linux-kernel - Re: [PATCH] powerpc/powernv: Clear LPCR[PECE1] via stop-api only for deep state offline

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Message-Id: <e75eda1b-c6ae-fe74-d45c-2c98e366be2a@linux.vnet.ibm.com>
Date:   Fri, 1 Sep 2017 15:38:59 +0530
From:   Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        Pavithra Prakash <pavirampu@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
Subject: Re: [PATCH] powerpc/powernv: Clear LPCR[PECE1] via stop-api only for
 deep state offline


On 08/31/2017 05:37 PM, Nicholas Piggin wrote:
> On Thu, 31 Aug 2017 17:17:41 +0530
> "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com> wrote:
>
> > From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
> >
> > commit 24be85a23d1f ("powerpc/powernv: Clear PECE1 in LPCR via
> > stop-api only on Hotplug") clears the PECE1 bit of the LPCR via
> > stop-api during CPU-Hotplug to prevent wakeup due to a decrementer on
> > an offlined CPU which is in a deep stop state.
> >
> > In the case where the stop-api support is found to be lacking, the
> > commit 785a12afdb4a ("powerpc/powernv/idle: Disable LOSE_FULL_CONTEXT
> > states when stop-api fails") disables deep states that lose hypervisor
> > context. Thus in this case, the offlined CPU will be put to some
> > shallow idle state.
> >
> > However, we currently unconditionally clear the PECE1 in LPCR via
> > stop-api during CPU-Hotplug even when deep states are disabled due to
> > stop-api failure.
> >
> > Fix this by clearing PECE1 of LPCR via stop-api during CPU-Hotplug
> > *only* when the offlined CPU will be put to a deep state that loses
> > hypervisor context.
>
> This looks okay to me. The bug is due to calling opal_slw_set_reg when
> firmware has not enabled that feature, right?
Yes,

In the case where the stop-api support is found to be lacking, the 
commit 785a12afdb4a ("powerpc/powernv/idle: Disable LOSE_FULL_CONTEXT 
states when stop-api fails") disables deep states that lose hypervisor 
context. Thus in this case, the offlined CPU will be put to some shallow 
idle state.

If a shallow state ( < stop4 ) is being chosen for cpu hotplug, then :
1) this opal call is not required.
2) may not be supported.

Hence  should call opal_slw_set_reg() only if a deep state chosen for 
cpu hotplug.
>
> >
> > Fixes: commit 24be85a23d1f ("powerpc/powernv: Clear PECE1 in LPCR via
> > stop-api only on Hotplug")
> >
> > Reported-by: Pavithra Prakash <pavirampu@...ux.vnet.ibm.com>
> > Signed-off-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
> > ---
> >  arch/powerpc/platforms/powernv/idle.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/platforms/powernv/idle.c
> b/arch/powerpc/platforms/powernv/idle.c
> > index 9f59041..23f8fba 100644
> > --- a/arch/powerpc/platforms/powernv/idle.c
> > +++ b/arch/powerpc/platforms/powernv/idle.c
> > @@ -393,7 +393,13 @@ static void
> pnv_program_cpu_hotplug_lpcr(unsigned int cpu, u64 lpcr_val)
> >      u64 pir = get_hard_smp_processor_id(cpu);
> >
> >      mtspr(SPRN_LPCR, lpcr_val);
> > -    opal_slw_set_reg(pir, SPRN_LPCR, lpcr_val);
> > +
> > +    /*
> > +     * Program the LPCR via stop-api only for deepest stop state
> > +     * can lose hypervisor context.
> > +     */
> > +    if (supported_cpuidle_states & OPAL_PM_LOSE_FULL_CONTEXT)
> > +        opal_slw_set_reg(pir, SPRN_LPCR, lpcr_val);
> >  }
> >
> >  /*
>