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

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Message-ID: <20170831220758.5a5d2171@roar.ozlabs.ibm.com>
Date:   Thu, 31 Aug 2017 22:07:58 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>,
        Pavithra Prakash <pavirampu@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/powernv: Clear LPCR[PECE1] via stop-api only
 for deep state offline

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?

> 
> 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);
>  }
>  
>  /*