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

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Date:   Thu, 1 Mar 2018 00:49:03 +0530
From:   Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>
To:     Stewart Smith <stewart@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        npiggin@...il.com
Subject: Re: [PATCH] cpuidle/powernv : Restore different PSSCR for idle and
 hotplug

On Mon, Feb 26, 2018 at 03:47:12PM +1100, Stewart Smith wrote:
> 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?

Yes I will resend and CC it to stable.
> 
> -- 
> Stewart Smith
> OPAL Architect, IBM.
>