linux-kernel - Re: [RFC 0/7] cpuidle: Add poking mechanism to support non-IPI wakeup

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Date:   Wed, 27 Mar 2019 17:06:07 +0100
From:   Lucas Stach <l.stach@...gutronix.de>
To:     Marc Zyngier <marc.zyngier@....com>, Abel Vesa <abel.vesa@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Rob Herring <robh@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <kernel@...gutronix.de>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Fabio Estevam <fabio.estevam@....com>,
        Aisheng Dong <aisheng.dong@....com>
Cc:     dl-linux-imx <linux-imx@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: Re: [RFC 0/7] cpuidle: Add poking mechanism to support non-IPI
 wakeup

Hi Marc,

Am Mittwoch, den 27.03.2019, 15:57 +0000 schrieb Marc Zyngier:
> On 27/03/2019 15:44, Lucas Stach wrote:
> > Hi Abel,
> > 
> > Am Mittwoch, den 27.03.2019, 13:21 +0000 schrieb Abel Vesa:
> > > This work is a workaround I'm looking into (more as a background task)
> > > in order to add support for cpuidle on i.MX8MQ based platforms.
> > > 
> > > The main idea here is getting around the missing GIC wake_request signal
> > > (due to integration design issue) by waking up a each individual core through
> > > some dedicated SW power-up bits inside the power controller (GPC) right before
> > > every IPI is requested for that each individual core.
> > 
> > Just a general comment, without going into the details of this series:
> > this issue is not only affecting IPIs, but also MSIs terminated at the
> > GIC. Currently MSIs are terminated at the PCIe core, but terminating
> > them at the GIC is clearly preferable, as this allows assigning CPU
> > affinity to individual MSIs and lowers IRQ service overhead.
> > 
> > I'm not sure what the consequences are for upstream Linux support yet,
> > but we should keep in mind that having a workaround for IPIs is only
> > solving part of the issue.
> 
> If this erratum is affecting more than just IPIs, then indeed I don't
> see how this patch series solves anything.
> 
> But the erratum documentation seems to imply that only SGIs are
> affected, and goes as far as suggesting to use an external interrupt
> would solve it. How comes this is not the case? Or is it that anything
> directly routed to a redistributor is also affected? This would break
> LPIs (and thus MSIs) and PPIs (the CPU timer, among others).
> 
> What is the *exact* status of this thing? I have the ugly feeling that
> the true workaround is just to disable cpuidle.

As far as I understand the erratum, the basic issue is that the GIC
wake_request signals are not connected to the GPC (the CPU/peripheral
power sequencer). The SPIs are routed through the GPC and thus are
visible as wakeup sources, which is why the workaround of using an
external SPI as wakeup trigger for the IPI works.

Anything that isn't visible to the GPC and requires the GIC
wake_request signal to behave as specified is broken by this erratum.
You probably know the GIC better than any of us to tell what this
means.

Regards,
Lucas