Re: [PATCH v2 4/4] cpuidle: psci: Allow WFI to be the only state for the hierarchical topology

From: Sudeep Holla <sudeep.holla@arm.com>
To: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Lorenzo Pieralisi <Lorenzo.Pieralisi@arm.com>,
	Benjamin Gaignard <benjamin.gaignard@st.com>,
	Linux PM <linux-pm@vger.kernel.org>,
	Stephen Boyd <sboyd@kernel.org>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Lina Iyer <ilina@codeaurora.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v2 4/4] cpuidle: psci: Allow WFI to be the only state for the hierarchical topology
Date: Fri, 6 Mar 2020 10:04:31 +0000	[thread overview]
Message-ID: <20200306100431.GA16541@bogus> (raw)
In-Reply-To: <CAPDyKFogjPG+mRsfPaxN7RjB7TQL9=qHNzA=K_t0F6M6Q9-TuA@mail.gmail.com>

On Fri, Mar 06, 2020 at 10:28:10AM +0100, Ulf Hansson wrote:
> On Thu, 5 Mar 2020 at 17:23, Sudeep Holla <sudeep.holla@arm.com> wrote:
> >

[...]

> > OK. The only state that cluster can enter when CPUs are in WFI are
> > cluster WFI and most hardware can handle it automatically. I don't see
> > the need to do any extra work for that.
>
> This isn't about cluster WFI, but about deeper cluster states, such as
> a cluster-clock-gated-state and a cluster-power-off-state. It's an ST
> platform, which Benjamin is working on.
>

Then definitely something is completely wrong. You can't enter deeper
cluster states(clock-gated and power-off to be specific) with CPU in
just WFI state. So, if the attempt here is to enter those states, I
disagree with the change.

Benjamin, please share the complete hierarchical topology for your platform.

> >
> > > Then, after we have called pm_runtime_put_sync_suspend() a few lines
> > > above, we may potentially have a "domain state" to use, instead of the
> > > WFI state.
> > >
> >
> > Are they any platforms with this potential "domain state" to use with
> > CPU WFI. I want to understand this better.
> >
> > > In this case, if we would have called psci_enter_state(), that would
> > > lead us to calling cpu_do_idle() from the __CPU_PM_CPU_IDLE_ENTER()
> > > macro, becuase idx is zero. In other words, the domain state would
> > > become unused.
> > >
> >
> > For a domain state to become unused with WFI, it needs to be available
> > and I am not 100% sure of that.
>
> With these changes from the series, we can fully conform to the
> hierarchical DT bindings for PSCI.
>

Theoretically may be, but may not confirm to the hardware states.

> I am not sure I understand your concern, is there a cost involved by
> applying this?
>

Yes as mentioned above.

--
Regards,
Sudeep

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