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

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

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.
> 
> 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.  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.

(1) enter_freeze() hooks are not strictly necessary to enable
    suspend-to-idle (they are if we want the tick to be frozen
    on suspend-to-idle, which is different)
(2) If I understand your code correctly you have to set the suspend
    ops hook to make sure suspend-to-idle is enabled. This is a core
    code issue rather than anything else, given that suspend-to-idle
    (hey it is based on CPUidle !) does NOT rely on suspend ops to
    function.

So the gist is: as far as I am concerned you do not need any of this
code to enable (yes you need PSCI idle states but no
qcom,idle-state-spc compatible string whatsoever) suspend-to-idle
on ARM64 on top of PSCI, let me know what I am missing.

Thanks,
Lorenzo