Re: [PATCH 14/18] drivers: firmware: psci: Manage runtime PM in the idle path for CPUs

[Lorenzo Pieralisi]

On Thu, Jul 18, 2019 at 11:49:11PM +0200, Ulf Hansson wrote:
> On Thu, 18 Jul 2019 at 19:41, Lina Iyer <ilina@xxxxxxxxxxxxxx> wrote:
> >
> > On Thu, Jul 18 2019 at 10:55 -0600, Ulf Hansson wrote:
> > >On Thu, 18 Jul 2019 at 15:31, Lorenzo Pieralisi
> > ><lorenzo.pieralisi@xxxxxxx> wrote:
> > >>
> > >> On Thu, Jul 18, 2019 at 12:35:07PM +0200, Ulf Hansson wrote:
> > >> > On Tue, 16 Jul 2019 at 17:53, Lorenzo Pieralisi
> > >> > <lorenzo.pieralisi@xxxxxxx> wrote:
> > >> > >
> > >> > > On Mon, May 13, 2019 at 09:22:56PM +0200, Ulf Hansson wrote:
> > >> > > > When the hierarchical CPU topology layout is used in DT, let's allow the
> > >> > > > CPU to be power managed through its PM domain, via deploying runtime PM
> > >> > > > support.
> > >> > > >
> > >> > > > To know for which idle states runtime PM reference counting is needed,
> > >> > > > let's store the index of deepest idle state for the CPU, in a per CPU
> > >> > > > variable. This allows psci_cpu_suspend_enter() to compare this index with
> > >> > > > the requested idle state index and then act accordingly.
> > >> > >
> > >> > > I do not see why a system with two CPU CPUidle states, say CPU retention
> > >> > > and CPU shutdown, should not be calling runtime PM on CPU retention
> > >> > > entry.
> > >> >
> > >> > If the CPU idle governor did select the CPU retention for the CPU, it
> > >> > was probably because the target residency for the CPU shutdown state
> > >> > could not be met.
> > >>
> > >> The kernel does not know what those cpu states represent, so, this is an
> > >> assumption you are making and it must be made clear that this code works
> > >> as long as your assumption is valid.
> > >>
> > >> If eg a "cluster" retention state has lower target_residency than
> > >> the deepest CPU idle state this assumption is wrong.
> > >
> > >Good point, you are right. I try to find a place to document this assumption.
> > >
> > >>
> > >> And CPUidle and genPD governor decisions are not synced anyway so,
> > >> again, this is an assumption, not a certainty.
> > >>
> > >> > In this case, there is no point in allowing any other deeper idle
> > >> > states for cluster/package/system, since those have even greater
> > >> > residencies, hence calling runtime PM doesn't make sense.
> > >>
> > >> On the systems you are testing on.
> > >
> > >So what you are saying typically means, that if all CPUs in the same
> > >cluster have entered the CPU retention state, on some system the
> > >cluster may also put into a cluster retention state (assuming the
> > >target residency is met)?
> > >
> > >Do you know of any systems that has these characteristics?
> > >
> > Many QCOM SoCs can do that. But with the hardware improving, the
> > power-performance benefits skew the results in favor of powering off
> > the cluster than keeping the CPU and cluster in retention.
> >
> > Kevin H and I thought of this problem earlier on. But that is a second
> > level problem to solve and definitely to be thought of after we have the
> > support for the deepest states in the kernel. We left that out for a
> > later date. The idea would have been to setup the allowable state(s) in
> > the DT for CPU and cluster state definitions and have the genpd take
> > that into consideration when deciding the idle state for the domain.
> 
> Thanks for confirming.
> 
> This more or less means we need to improve the hierarchical support in
> genpd to support more levels, such that it makes sense to have a genpd
> governor assigned at more than one level. This doesn't work well
> today. As I also have stated, this is on my todo list for genpd.
> 
> However, I also agree with your standpoint, that let's start simple to
> enable the deepest state as a start with, then we can improve things
> on top.

How to solve this in the kernel I don't know but please do make sure
that the DT bindings allow you to describe what's needed, once they are
merged you won't be able to change them and I won't bodge the code to
make things fit, so if anything let's focus on getting them right as a
matter of priority to get this done please.

Thanks,
Lorenzo