Re: [PATCH 2/2] ARM: vf610: Use ARM Global Timer as clocksource

Shawn Guo
2014-09-25T15:11:05Z

On Thu, Sep 25, 2014 at 10:25:13AM +0200, Stefan Agner wrote:
> Am 2014-09-25 09:50, schrieb Shawn Guo:
> > On Thu, Sep 11, 2014 at 02:06:15PM +0200, Stefan Agner wrote:
> >> Use ARM Global Timer as clocksource instead of the PIT timer. This
> >> leaves the PIT timer for other users e.g. the secondary Cortex-M4
> >> core. Also, the Global Timer has double the precission (running at
> >> pheripheral clock compared to IPG clock) and a 64-bit incrementing
> >> counter register.
> >
> > I just think of one thing. Will this change cause a problem of the low
> > power idle support in case we want to power down ARM core in there?
> >
>
> I'm not sure what really happend to the Global Timer when we power down
> the ARM core. We use a clocksoure of different power domain now, so it
> might make a difference in low power modes, but I think it will improve
> things: The PIT timer's clock currently have been clock gated even in
> STOP mode, which does not power down the ARM core. And it would be shut
> down completely in LP-Mode 1-3 which since PIT is part of the big power
> domain 1.
>
> But AFAIK, its not required that the clocksource is running while in low
> power modes. The time should just not jump, and if the timers registers
> are lost during suspend, a proper suspend/resume support need to be
> implemented.

Sorry, I should be more specific in the first place. What I'm concerned
is more about clockevent than clocksource. If some day we have a
cpuidle driver for vf610, which powers off ARM core in a deep C-state,
the clockevent device will be gone as long as system enters the C-state,
and no timer interrupt can wake up the core from idle state.

Shawn