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

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Message-ID: <61dda183-cd53-153b-8ed1-e2ecb8f96e03@st.com>
Date:   Mon, 14 Oct 2019 09:31:14 +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 2:59 PM, Marc Zyngier wrote:
> On 2019-09-27 13:44, Benjamin GAIGNARD wrote:
>> On 9/27/19 2:41 PM, Marc Zyngier wrote:
>>> On 2019-09-27 13:36, Benjamin GAIGNARD wrote:
>>>> 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.
>>>
>>> And you don't have any form of power management either? What happens 
>>> when
>>> your CPU goes into idle? If your system does any form of power 
>>> management
>>> *and* doesn't have a separate timer, it is remarkably broken.
>>
>> Even in low-power modes this timer is always powered and clocked so it
>> is working fine.
>
> You're missing the point again. It is not about the clock, but the 
> comparator
> that is internal to the CPU, and not functional when the CPU is in its 
> lowest
> power mode. See also the verbiage in [1] (44.3 STGEN functional 
> description),
> which indicates that the clock source actually dies in low-power mode 
> (going
> against the architecture which mandates it to be always-on).
>
> Also, coming back to your earlier assertion ("This SoC doesn't have 
> any other
> global timer"): The documentation at [1] shows at least 17 timers that 
> could
> be used and avoid this dirty hack.
>
> So for what it is worth, NAK to this patch.


Hi Marc,

I have listen your remarks and propose another way to solve this issue:

https://lkml.org/lkml/2019/10/9/690

Thanks,

Benjamin


>
>         M.
>
> [1] https://www.st.com/resource/en/reference_manual/dm00366355.pdf