpack-CMM BMC hardware (AST2500) shows
> the issue is fixed: without the patch, usleep(100) suspends the process
> for several milliseconds (and sometimes even over 40 milliseconds);
> after applying the fix, usleep(100) takes averagely 240 microseconds to
> return under the same workload level.
> 
> Signed-off-by: Tao Ren <taoren@fb.com>

Linus, any comments ?

> ---
>  drivers/clocksource/timer-fttmr010.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/clocksource/timer-fttmr010.c b/drivers/clocksource/timer-fttmr010.c
> index c020038ebfab..cf93f6419b51 100644
> --- a/drivers/clocksource/timer-fttmr010.c
> +++ b/drivers/clocksource/timer-fttmr010.c
> @@ -130,13 +130,17 @@ static int fttmr010_timer_set_next_event(unsigned long cycles,
>  	cr &= ~fttmr010->t1_enable_val;
>  	writel(cr, fttmr010->base + TIMER_CR);
>  
> -	/* Setup the match register forward/backward in time */
> -	cr = readl(fttmr010->base + TIMER1_COUNT);
> -	if (fttmr010->count_down)
> -		cr -= cycles;
> -	else
> -		cr += cycles;
> -	writel(cr, fttmr010->base + TIMER1_MATCH1);
> +	if (fttmr010->count_down) {
> +		/*
> +		 * ASPEED Timer Controller will load TIMER1_LOAD register
> +		 * into TIMER1_COUNT register when the timer is re-enabled.
> +		 */
> +		writel(cycles, fttmr010->base + TIMER1_LOAD);
> +	} else {
> +		/* Setup the match register forward in time */
> +		cr = readl(fttmr010->base + TIMER1_COUNT);
> +		writel(cr + cycles, fttmr010->base + TIMER1_MATCH1);
> +	}
>  
>  	/* Start */
>  	cr = readl(fttmr010->base + TIMER_CR);
> 


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