linux-kernel - Re: [PATCH v8 4/6] clocksource: stm32: only use 32 bits timers

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Message-ID: <2afa343b-3a23-3352-33b3-2133678a4122@linaro.org>
Date:   Thu, 7 Dec 2017 17:49:55 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Benjamin Gaignard <benjamin.gaignard@...aro.org>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ludovic Barre <ludovic.barre@...com>,
        Julien Thierry <julien.thierry@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Arnd Bergmann <arnd@...db.de>, devicetree@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 4/6] clocksource: stm32: only use 32 bits timers

On 07/12/2017 17:33, Benjamin Gaignard wrote:
> 2017-12-07 16:27 GMT+01:00 Daniel Lezcano <daniel.lezcano@...aro.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@...aro.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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