On Fri, Aug 24, 2012 at 04:23:39PM +0800, Bill Huang wrote:

When doing shutdown on Tegra20/Tegra30, we need to read/write PMIC
registers through I2C to perform the power off sequence. Unfortunately,
sometimes we'll fail to shutdown due to I2C timeout on Tegra20. And the
cause of the timeout is due to the CPU which I2C controller IRQ affined
to will have chance to be offlined without migrating all irqs affined 
to it, so the following I2C transactions will fail (no any CPU will
handle that interrupt since then).

Some snippet of the shutdown codes:

void kernel_power_off(void)
{
	kernel_shutdown_prepare(SYSTEM_POWER_OFF);
	:
	disable_nonboot_cpus();
	:
	machine_power_off();
}

void machine_power_off(void)
{
	machine_shutdown();
	if (pm_power_off)
		pm_power_off(); /* this is where we send I2C write to shutdown */
}

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

In "smp_send_stop()", it will send "IPI_CPU_STOPS" to offline other cpus except
current cpu (smp_processor_id()), however, current cpu will not always be cpu0 at
least at Tegra20, that said for example cpu1 might be the current cpu and cpu0 will
be offlined and this is the case why the I2C transaction will timeout. 

For normal case, "disable_nonboot_cpus()" call will disable all other Cpus except
cpu0, that means we won't hit the problem mentioned here since cpu0 will always be
the current cpu in the call "smp_send_stop", but the call to "disable_nonboot_cpus" 
will happen only when "CONFIG_PM_SLEEP_SMP" is enabled which is not the case for
Tegra20/Tegra30, we don't support suspend yet so this can't be enabled.

So what you're asking for is a feature to do what CONFIG_PM_SLEEP_SMP
does, but without CONFIG_PM_SLEEP_SMP enabled?

Why not just ensure that CONFIG_PM_SLEEP_SMP is enabled if your platform
requires that the lowest CPU number be the CPU dealing with reboot?