quoted

@@ -258,6 +256,9 @@ int __init tegra20_cpuidle_init(void)

quoted

+#ifdef CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED
+		dev->coupled_cpus = *cpu_online_mask;
+#endif

What if that changes; I assume couple cpuidle handles that by
registering a notifier?

Yes. see drivers/cpuidle/coupled.c:651

Is there any way that the kernel can boot with only CPU 0 "plugged in",
but later have the other CPU hotplugged in? In other words, should that
hard-code a mask (3?) that describes the HW, rather than relying on the
runtime hotplug status? (think about what happens when this idle code is
moved into drivers/cpuidle/ and built as a module, and hence could be
initialized with only 1 CPU hotplugged in).

Using the cpu_possible_mask might be a better idea. The only other user
of coupled cpuidle (OMAP4) also uses cpu_online_mask however.

Cheers,

Peter.