[PATCH] arm64: dts: Add idle-states for Juno

[Liviu Dudau]

On Fri, May 01, 2015 at 12:55:07PM +0100, Leo Yan wrote:
> On Fri, May 01, 2015 at 11:45:06AM +0100, Lorenzo Pieralisi wrote:
> > On Fri, May 01, 2015 at 11:22:31AM +0100, Jon Medhurst (Tixy) wrote:
> > > On Fri, 2015-05-01 at 09:55 +0800, Leo Yan wrote:
> > > [...]
> > > > >  arch/arm64/boot/dts/arm/juno.dts | 28 ++++++++++++++++++++++++++++
> > > > >  1 file changed, 28 insertions(+)
> > > > > 
> > > > > diff --git a/arch/arm64/boot/dts/arm/juno.dts b/arch/arm64/boot/dts/arm/juno.dts
> > > > > index 133ee59..7a9a449 100644
> > > > > --- a/arch/arm64/boot/dts/arm/juno.dts
> > > > > +++ b/arch/arm64/boot/dts/arm/juno.dts
> > > > > @@ -34,12 +34,35 @@
> > > > >  		#address-cells = <2>;
> > > > >  		#size-cells = <0>;
> > > > >  
> > > > > +		idle-states {
> > > > > +			entry-method = "arm,psci";
> > > > > +
> > > > > +			CPU_SLEEP_0: cpu-sleep-0 {
> > > > > +				compatible = "arm,idle-state";
> > > > > +				arm,psci-suspend-param = <0x0010000>;
> > > > > +				local-timer-stop;
> > > > 
> > > > Just want to figure out the best way for big.LITTLE system; so have
> > > > one question: CA53 and CA57 have different power domain for arch
> > > > timer, right?
> > > 
> > > I'm not sure of the answer to that. The documentation I have does seem
> > > to state the timer is lost on cluster power down, which would imply that
> > > it's not when just powering down a cpu, but I'm not at all clear on the
> > > matter.
> > > 
> > > >  If this is the case, should we define two kinds of cpu
> > > > sleep states, one of them will not migrate to broadcast timer and
> > > > keep using arch timer after cpu has been powered down?
> > > 
> > > Do you mean that if the local timer is not lost (and so we should not
> > > have local-timer-stop above), then we should have another identical idle
> > > state except that it _does_ specify local-timer-stop to force the
> > > broadcast time to be used? If so, would that second state ever be more
> > > power efficient than the first? (I don't know the answer to that, this
> > > whole area is pretty new to me).
> > 
> > Ok, to start with, if we want the generic CPUidle driver to work on bL
> > systems, we should get this patch merged:
> > 
> > http://www.spinics.net/lists/arm-kernel/msg412790.html
> 
> Thanks pointing out this.
> 
> > Architected timer is always lost on power down, unless we are talking
> > about a logic retention state (or if we are talking about local timers
> > that are not the architected ones). Anyway, with the patch above, different
> > cpus can have different idle states, so different behaviours when it
> > comes to the local timer behaviour. If, say, on a A53/A57 system, A53
> > cpus lose the local timer on idle state entry and A57 cpus do not
> > (that's a non-existing example though), those cpus will have different
> > idle states so different local-timer-stop flags.
> 
> For A57, not sure if arch timer will be powered off if cpu has been
> powered off.
> 
> From A53's TRM figure 2-1 Cortex-A53 processor block diagram, the arch
> timer is not within CPU power domain, so likey arch timer still will
> work even cpu has been powered off (PDCPU/PDADVSIMD both have been off).
> This looks like is not aligning w/t your upper description. Could u
> confirm this understanding is right?

Not quite. Behind the architected timers there is a system shared counter
that is used as a basis for the local CPU's counter. So when a CPU is being
powered off the system counter still counts and the CPU's copy gets updated
on power on by the firmware (hopefully).

Best regards,
Liviu

> 
> Thanks,
> Leo Yan
> 

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