Subject: [PATCH 06/16] ARM: b.L: generic SMP secondary bringup and hotplug support

From: [email-masked] (Santosh Shilimkar)
Date: Tue, 15 Jan 2013 16:56:52 +0530

On Tuesday 15 January 2013 04:48 PM, Achin Gupta wrote:
> Hi Santosh,
>
> On Tue, Jan 15, 2013 at 6:32 AM, Santosh Shilimkar
> <[email-masked]> wrote:
>> On Monday 14 January 2013 11:35 PM, Achin Gupta wrote:
>>>
>>> Hi Santosh,
>>>
>>> On Fri, Jan 11, 2013 at 6:02 PM, Santosh Shilimkar
>>> <[email-masked]> wrote:
>>>>
>>>> On Thursday 10 January 2013 05:50 AM, Nicolas Pitre wrote:
>>>>>
>>>>>
>>>>> Now that the b.L power API is in place, we can use it for SMP secondary
>>>>> bringup and CPU hotplug in a generic fashion.
>>>>>
>>>>> Signed-off-by: Nicolas Pitre <[email-masked]>
>>>>> ---
>>>>>     arch/arm/common/Makefile     |  2 +-
>>>>>     arch/arm/common/bL_platsmp.c | 79
>>>>> ++++++++++++++++++++++++++++++++++++++++++++
>>>>>     2 files changed, 80 insertions(+), 1 deletion(-)
>>>>>     create mode 100644 arch/arm/common/bL_platsmp.c
>>>>>
>>>>> diff --git a/arch/arm/common/Makefile b/arch/arm/common/Makefile
>>>>> index 894c2ddf9b..59b36db7cc 100644
>>>>> --- a/arch/arm/common/Makefile
>>>>> +++ b/arch/arm/common/Makefile
>>>>> @@ -15,4 +15,4 @@ obj-$(CONFIG_PCI_HOST_ITE8152)  += it8152.o
>>>>>     obj-$(CONFIG_ARM_TIMER_SP804) += timer-sp.o
>>>>>     obj-$(CONFIG_FIQ_GLUE)                += fiq_glue.o fiq_glue_setup.o
>>>>>     obj-$(CONFIG_FIQ_DEBUGGER)    += fiq_debugger.o
>>>>> -obj-$(CONFIG_BIG_LITTLE)       += bL_head.o bL_entry.o vlock.o
>>>>> +obj-$(CONFIG_BIG_LITTLE)       += bL_head.o bL_entry.o bL_platsmp.o
>>>>> vlock.o
>>>>> diff --git a/arch/arm/common/bL_platsmp.c b/arch/arm/common/bL_platsmp.c
>>>>> new file mode 100644
>>>>> index 0000000000..0acb9f4685
>>>>> --- /dev/null
>>>>> +++ b/arch/arm/common/bL_platsmp.c
>>>>> @@ -0,0 +1,79 @@
>>>>> +/*
>>>>> + * linux/arch/arm/mach-vexpress/bL_platsmp.c
>>>>> + *
>>>>> + * Created by:  Nicolas Pitre, November 2012
>>>>> + * Copyright:   (C) 2012  Linaro Limited
>>>>> + *
>>>>> + * This program is free software; you can redistribute it and/or modify
>>>>> + * it under the terms of the GNU General Public License version 2 as
>>>>> + * published by the Free Software Foundation.
>>>>> + *
>>>>> + * Code to handle secondary CPU bringup and hotplug for the bL power
>>>>> API.
>>>>> + */
>>>>> +
>>>>> +#include <linux/init.h>
>>>>> +#include <linux/smp.h>
>>>>> +
>>>>> +#include <asm/bL_entry.h>
>>>>> +#include <asm/smp_plat.h>
>>>>> +#include <asm/hardware/gic.h>
>>>>> +
>>>>> +static void __init simple_smp_init_cpus(void)
>>>>> +{
>>>>> +       set_smp_cross_call(gic_raise_softirq);
>>>>> +}
>>>>> +
>>>>> +static int __cpuinit bL_boot_secondary(unsigned int cpu, struct
>>>>> task_struct *idle)
>>>>> +{
>>>>> +       unsigned int pcpu, pcluster, ret;
>>>>> +       extern void secondary_startup(void);
>>>>> +
>>>>> +       pcpu = cpu_logical_map(cpu) & 0xff;
>>>>> +       pcluster = (cpu_logical_map(cpu) >> 8) & 0xff;
>>>>> +       pr_debug("%s: logical CPU %d is physical CPU %d cluster %d\n",
>>>>> +                __func__, cpu, pcpu, pcluster);
>>>>> +
>>>>> +       bL_set_entry_vector(pcpu, pcluster, NULL);
>>>>> +       ret = bL_cpu_power_up(pcpu, pcluster);
>>>>> +       if (ret)
>>>>> +               return ret;
>>>>> +       bL_set_entry_vector(pcpu, pcluster, secondary_startup);
>>>>> +       gic_raise_softirq(cpumask_of(cpu), 0);
>>>>> +       sev();
>>>>
>>>>
>>>> softirq() should be enough to break a CPU if it is in standby with
>>>> wfe state. Is that additional sev() needed here ?
>>>
>>>
>>> Not if the target cpu has its I & F bits disabled and that would be the
>>> case with a secondary waiting to be woken up
>>>
>> This is interesting since CPU is actually in standby state and this
>> was not my understanding so far. Your statement at least contradicts
>> the ARM ARM (B1.8.12 Wait For Interrupt)
>> -----------------------
>> The processor can remain in the WFI low-power state until it is reset, or it
>> detects one of the following WFI wake-up
>> events:
>> ? a physical IRQ interrupt, regardless of the value of the CPSR.I bit
>> ? a physical FIQ interrupt, regardless of the value of the CPSR.F bit
>> ----------------------------------
>>
>> Are you referring to some new behavior on latest ARMv7 CPUs ?
>
> You are abs right about the 'wfi' behaviour. I was talking about the effect
> of interrupts on a cpu thats in 'wfe'.
>
> The power up process takes place in two steps. The first step involves
> sending an ipi which will either:
>
> a. cause the power controller to bring the processor out of reset
> b. cause the processor to exit from wfi (most probably in the bootloader code)
>
> The cpu then enters Linux (bL_entry_point) and after doing any cluster setup
> waits in 'wfe' if its 'bL_entry_vector' has not been set as yet. The
> 'sev' is meant
> to poke the cpu once this has been done.
>
Thanks for additional information. Its clear to me now.

Regards
Santosh