Message-ID: <CAPDyKFrgJf05H8S8_p9+w3V3ND7NPpHSU=bpEBk75-goO+FUjA@mail.gmail.com>
Date: Tue, 4 Nov 2025 17:52:01 +0100
From: Ulf Hansson <ulf.hansson@...aro.org>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Kevin Hilman <khilman@...libre.com>, linux-pm@...r.kernel.org, 
	Vincent Guittot <vincent.guittot@...aro.org>, Peter Zijlstra <peterz@...radead.org>, 
	Pavel Machek <pavel@...nel.org>, Len Brown <len.brown@...el.com>, 
	Daniel Lezcano <daniel.lezcano@...aro.org>, Saravana Kannan <saravanak@...gle.com>, 
	Maulik Shah <quic_mkshah@...cinc.com>, Prasad Sodagudi <psodagud@...cinc.com>, 
	Dhruva Gole <d-gole@...com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/4] pmdomain: Respect the CPU system-wakeup QoS limit
 during s2idle

On Tue, 4 Nov 2025 at 17:37, Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Tue, Nov 4, 2025 at 5:10 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> >
> > On Sat, 1 Nov 2025 at 01:11, Kevin Hilman <khilman@...libre.com> wrote:
> > >
> > > Ulf Hansson <ulf.hansson@...aro.org> writes:
> > >
> > > > A CPU system-wakeup QoS limit may have been requested by user-space. To
> > > > avoid breaking this constraint when entering a low-power state during
> > > > s2idle through genpd, let's extend the corresponding genpd governor for
> > > > CPUs. More precisely, during s2idle let the genpd governor select a
> > > > suitable low-power state, by taking into account the QoS limit.
> > >
> > > In addition to a QoS limit requested by userspace, shouldn't any
> > > per-device QoS limits from devices in the PM domain with CPUs/clusters
> > > also be considered?
> > >
> > > Right now, if a device is in a PM domain that also contains CPUs, any
> > > per-device QoS constraints for those devices should also impact the
> > > state chosen by s2idle.
> >
> > I am not so sure about that. The existing dev PM QoS latency is
> > targeted towards runtime suspend of a device and the genpd governor
> > also takes it into account for this use case.
> >
> > If we would start to take the same dev PM QoS latency constraint into
> > account for system suspend (s2idle), it may not be what the user
> > really intended. Instead, I think we should consider extending the dev
> > PM QoS interface, to allow the user to set a specific latency limit
> > for system wakeup. Then the genpd governor should take that into
> > account for s2idle.
> >
> > >
> > > I just tried a quick hack of extending you cpu_system_power_down_ok()
> > > function to look for any per-device QoS constraints set all devices in
> > > the PM domain (and subdomains).  It takes the min of all the per-device
> > > QoS constratins, compares it to the one from userspace, and uses the min
> > > of those to decide the genpd state.
> > >
> > > That has the effect I'm after, but curious what you think about the
> > > usecase and the idea?
> >
> > It makes sense, but as stated above, I think we need a new QoS limit
> > specific for system suspend.
> >
> > Rafael, what's your thoughts around this?
>
> Well, it's analogous to the CPU latency limit case, so if a new
> "system suspend" QoS limit is introduced for CPUs, that also needs to
> be done for the other devices.

Agreed!

>
> However, as in the CPU case, my personal view is that the "system
> suspend" latency limits should be greater than or equal to the
> corresponding latency limits for runtime PM.

Right, we should treat general devices similar to CPUs.

>
> One more thing that has just occurred to me is that there are systems
> in which I don't want to enable the "system suspend" limits at all.
> IOW, all of this needs to be disabled unless the platform opts in.

Okay. So are you thinking of using a Kconfig for this or better to
manage this in runtime?

Kind regards
Uffe

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