linux-kernel - Re: [PATCH RFC 1/1] arm64: Use PSCI calls for CPU stop when hotplug is supported

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Message-ID: <20190123165607.GB55887@lakrids.cambridge.arm.com>
Date:   Wed, 23 Jan 2019 16:56:07 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Pramod Kumar <pramod.kumar@...adcom.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Suzuki K Poulose <Suzuki.Poulose@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Dave Martin <dave.martin@....com>,
        Rob Herring <robh@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Steve Capper <steve.capper@....com>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 1/1] arm64: Use PSCI calls for CPU stop when hotplug
 is supported

On Fri, Jan 18, 2019 at 11:16:20AM +0530, Pramod Kumar wrote:
> If CPU hotplug is supported, ipi_cpu_stop should use PSCI cpudie
> call to stop the CPU. This call ensures L1/L2 cache flush,
> CPUs cache-cohenrecy setting w.r.to interconnect.
> 
> Apart from this, this gives control to f/w to reduce power consumption
> by take appropriate decesion on power rails for plugging-out core.
> 
> Signed-off-by: Pramod Kumar <pramod.kumar@...adcom.com>
> Reviewed-by: Ray Jui <ray.jui@...adcom.com>
> Reviewed-by: Scott Branden <scott.branden@...adcom.com>
> ---
>  arch/arm64/kernel/smp.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 1598d6f..360e52b 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -822,8 +822,13 @@ static void ipi_cpu_stop(unsigned int cpu)
>  	local_daif_mask();
>  	sdei_mask_local_cpu();
>  
> +#ifdef CONFIG_HOTPLUG_CPU
> +	if (cpu_ops[cpu]->cpu_die)
> +		cpu_ops[cpu]->cpu_die(cpu);
> +#else
>  	while (1)
>  		cpu_relax();
> +#endif

If cpu_ops[cpu]->cpu_die is NULL, this change makes ipi_cpu_stop()
return, which is not correct.

Regardless, I don't think that there is sufficient rationale for this
change, especially given that your commit message describes
platform-specific assumptions which do not hold in general.

Thanks,
Mark.