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SubjectRe: [PATCH v8 4/6] clocksource: stm32: only use 32 bits timers
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On 07/12/2017 17:33, Benjamin Gaignard wrote:
> 2017-12-07 16:27 GMT+01:00 Daniel Lezcano <daniel.lezcano@linaro.org>:
>> On 14/11/2017 09:52, Benjamin Gaignard wrote:
>>> The clock driving counters is at 90MHz so the maximum period
>>> for 16 bis counters is around 750 ms which is a short period
>>> for a clocksource.
>>
>> Isn't it 728us ?
>
> yes it is: 2^16 / 90.000.000 => 728us

Ok, now I can do the connection with the previous patch.

So, the real issue of all this is the 16bits clocksource is wrapping up
every 728us, hence the clockevent periodically expires every ~728us to
keep the timekeeping consistent. Unfortunately, the kernel has a too
high overhead for this as the system is consistently processing this
timer leading to a CPU time resource starvation.

Is that correct ?


>>> For 32 bits counters this period is close
>>> 47 secondes which is more acceptable.
>>>
>>> This patch remove 16 bits counters support and makes sure that
>>> they won't be probed anymore.
>>>
>>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
>>> ---
>>> drivers/clocksource/timer-stm32.c | 26 ++++++++++++--------------
>>> 1 file changed, 12 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/drivers/clocksource/timer-stm32.c b/drivers/clocksource/timer-stm32.c
>>> index ae41a19..8173bcf 100644
>>> --- a/drivers/clocksource/timer-stm32.c
>>> +++ b/drivers/clocksource/timer-stm32.c
>>> @@ -83,9 +83,9 @@ static irqreturn_t stm32_clock_event_handler(int irq, void *dev_id)
>>> static int __init stm32_clockevent_init(struct device_node *node)
>>> {
>>> struct reset_control *rstc;
>>> - unsigned long max_delta;
>>> - int ret, bits, prescaler = 1;
>>> + unsigned long max_arr;
>>> struct timer_of *to;
>>> + int ret;
>>>
>>> to = kzalloc(sizeof(*to), GFP_KERNEL);
>>> if (!to)
>>> @@ -115,29 +115,27 @@ static int __init stm32_clockevent_init(struct device_node *node)
>>>
>>> /* 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;
>>> - bits = 32;
>>> - } else {
>>> - prescaler = 1024;
>>> - bits = 16;
>>> + max_arr = readl_relaxed(timer_of_base(to) + TIM_ARR);
>>> + if (max_arr != ~0U) {
>>> + pr_err("32 bits timer is needed\n");
>>> + ret = -EINVAL;
>>> + goto deinit;
>>> }
>>> +
>>> writel_relaxed(0, timer_of_base(to) + TIM_ARR);
>>>
>>> - writel_relaxed(prescaler - 1, timer_of_base(to) + TIM_PSC);
>>> + writel_relaxed(0, timer_of_base(to) + TIM_PSC);
>>> writel_relaxed(TIM_EGR_UG, timer_of_base(to) + TIM_EGR);
>>> writel_relaxed(TIM_DIER_UIE, timer_of_base(to) + TIM_DIER);
>>> writel_relaxed(0, timer_of_base(to) + TIM_SR);
>>>
>>> clockevents_config_and_register(&to->clkevt,
>>> - timer_of_period(to), MIN_DELTA, max_delta);
>>> -
>>> - pr_info("%pOF: STM32 clockevent driver initialized (%d bits)\n",
>>> - node, bits);
>>> + timer_of_period(to), MIN_DELTA, ~0U);
>>>
>>> return 0;
>>>
>>> +deinit:
>>> + timer_of_exit(to);
>>> err:
>>> kfree(to);
>>> return ret;
>>>
>>
>>
>> --
>> <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs
>>
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>>
>
>
>


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