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

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Date:   Wed, 12 Jun 2019 08:35:41 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
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 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.

Thanks,

	M.

-- 
Jazz is not dead, it just smells funny.