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

From: [email-masked] (Dave Martin)
Date: Tue, 15 Jan 2013 18:53:06 +0000

On Tue, Jan 15, 2013 at 11:18:44AM +0000, 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.
> 
> Its not required in this case as we have already set 'bL_entry_vector' , issued
> a barrier & flushed the cache line. So if the incoming cpu sees a 0 in
> its vector
> then that would be a symptom of a different problem.

Perhaps this could be made a bit clearer by defining two helpers,
bL_entry_close_gate() and bL_entry_open_gate().  The sev() is only
applicable for opening the gate, and could be buried in
bL_entry_open_gate().

This is pretty specialised low-level code though, so I'm not sure that
the abstraction is worth it.

Cheers
---Dave