[PATCH 2/2] ARM: tegra: moving the clock gating procedure to tegra_cpu_kill

[swarren@wwwdotorg.org (Stephen Warren)]

On 12/21/2012 02:58 AM, Joseph Lo wrote:
> On Fri, 2012-12-21 at 01:01 +0800, Stephen Warren wrote:
>> On 12/19/2012 07:28 PM, Joseph Lo wrote:
>>> The tegra_cpu_die was be executed by the CPU itslf. So the clock gating
>>> procedure won't be executed after the CPU hardware shutdown code. Moving
>>> the clock gating procedure to tegra_cpu_kill that will be run by another
>>> CPU after the CPU died.
>>
>> Hmmm. I wonder if this is enough to make kexec-on-Tegra-with-SMP-enabled
>> work without explicitly hot-unplugging all the CPUs first... An
>> implementation of cpu_kill() was a major part of what was missing. I
>> thought an implementation of cpu_kill() would require a bunch of code
>> from cpu_die() too. Does this patch assume cpu_die() has executed first,
>> and only then cpu_kill() will work, or can cpu_kill() be used on its own?
>>
> Yes, the tegra_cpu_die must be run first then tegra_cpu_kill. Because
> the tegra_cpu_die shutdown the CPU by itself, then we can clock gate the
> CPU in tegra_cpu_kill. If the tegra_cpu_kill be run itself, the system
> will be hung in wait_for_reset(CPU).
> 
> About the kexec issue, it's complicate. Your solution should be the
> right solution. And we should implement the hardware shutdown code in
> cpu_die not cpu_kill. I suspect this issue can be reproduced on all ARM
> platform. Because the smp_send_stop can't really shutdown the secondary
> CPU core if just call machine_shutdown without disable_nonboot_cpu. And
> in platform_shutdown and platform_reboot case, the secondary CPU already
> been offlined and killed before this function be called. Does any other
> ARM SMP platform can run kexec without any issue?
> 
> (I will check if we can do some HW shutdown in tegra_cpu_kill. For ex,
> just clock gate the CPU without wait_for_reset.)

Will, Joseph's thoughts above pretty much match mine re: kexec. In the
kexec thread I started earlier, I talked about replacing:

void machine_shutdown(void)
{
#ifdef CONFIG_SMP
	smp_send_stop();
#endif
}

with something like:

void machine_shutdown(void)
{
#ifdef CONFIG_HOTPLUG_CPU
 	disable_nonboot_cpus();
#elifdef CONFIG_SMP
	smp_send_stop();
#endif
}

and you'd responded "I think you're better off using what we currently
have and hanging your code off platform_cpu_kill.".

What exactly did you mean by "what we currently have"; did you mean that
cpu_kill() should work without cpu_die() having executed on the target
CPU itself first, so Tegra should simply implement cpu_kill()? As Joseph
says above, I'm not sure that will work. Any more detailed thoughts you
have here would be greatly appreciated. Thanks.