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

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Date:   Wed, 27 Mar 2019 17:00:28 +0000
From:   Leonard Crestez <leonard.crestez@....com>
To:     "l.stach@...gutronix.de" <l.stach@...gutronix.de>,
        "marc.zyngier@....com" <marc.zyngier@....com>,
        Richard Zhu <hongxing.zhu@....com>
CC:     Fabio Estevam <fabio.estevam@....com>,
        Cosmin Samoila <cosmin.samoila@....com>,
        Robin Gong <yibin.gong@....com>,
        Mircea Pop <mircea.pop@....com>,
        Daniel Baluta <daniel.baluta@....com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        Aisheng Dong <aisheng.dong@....com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        Robert Chiras <robert.chiras@....com>,
        Anson Huang <anson.huang@....com>, Jun Li <jun.li@....com>,
        Abel Vesa <abel.vesa@....com>,
        "robh@...nel.org" <robh@...nel.org>,
        Zening Wang <zening.wang@....com>,
        dl-linux-imx <linux-imx@....com>,
        BOUGH CHEN <haibo.chen@....com>,
        Horia Geanta <horia.geanta@....com>,
        Leonard Crestez <leonard.crestez@....com>,
        Peter Chen <peter.chen@....com>,
        Joakim Zhang <qiangqing.zhang@....com>,
        "rjw@...ysocki.net" <rjw@...ysocki.net>,
        Leo Zhang <leo.zhang@....com>,
        Shenwei Wang <shenwei.wang@....com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Ranjani Vaidyanathan <ranjani.vaidyanathan@....com>,
        Han Xu <han.xu@....com>,
        "will.deacon@....com" <will.deacon@....com>,
        Iuliana Prodan <iuliana.prodan@....com>,
        "sudeep.holla@....com" <sudeep.holla@....com>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        Jacky Bai <ping.bai@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        Peng Fan <peng.fan@....com>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        Viorel Suman <viorel.suman@....com>
Subject: Re: [RFC 0/7] cpuidle: Add poking mechanism to support non-IPI wakeup

On Wed, 2019-03-27 at 17:06 +0100, Lucas Stach wrote:
> Am Mittwoch, den 27.03.2019, 15:57 +0000 schrieb Marc Zyngier:
> > On 27/03/2019 15:44, Lucas Stach wrote:
> > > 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.

We had a kernel workaround for IPIs in our internal tree for a long
time and I don't think we do anything special for PCI. Does PCI MSI
really bypass the GPC on 8mq?

Adding Richard/Jacky, they might know about this.

This seems like something of a corner case to me, don't many imx boards
ship without PCI; especially for low-power scenarios? If required it
might be reasonable to add an additional workaround to disable all
cpuidle if pci msis are used.

--
Regards,
Leonard