linux-kernel - Re: [PATCH v2] clocksource: atmel-pit: register as a sched

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Date:	Wed, 24 Feb 2016 17:44:59 +0100
From:	Romain Izard <romain.izard.pro@...il.com>
To:	Sylvain Rochet <sylvain.rochet@...secur.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org,
	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] clocksource: atmel-pit: register as a sched_clock

2016-02-24 17:20 GMT+01:00 Sylvain Rochet <sylvain.rochet@...secur.com>:
> Hi,
>
> On Wed, Feb 24, 2016 at 05:04:42PM +0100, Romain Izard wrote:
>> Register the counter of the Periodic Interval Timer as a possible
>> source for sched_clock. Keep the timer running even if the related
>> clockevent is disabled.
>>
>> This provides a better precision than the jiffies-based default. The
>> TCB clocksource does not work, as it is registered too late in the
>> initialization sequence.
>
> I have mixed feelings about that, but this is probably just a
> misunderstanding from my part.
>
My goal is to get a better precision on my printk and trace logs,
because the precision of jiffies is really very bad compared to
everything else I have encountered until now. But it looks like reading
a timer is quite complicated on AT91...

> The PIT timer should not be used for systems with PM_SUSPEND enabled
> and used because it takes ages to synchronize on resume, how does this
> patch affect that ?
>
> Ref: commit ac34ad27fc ("clockevents: Do not suspend/resume if
> unused")

So your advice would be to stay clear of the PIT, because it is
(globally) useless.

I'll keep it in mind.

Best regards,
-- 
Romain Izard