[PATCH 3/5] arm64: dts: msm8916: Add spc compat tag

[mark.rutland@arm.com (Mark Rutland)]

On Fri, Jun 10, 2016 at 11:47:21AM -0500, Andy Gross wrote:
> On Fri, Jun 10, 2016 at 05:25:55PM +0100, Mark Rutland wrote:
> > On Fri, Jun 10, 2016 at 11:12:34AM -0500, Andy Gross wrote:
> > > On Fri, Jun 10, 2016 at 04:48:57PM +0100, Mark Rutland wrote:
> > > > [+ Lorenzo]
> > > > 
> > > > On Thu, May 19, 2016 at 12:00:18AM -0500, Andy Gross wrote:
> > > > > This patch adds the qcom,idle-state-spc compatible to the SPC idle
> > > > > state.  This compatible indicates that the state is one which supports
> > > > > freeze.
> > > > > 
> > > > > Signed-off-by: Andy Gross <andy.gross@linaro.org>
> > > > > ---
> > > > >  arch/arm64/boot/dts/qcom/msm8916.dtsi | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/arch/arm64/boot/dts/qcom/msm8916.dtsi b/arch/arm64/boot/dts/qcom/msm8916.dtsi
> > > > > index 208af00..032e411 100644
> > > > > --- a/arch/arm64/boot/dts/qcom/msm8916.dtsi
> > > > > +++ b/arch/arm64/boot/dts/qcom/msm8916.dtsi
> > > > > @@ -104,7 +104,7 @@
> > > > >  
> > > > >  		idle-states {
> > > > >  			CPU_SPC: spc {
> > > > > -				compatible = "arm,idle-state";
> > > > > +				compatible = "qcom,idle-state-spc", "arm,idle-state";
> > > > >  				arm,psci-suspend-param = <0x40000002>;
> > > > >  				entry-latency-us = <130>;
> > > > >  				exit-latency-us = <150>;
> > > > 
> > > > This looks suspicious.
> > > > 
> > > > This is a PSCI idle state, and we have a PSCI driver driven by the
> > > > generic ARM cpuidle driver.
> > > > 
> > > > Why do we need a qcom-specific compatible here?
> > > > 
> > > > Surely we should be able to use the idle code in a generic fashion to
> > > > driver suspend-to-idle?
> > > 
> > > We need a way to identify specific idle states that support suspend-to-idle.  In
> > > addition, when we have identified the states, we may have to configure the
> > > enter_freeze() function.
> > 
> > Could you elaborate on what you mean by a state supporting
> > suspend-to-idle? It was my understanding that any idle state should
> > function for suspend-to-idle (and the choice of state is potentially
> > subjective).
> 
> when you freeze the system, cpuidle will try to find the deepest state which
> supports freeze (by checking if a enter_freeze() exists).  If it does exist,
> then the tick is frozen and the enter_freeze is called as each cpu goes idle.

Per Lorenzo's replies in another thread, it sounds like this is a
generic issue, and not specific to Qualcomm. My understanding is that
the only issue is coupled idle states, and further, that issue is really
an implementation detail within Linux w.r.t. IRQ management.

So it sounds like we need to rework things to be robust in the case of
coupled idle states, and we can wire up enter_freeze for all states
generically.

If there is some peroblem with making things robust, I assume we can
identify coupled idle states today in some generic manner. I don't
currently see the need for any DT binding.

Lorenzo, does the above make sense to you?

> > > I chose to do this outside of the arm cpuidle driver because I didn't want to
> > > add any more DT information aside from the compatible, and I needed a separate
> > > place for the Qualcomm specific suspend code. 
> > 
> > Which suspend code is Qualcomm-specific? There shouldn't be anything on
> > the PSCI side, so I can only imagine that device management is left.
> > What am I missing?
> 
> Qualcomm won't be supporting the psci suspend feature and will be doing their
> own thing.  As such, they need their own suspend ops.  In addition, we have
> platforms that don't use psci (32 bit).

That's orthogonal to suspend-to-idle, which can be handled in a generic
fashion. It's also orthogonal to 32-bit !PSCI platforms, as my
complaint was regarding the use of this with PSCI.

> > > With the compatible, this makes
> > > my 32 and 64 bit processor suspend code identical, as we have our own cpuidle
> > > driver for the 32 bit procs.
> > > 
> > > An alternative would be to add some facilities to communicate this to the arm
> > > cpuidle driver and configure the enter_freeze() function at some later point.
> > 
> > I would prefer that suspend-to-idle using PSCI occurred via the generic
> > PSCI and cpuidle code. I am happy to extend that code if there is
> > something lacking, and I would prefer to not have platform-specific
> > hooks.
> 
> Right, I briefly looked at that in the beginning and decided against it.  But if
> we can at least add some way to identify freezeable idle states and configure
> the freeze function, that'd cover all the requirements.

Ok, per the above I believe that is covered.

Thanks,
Mark.