linux-kernel - Re: [PATCH] cpuidle: riscv-sbi: Add cluster_pm

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Date: Fri, 24 May 2024 12:41:03 +0200
From: Ulf Hansson <ulf.hansson@...aro.org>
To: Anup Patel <apatel@...tanamicro.com>, Nick Hu <nick.hu@...ive.com>
Cc: Anup Patel <anup@...infault.org>, palmer@...belt.com, rafael@...nel.org, 
	daniel.lezcano@...aro.org, paul.walmsley@...ive.com, linux-pm@...r.kernel.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	zong.li@...ive.com, Cyan Yang <cyan.yang@...ive.com>
Subject: Re: [PATCH] cpuidle: riscv-sbi: Add cluster_pm_enter()/exit()

On Fri, 17 May 2024 at 06:39, Anup Patel <apatel@...tanamicro.com> wrote:
>
> On Thu, May 16, 2024 at 9:40 AM Nick Hu <nick.hu@...ive.com> wrote:
> >
> > Hi Anup
> >
> > On Wed, May 15, 2024 at 9:46 PM Anup Patel <anup@...infault.org> wrote:
> > >
> > > Hi Nick,
> > >
> > > On Wed, May 15, 2024 at 5:45 PM Nick Hu <nick.hu@...ive.com> wrote:
> > > >
> > > > Hi Anup,
> > > >
> > > > Thank you for your guidance.
> > > > After enabling the debug message, we found a way to solve the problem
> > > > by the following steps:
> > > > 1. Add a compatible string in 'power-domains' node otherwise it won't
> > > > be the supplier of the consumers. (See of_link_to_phandle())
> > >
> > > Hmm, requiring a compatible string is odd. Where should we document
> > > this compatible string ?
> > >
> > Sorry, this is my fault. I didn't include some updates in
> > of_link_to_phandle(). This led some misunderstandings here.
> > You are right, we don't need it.
> > The supplier will be linked to the CLUSTER_PD node.
> >
> > > > 2. Move the 'power-domains' node outside the 'cpus' node otherwise it
> > > > won't be added to the device hierarchy by device_add().
> > > > 3. Update the cpuidle-riscv-sbi driver to get the pds_node from
> > > > '/power-domains'.
> > >
> > > By adding a compatible string and moving the "power-domains" node
> > > outside, you are simply forcing the OF framework to populate devices.
> > >
> > > How about manually creating platform_device for each power-domain
> > > DT node using of_platform_device_create() in sbi_pd_init() ?
> > >
> > Thanks for the suggestion! We have test the solution and it could work.
> > We was wondering if it's feasible for us to relocate the
> > 'power-domains' node outside of the /cpus? The CLUSTER_PD might
> > encompass not only the CPUs but also other components within the
> > cluster.
>
> The cpuidle-riscv-sbi driver expects "power-domains" DT node
> under "/cpus" DT node because this driver only deals with power
> domains related to CPU cluster or CPU cache-hierarchy. It does
> make sense to define L2/L3 power domains under
> "/cpus/power-domain" since these are related to CPUs.
>
> Moving the CPU "power-domains" DT node directly under "/" or
> somewhere else would mean that it covers system-wide power
> domains which is not true.

I understand your point, but I am not convinced that the power-domains
need to belong to the "cpus" node. Ideally, the power-domain describes
the power-rail and the interface to manage the CPUs, this can surely
be described outside the "cpus" node - even if there are only CPUs
that are using it.

Moreover, moving forward, one should not be surprised if it turns out
that a platform has other devices than the CPUs, sharing the same
power-rail as the CPU cluster. At least, we have that for arm/psci
[1].

>
> I suggest we continue using "/cpus/power-domains" DT node
> only for power domains related to CPU clusters or CPU
> cache-hierarchy.
>
> For system wide power domains of SoC devices, we can either:
> 1) Use device power domains through the SBI MPXY extension
>     via different driver
> 2) Use a platform specific driver
>
> >
> > We also look at cpuidle_psci_domain driver and it seems Arm doesn't
> > create the devices for each subnode of psci domain.
> > Is there any reason that they don't need it?

We don't need it for arm as we have a separate node for PSCI and its
power-domains [2]. Moreover, we have a separate driver that manages
the power-domain (cpuidle-psci-domain).

[...]

[1] arch/arm64/boot/dts/qcom/sc7280.dtsi (search for "CLUSTER_PD")
[2] Documentation/devicetree/bindings/arm/psci.yaml

Kind regards
Uffe