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

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Date:   Fri, 27 Sep 2019 12:44:55 +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 2:41 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.

>
>         M.