powerpc/book3e-64/kexec: Enable SMP release

The SMP release mechanism for FSL book3e is different from when booting
with normal hardware.  In theory we could simulate the normal spin
table mechanism, but not at the addresses U-Boot put in the device tree
-- so there'd need to be even more communication between the kernel and
kexec to set that up.  Instead, kexec-tools will set a boolean property
linux,booted-from-kexec in the /chosen node.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: devicetree@vger.kernel.org

Documentation/devicetree/bindings/chosen.txt

@@ -44,3 +44,11 @@ Implementation note: Linux will look for the property "linux,stdout-path" or
 on PowerPC "stdout" if "stdout-path" is not found.  However, the
 "linux,stdout-path" and "stdout" properties are deprecated. New platforms
 should only use the "stdout-path" property.
+
+linux,booted-from-kexec
+-----------------------
+
+This property is set (currently only on PowerPC, and only needed on
+book3e) by some versions of kexec-tools to tell the new kernel that it
+is being booted by kexec, as the booting environment may differ (e.g.
+a different secondary CPU release mechanism)

arch/powerpc/kernel/setup_64.c

@@ -340,11 +340,26 @@ void early_setup_secondary(void)
 #endif /* CONFIG_SMP */
 
 #if defined(CONFIG_SMP) || defined(CONFIG_KEXEC)
+static bool use_spinloop(void)
+{
+	if (!IS_ENABLED(CONFIG_PPC_BOOK3E))
+		return true;
+
+	/*
+	 * When book3e boots from kexec, the ePAPR spin table does
+	 * not get used.
+	 */
+	return of_property_read_bool(of_chosen, "linux,booted-from-kexec");
+}
+
 void smp_release_cpus(void)
 {
 	unsigned long *ptr;
 	int i;
 
+	if (!use_spinloop())
+		return;
+
 	DBG(" -> smp_release_cpus()\n");
 
 	/* All secondary cpus are spinning on a common spinloop, release them
@@ -524,7 +539,7 @@ void __init setup_system(void)
 	 * Freescale Book3e parts spin in a loop provided by firmware,
 	 * so smp_release_cpus() does nothing for them
 	 */
-#if defined(CONFIG_SMP) && !defined(CONFIG_PPC_FSL_BOOK3E)
+#if defined(CONFIG_SMP)
 	/* Release secondary cpus out of their spinloops at 0x60 now that
 	 * we can map physical -> logical CPU ids
 	 */