Re: [PATCH 2/2] arm64: dts: mt8173: add timer node

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Sep 17, 2015 at 03:56:56PM +0100, Yingjoe Chen wrote:
> On Thu, 2015-09-17 at 14:51 +0100, Sudeep Holla wrote:
> > 
> > On 16/09/15 03:04, Yingjoe Chen wrote:
> > > From: Daniel Kurtz <djkurtz@chromium.org>
> > >
> > > Add device node to enable GPT timer. This timer will be
> > > used as sched clock source.
> > >
> > 
> > Interesting any known issues with or advantage over the arch timers
> > to prefer it as sched clock source. I see even arch timers are present
> > in DT, hence the question. Or is it just a incorrect commit log ?
> > 
> > How does this get selected as sched clock source ? I don't see
> > sched_clock_register in mtk_timer.c
> > 
> > To be clear, I am not against adding this timer support, but just want
> > to know is it preferred for sched clock source ? if yes why ? better
> > resolution ?
> 
> Hi Sudeep,
> 
> Thanks for your review.
> 
> I hit the send too soon and missed cover letter, please see:
> http://lists.infradead.org/pipermail/linux-mediatek/2015-September/002303.html
> 
> The main reason to use GPT as sched clock is it won't stop during idle.

You don't mean sched clock, you just mean a clock_event_device.

A sched_clock is a high-precision clocksource that is read from (which
by definition requires the CPUs to be non-idle). It doesn't have
anything to do with interrupts and therefore cannot wake devices from
idle.

While the clock_event_device for the generic timer can't necessarily
wake CPUs from idle. The generic timer system counter counts even if
CPUs are idle, so the generic timer is fine as a sched_clock. 

Thanks,
Mark.