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

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Date:   Thu, 01 Mar 2018 12:43:34 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Stewart Smith <stewart@...ux.vnet.ibm.com>,
        Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Cc:     npiggin@...il.com, Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>
Subject: Re: [PATCH] cpuidle/powernv : Restore different PSSCR for idle and hotplug

Stewart Smith <stewart@...ux.vnet.ibm.com> writes:

> 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?

But is Cc'ing this to stable even sufficient to enable stop11?

If firmware starts enabling stop11 then every existing kernel will be
broken, so eg. bisecting anything prior to now will be impossible.

cheers