linux-kernel - Re: [PATCH v2] ARM: smp: Only expose /sys/.../cpuX/online if hotpluggable

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Date:	Fri, 13 Feb 2015 20:54:22 +0000
From:	Magnus Damm <magnus.damm@...il.com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Simon Horman <horms@...ge.net.au>,
	Russell King <linux@....linux.org.uk>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Mark Rutland <mark.rutland@....com>,
	Nicolas Pitre <nico@...aro.org>,
	Dave Martin <Dave.Martin@....com>,
	SH-Linux <linux-sh@...r.kernel.org>
Subject: Re: [PATCH v2] ARM: smp: Only expose /sys/.../cpuX/online if hotpluggable

Hi Stephen,

On Fri, Feb 13, 2015 at 8:23 PM, Stephen Boyd <sboyd@...eaurora.org> wrote:
> On 02/13/15 12:20, Simon Horman wrote:
>> Hi Stephen,
>>
>> On Fri, Feb 13, 2015 at 10:06:39AM -0800, Stephen Boyd wrote:
>>> diff --git a/arch/arm/mach-shmobile/smp-r8a7779.c b/arch/arm/mach-shmobile/smp-r8a7779.c
>>> index 3f761f839043..b45206f93ddf 100644
>>> --- a/arch/arm/mach-shmobile/smp-r8a7779.c
>>> +++ b/arch/arm/mach-shmobile/smp-r8a7779.c
>>> @@ -124,19 +124,12 @@ static int r8a7779_cpu_kill(unsigned int cpu)
>>>
>>>      return 0;
>>>  }
>>> -
>>> -static int r8a7779_cpu_disable(unsigned int cpu)
>>> -{
>>> -    /* only CPU1->3 have power domains, do not allow hotplug of CPU0 */
>>> -    return cpu == 0 ? -EPERM : 0;
>>> -}
>>>  #endif /* CONFIG_HOTPLUG_CPU */
>>>
>>>  struct smp_operations r8a7779_smp_ops  __initdata = {
>>>      .smp_prepare_cpus       = r8a7779_smp_prepare_cpus,
>>>      .smp_boot_secondary     = r8a7779_boot_secondary,
>>>  #ifdef CONFIG_HOTPLUG_CPU
>>> -    .cpu_disable            = r8a7779_cpu_disable,
>>>      .cpu_die                = shmobile_smp_scu_cpu_die,
>>>      .cpu_kill               = r8a7779_cpu_kill,
>>>  #endif
>> Its not clear to me why r8a7779_cpu_disable() has been
>> removed rather than replaced by r8a7779_cpu_can_disable()
>
> By default ARM's smp.c assumes that CPU0 can't be hotplugged. The
> function is redundant. I guess I should have mentioned that in the
> commit text.

Thanks for your efforts. Can you please tell me where that limitation
is located? I'm quite sure I've brought CPU cores up and down
including CPU0, but maybe something is missing?

Cheers,

/ magnus
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