linux-kernel - Re: [PATCH] ARM: dts: stm32: Enable high resolution timer

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Message-ID: <341949c8-7864-5d65-2797-988022724a4c@st.com>
Date:   Fri, 27 Sep 2019 12:36:20 +0000
From:   Benjamin GAIGNARD <benjamin.gaignard@...com>
To:     Marc Zyngier <maz@...nel.org>
CC:     Alexandre TORGUE <alexandre.torgue@...com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: dts: stm32: Enable high resolution timer


On 9/27/19 1:22 PM, Marc Zyngier wrote:
> On 2019-09-27 09:48, Benjamin Gaignard wrote:
>> Adding always-on makes arm arch_timer claim to be an high resolution 
>> timer.
>> That is possible because power mode won't stop clocking the timer.
>
> The "always-on" is not about the clock. It is about the comparator.
> The clock itself is *guaranteed* to always tick. If it didn't, that'd be
> an integration bug, and a pretty bad one.
>
> What you're claiming here is that your CPU never enters a low-power mode?
> Ever? I find this very hard to believe.
>
> Furthermore, claiming that always-on is the way to force the arch-timer
> to be an hrtimer is factually wrong. This is what happens *if* this is
> the only timer in the system. The only case this is true is for virtual
> machines. Anything else has a global timer somewhere that will allow
> the arch timers to be used as an hrtimer.
>
> I'm pretty sure you too have a global timer somewhere in your system.
> Enable it, and enjoy hrtimers without having to lie about the properties
> of your system! ;-)

Hi Marc,

This SoC doesn't have any other global timer. Use arch_time is the only
we have to provide hrtimer on this system.

Benjamin

>
>         M.
>
>>
>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...com>
>> ---
>>  arch/arm/boot/dts/stm32mp157c.dtsi | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/arm/boot/dts/stm32mp157c.dtsi
>> b/arch/arm/boot/dts/stm32mp157c.dtsi
>> index 9b11654a0a39..74f64745d60d 100644
>> --- a/arch/arm/boot/dts/stm32mp157c.dtsi
>> +++ b/arch/arm/boot/dts/stm32mp157c.dtsi
>> @@ -50,6 +50,7 @@
>>                   <GIC_PPI 11 (GIC_CPU_MASK_SIMPLE(4) | 
>> IRQ_TYPE_LEVEL_LOW)>,
>>                   <GIC_PPI 10 (GIC_CPU_MASK_SIMPLE(4) | 
>> IRQ_TYPE_LEVEL_LOW)>;
>>          interrupt-parent = <&intc>;
>> +        always-on;
>>      };
>>
>>      clocks {
>