linux-kernel - Re: [RFC 0/2] Add workaround for core wake-up on IPI for i.MX8MQ

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Message-ID: <alpine.DEB.2.21.1906120937430.2214@nanos.tec.linutronix.de>
Date:   Wed, 12 Jun 2019 09:37:58 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Marc Zyngier <marc.zyngier@....com>
cc:     Leonard Crestez <leonard.crestez@....com>,
        Abel Vesa <abel.vesa@....com>,
        Lucas Stach <l.stach@...gutronix.de>,
        Mark Rutland <mark.rutland@....com>,
        Abel Vesa <abelvesa@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        Jacky Bai <ping.bai@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        dl-linux-imx <linux-imx@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Carlo Caione <ccaione@...libre.com>
Subject: Re: [RFC 0/2] Add workaround for core wake-up on IPI for i.MX8MQ

On Wed, 12 Jun 2019, Marc Zyngier wrote:
> On Wed, 12 Jun 2019 08:14:16 +0100,
> Thomas Gleixner <tglx@...utronix.de> wrote:
> > On Mon, 10 Jun 2019, Leonard Crestez wrote:
> > > On 6/10/2019 5:08 PM, Marc Zyngier wrote:
> > > > Nobody is talking about performance here. It is strictly about
> > > > correctness, and what I read about this system is that it cannot
> > > > reliably use cpuidle.
> > > My argument was that it's fine if PPIs and LPIs are broken as long as 
> > > they're not used:
> > > 
> > >   * PPIs are only used for local timer which is not used for wakeup.
> > 
> > Huch? The timer has to bring the CPU out of idle as any other
> > interrupt.
> 
> They use a separate hack for that, pretending that the timer is
> stopped during idle (it isn't), and setup a broadcast timer when
> entering idle. That timer uses an interrupt that can wake-up the
> target CPU, and all is well in the world. Sort of.
> 
> Of course, this breaks as PPIs are not only used by the timer, but
> also by a number of other HW bits (PMU, GIC, guest and hypervisor
> timers), and they don't have corresponding hacks to back them up.

Eew.