linux-kernel - Re: [PATCH 2/4] clocksource: stm32: use prescaler to adjust the resolution

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Date:   Mon, 18 Dec 2017 10:26:10 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        mark.rutland@....com, linux@...linux.org.uk,
        mcoquelin.stm32@...il.com, alexandre.torgue@...com,
        tglx@...utronix.de
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Benjamin Gaignard <benjamin.gaignard@...com>
Subject: Re: [PATCH 2/4] clocksource: stm32: use prescaler to adjust the
 resolution

On 15/12/2017 09:52, Benjamin Gaignard wrote:
> Rather than use fixed prescaler values compute it to get a clock
> as close as possible of 10KHz and a resolution of 0.1ms.
> 
> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...com>
> ---
>  drivers/clocksource/timer-stm32.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/clocksource/timer-stm32.c b/drivers/clocksource/timer-stm32.c
> index 23a321cca45b..de721d318065 100644
> --- a/drivers/clocksource/timer-stm32.c
> +++ b/drivers/clocksource/timer-stm32.c
> @@ -37,6 +37,11 @@
>  
>  #define TIM_EGR_UG	BIT(0)
>  
> +#define MAX_TIM_PSC	0xFFFF
> +
> +/* Target a 10KHz clock to get a resolution of 0.1 ms */
> +#define TARGETED_CLK_RATE 10000
> +
>  static int stm32_clock_event_shutdown(struct clock_event_device *evt)
>  {
>  	struct timer_of *to = to_timer_of(evt);
> @@ -83,7 +88,7 @@ static irqreturn_t stm32_clock_event_handler(int irq, void *dev_id)
>  static void __init stm32_clockevent_init(struct timer_of *to)
>  {
>  	unsigned long max_delta;
> -	int prescaler;
> +	unsigned long prescaler;
>  
>  	to->clkevt.name = "stm32_clockevent";
>  	to->clkevt.features = CLOCK_EVT_FEAT_PERIODIC;
> @@ -96,13 +101,17 @@ static void __init stm32_clockevent_init(struct timer_of *to)
>  	/* Detect whether the timer is 16 or 32 bits */
>  	writel_relaxed(~0U, timer_of_base(to) + TIM_ARR);
>  	max_delta = readl_relaxed(timer_of_base(to) + TIM_ARR);
> -	if (max_delta == ~0U) {
> -		prescaler = 1;
> +	to->clkevt.rating = 50;
> +	if (max_delta == ~0U)
>  		to->clkevt.rating = 250;
> -	} else {
> -		prescaler = 1024;
> -		to->clkevt.rating = 50;
> -	}
> +
> +	/*
> +	 * Get the highest possible prescaler value to be as close
> +	 * as possible of TARGETED_CLK_RATE
> +	 */
> +	prescaler = DIV_ROUND_CLOSEST(timer_of_rate(to), TARGETED_CLK_RATE);

With a 90MHz or 125MHz, the prescaler will be 9000 or 12500, so much
more than the 1024 we have today for 16b, and 1 for 32b.

Shouldn't the computation be weighted with the bits width ?

Otherwise the timer will wrap like:

32bits:

before:	(2^32 / 90e6) x 1 = 47.72 seconds
after:	(2^32 / 90e6) x 9000 = 119.3 *hours* ~= 5days

16bits:

before:	(2^16 / 90e6) x 1024 = 0.745 seconds
after:	(2^16 / 90e6) x 9000 = 6.55 seconds

The patch is ok to target the 10KHz timer rate for 16b with a 1ms
resolution wrapping up after 6.55 seconds. But not for the 32bits timer.
Furthermore, we can't tell anymore the 32bits timers have a rating of
250 after this patch.

Leave the 32bits part as it is and compute the prescaler only in case of
16bits with the target rate, which sounds a reasonable approach.

> +	if (prescaler > MAX_TIM_PSC)
> +		prescaler = MAX_TIM_PSC;

That can happen only if the clock rate is greater than ~655MHz, that
could not happen today as far as I can tell regarding the DT. So if we
hit this condition, we should speak up in the log (pr_warn).

>  	writel_relaxed(0, timer_of_base(to) + TIM_ARR);
>  	writel_relaxed(prescaler - 1, timer_of_base(to) + TIM_PSC);

Can you fix this prescaler - 1 in order to be consistent with the
computation with 16b ? (32b prescaler = 0, 16b prescaler = clk_rate /
target ).

Thanks.

  -- Daniel

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