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

Lucas Stach
2019-03-27T17:55:17Z

Am Mittwoch, den 27.03.2019, 17:45 +0000 schrieb Marc Zyngier:
> On 27/03/2019 16:06, Lucas Stach wrote:
> > 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.
>
> Are all SPIs connected to the GPC?

AFAICS yes.

> > Anything that isn't visible to the GPC and requires the GIC
> > wake_request signal to behave as specified is broken by this erratum.
>
> I really wonder how a timer interrupt (a PPI, hence not routed through
> the GPC) can wake up the CPU in this case. It really feels like
> something like "program CNTV_CVAL_EL0 to expire at some later point;
> WFI" could result in the CPU going to a deep sleep state, and not
> wake-up at all.

I guess it's broken in the same way. The downstream DT claims
"local-timer-stop" for the CPU sleep state and "arm,no-tick-in-suspend"
for the armv8-timer, which I guess is not the timer actually stopping
in suspend, but the CPU being unable to wake up due to the timer IRQ.

> This would indicate that not only cpuidle is broken with this, but
> absolutely every interrupt that is not routed through the GPC.

That's my understanding as well. Note that I have no NXP internal
information and can only infer from the published reference manual,
errata notice and downstream kernel.

Regards,
Lucas