linux-kernel - Re: [PATCH] clocksource: arch_timer: Mark ARMv8 system timer as 'always-on'

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Message-ID: <CAOMFOmWbuaZ4kjPTojfF_h0gF-eh2iEzE44OKEfSQYRwAUrg2A@mail.gmail.com>
Date:	Thu, 6 Nov 2014 10:14:49 -0800
From:	Anatol Pomozov <anatol.pomozov@...il.com>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	daniel.lezcano@...aro.org, lorenzo.pieralisi@....com,
	tglx@...utronix.de, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] clocksource: arch_timer: Mark ARMv8 system timer as 'always-on'

Hi

On Thu, Nov 6, 2014 at 10:09 AM, Marc Zyngier <marc.zyngier@....com> wrote:
> Hi Anatol,
>
> On 2014-11-06 17:57, Anatol Pomozov wrote:
>>
>> Quoting ARMv8 Reference Manual section D6.1:
>> "The system counter must be implemented in an always-on power domain."
>
>
> Yes. And the key words here are "system counter". That's the global counter,
> not the per-cpu view of that, and critically nor comparators.
>
> The timers (per-cpu counter+comparator) will happily go down with the CPU,
> and won't be able to wake it up. Exactly like ARMv7, by the way.
>
>> There is no need to keep 'always-on' configurable on ARMv8. We ignore
>> this dts property value and unconditionally set it to true.
>>
>> The issue was discovered while working on ARMv8 board with only one timer
>> (arm arch timer). If 'always-on' is false then it disables high-resolution
>> timer functionality.
>
>
> Yup, and that's by design (see above). The *only* case where it is really
> "always-on" is when a virtual machine is running (if the vcpu is running
> then the physical CPU is, and so is the timer).
>
> If your system really never does any form of PM, then fine, just put the DT
> property in. Removing it is, I'm sorry to say, a bit short sighted.

Thanks for the clarification Marc. I will look if it possible to get
the other (always-on) timer back.

Ignore my patch.
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