linux-kernel - Re: [PATCH v5 1/3] cpuidle: psci: Call cpu_cluster_pm

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Date:   Thu, 17 Feb 2022 15:31:32 +0800
From:   Shawn Guo <shawn.guo@...aro.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Maulik Shah <quic_mkshah@...cinc.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/3] cpuidle: psci: Call cpu_cluster_pm_enter() on the
 last CPU

On Wed, Feb 16, 2022 at 03:58:41PM +0000, Marc Zyngier wrote:
> On 2022-02-16 14:49, Sudeep Holla wrote:
> > +Ulf (as you he is the author of cpuidle-psci-domains.c and can help you
> > with that if you require)

Thanks, Sudeep!

> > 
> > On Wed, Feb 16, 2022 at 09:28:28PM +0800, Shawn Guo wrote:
> > > Make a call to cpu_cluster_pm_enter() on the last CPU going to low
> > > power
> > > state (and cpu_cluster_pm_exit() on the firt CPU coming back), so that
> > > platforms can be notified to set up hardware for getting into the
> > > cluster
> > > low power state.
> > > 
> > 
> > NACK. We are not getting the notion of CPU cluster back to cpuidle
> > again.
> > That must die. Remember the cluster doesn't map to idle states
> > especially
> > in the DSU systems where HMP CPUs are in the same cluster but can be in
> > different power domains.

The 'cluster' in cpu_cluster_pm_enter() doesn't necessarily means
a physical CPU cluster.  I think the documentation of the function has a
better description.

 * Notifies listeners that all cpus in a power domain are entering a low power
 * state that may cause some blocks in the same power domain to reset.

So cpu_domain_pm_enter() might be a better name?  Anyways ...

> > 
> > You need to decide which PSCI CPU_SUSPEND mode you want to use first. If
> > it is
> > Platform Co-ordinated(PC), then you need not notify anything to the
> > platform.
> > Just request the desired idle state on each CPU and platform will take
> > care
> > from there.
> > 
> > If for whatever reason you have chosen OS initiated mode(OSI), then
> > specify
> > the PSCI power domains correctly in the DT which will make use of the
> > cpuidle-psci-domains and handle the so called "cluster" state correctly.

Yes, I'm running a Qualcomm platform that has OSI supported in PSCI.

> 
> My understanding is that what Shawn is after is a way to detect the "last
> man standing" on the system to kick off some funky wake-up controller that
> really should be handled by the power controller (because only that guy
> knows for sure who is the last CPU on the block).
> 
> There was previously some really funky stuff (copy pasted from the existing
> rpmh_rsc_cpu_pm_callback()), which I totally objected to having hidden in
> an irqchip driver.
> 
> My ask was that if we needed such information, and assuming that it is
> possible to obtain it in a reliable way, this should come from the core
> code, and not be invented by random drivers.

Thanks Marc for explain my problem!

Right, all I need is a notification in MPM irqchip driver when the CPU
domain/cluster is about to enter low power state.  As cpu_pm -
kernel/cpu_pm.c, already has helper cpu_cluster_pm_enter() sending
CPU_CLUSTER_PM_ENTER event, I just need to find a caller to this cpu_pm
helper.  

Is .power_off hook of generic_pm_domain a better place for calling the
helper?

Shawn

----8<------------
diff --git a/drivers/cpuidle/cpuidle-psci-domain.c b/drivers/cpuidle/cpuidle-psci-domain.c
index ff2c3f8e4668..58aad15851f9 100644
--- a/drivers/cpuidle/cpuidle-psci-domain.c
+++ b/drivers/cpuidle/cpuidle-psci-domain.c
@@ -10,6 +10,7 @@
 #define pr_fmt(fmt) "CPUidle PSCI: " fmt
 
 #include <linux/cpu.h>
+#include <linux/cpu_pm.h>
 #include <linux/device.h>
 #include <linux/kernel.h>
 #include <linux/platform_device.h>
@@ -33,6 +34,7 @@ static int psci_pd_power_off(struct generic_pm_domain *pd)
 {
        struct genpd_power_state *state = &pd->states[pd->state_idx];
        u32 *pd_state;
+       int ret;
 
        if (!state->data)
                return 0;
@@ -44,6 +46,16 @@ static int psci_pd_power_off(struct generic_pm_domain *pd)
        pd_state = state->data;
        psci_set_domain_state(*pd_state);
 
+       if (list_empty(&pd->child_links)) {
+               /*
+                * The top domain (not being a child of anyone) should be the
+                * best one triggering PM notification.
+                */
+               ret = cpu_cluster_pm_enter();
+               if (ret)
+                       return ret;
+       }
+
        return 0;
 }