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Date:   Fri, 7 Feb 2020 14:48:50 +0000
From:   Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
To:     Ulf Hansson <ulf.hansson@linaro.org>
Cc:     Sudeep Holla <sudeep.holla@arm.com>,
        Lina Iyer <ilina@codeaurora.org>,
        Maulik Shah <mkshah@codeaurora.org>,
        Stephen Boyd <swboyd@chromium.org>,
        Andy Gross <agross@kernel.org>,
        David Brown <david.brown@linaro.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Evan Green <evgreen@chromium.org>,
        Doug Anderson <dianders@chromium.org>,
        Rajendra Nayak <rnayak@codeaurora.org>, lsrao@codeaurora.org,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>
Subject: Re: [PATCH v3 5/7] drivers: firmware: psci: Add hierarchical domain
 idle states converter
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On Fri, Feb 07, 2020 at 01:32:28PM +0100, Ulf Hansson wrote:
> [...]
> 
> > > 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.

In PC mode not only there is no need but it is wrong to manage
any last-man activity in the kernel. I wonder why we are still
talking about this to be honest.

Code to handle PSCI platform coordinated mode has been/is in
the kernel today and that's all is needed according to the PSCI
specifications.

Thanks,
Lorenzo