linux-kernel - Re: [PATCH] cpuidle/powernv : Restore different PSSCR for idle and hotplug

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Message-Id: <87vaekmk6n.fsf@linux.vnet.ibm.com>
Date:   Mon, 26 Feb 2018 15:47:12 +1100
From:   Stewart Smith <stewart@...ux.vnet.ibm.com>
To:     Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Cc:     npiggin@...il.com, mpe@...erman.id.au,
        Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>
Subject: Re: [PATCH] cpuidle/powernv : Restore different PSSCR for idle and hotplug

Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com> writes:
> commit 1e1601b38e6e ("powerpc/powernv/idle: Restore SPRs for deep idle
> states via stop API.") uses stop-api provided by the firmware to restore
> PSSCR. PSSCR restore is required for handling special wakeup. When special
> wakeup is completed, the core enters stop state based on restored PSSCR.
>
> Currently PSSCR is restored to deepest available stop state, causing
> a idle cpu to enter deeper stop state on a special wakeup, which causes
> the cpu to hang on wakeup.
>
> A "sensors" command which reads temperature (through DTS sensors) on idle
> cpu can trigger special wakeup.
>
> Failed Scenario :
> Request restore of PSSCR with RL = 11
> cpu enters idle state (stop5)
>   user triggers "sensors" command
>    Assert special wakeup on cpu
>      Restores PSSCR with RL = 11  <---- Done by firmware
>       Read DTS sensor
>    Deassert special wakeup
>   cpu enters idle state (stop11) <-- Instead of stop5
>
> Cpu hang is caused because cpu ended up in a deeper state than it requested
>
> This patch fixes instability caused by special wakeup when stop11 is
> enabled. Requests restore of PSSCR to deepest stop state used by cpuidle.
> Only when offlining cpu, request restore of PSSCR to deepest stop state.
> On onlining cpu, request restore of PSSCR to deepest stop state used by
> cpuidle.
>
> Fixes : 1e1601b38e6e ("powerpc/powernv/idle: Restore SPRs for deep idle
> states via stop API.")

This should CC stable ?

We'll need this to enable stop11 in firmware and not break things, right?

-- 
Stewart Smith
OPAL Architect, IBM.