linux-kernel - Re: [PATCH v3 5/7] drivers: firmware: psci: Add hierarchical domain idle states converter

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Date:   Fri, 7 Feb 2020 13:32:28 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     Lina Iyer <ilina@...eaurora.org>,
        Maulik Shah <mkshah@...eaurora.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Andy Gross <agross@...nel.org>,
        David Brown <david.brown@...aro.org>,
        Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Evan Green <evgreen@...omium.org>,
        Doug Anderson <dianders@...omium.org>,
        Rajendra Nayak <rnayak@...eaurora.org>, lsrao@...eaurora.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH v3 5/7] drivers: firmware: psci: Add hierarchical domain
 idle states converter

[...]

> > I understand the arguments for using PC vs OSI and agree with it. But
> > what in PSCI is against Linux knowing when the last core is powering
> > down when the PSCI is configured to do only Platform Cordinated.
>
> Nothing :D. But knowing the evolution and reasons for adding OSI in the
> PSCI specification and having argued about benefits of OSI over PC for
> years and finally when we have it in mainline, this argument of using
> PC for exact reasons why OSI evolved is something I can't understand
> and I am confused.
>
> > There should not be any objection to drivers knowing when all the cores
> > are powered down, be it reference counting CPU PM notifications or using
> > a cleaner approach like this where GendPD framwork does everything
> > cleanly and gives a nice callback. ARM architecture allows for different
> > aspects of CPU access be handled at different levels. I see this as an
> > extension of that approach.
> >
>
> One thing that was repeatedly pointed out during OSI patch review was no
> extra overhead for PC mode where firmware can make decisions. So, just
> use OSI now and let us be done with this discussion of OSI vs PC. If PC
> is what you think you need for future, we can revert all OSI changes and
> start discussing again :-)

Just to make it clear, I fully agree with you in regards to overhead
for PC-mode. This is especially critical for ARM SoCs with lots of
cores, I assume.

However, the overhead you refer to, is *only* going to be present in
case when the DTS has the hierarchical CPU topology description with
"power-domains". Because, that is *optional* to use, I am expecting
only those SoC/platforms that needs to manage last-man activities to
use this layout, the others will remain unaffected.

That said, does that address your concern?

Kind regards
Uffe