ARM: 8338/1: kexec: Relax SMP validation to improve DT compatibility

When trying to kexec into a new kernel on a platform where multiple CPU
cores are present, but no SMP bringup code is available yet, the
kexec_load system call fails with:

    kexec_load failed: Invalid argument

The SMP test added to machine_kexec_prepare() in commit 2103f6cb
("ARM: 7807/1: kexec: validate CPU hotplug support") wants to prohibit
kexec on SMP platforms where it cannot disable secondary CPUs.
However, this test is too strict: if the secondary CPUs couldn't be
enabled in the first place, there's no need to disable them later at
kexec time.  Hence skip the test in the absence of SMP bringup code.

This allows to add all CPU cores to the DTS from the beginning, without
having to implement SMP bringup first, improving DT compatibility.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Acked-by: NStephen Warren <swarren@nvidia.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/include/asm/smp_plat.h

@@ -104,6 +104,7 @@ static inline u32 mpidr_hash_size(void)
 	return 1 << mpidr_hash.bits;
 }
 
+extern int platform_can_secondary_boot(void);
 extern int platform_can_cpu_hotplug(void);
 
 #endif

arch/arm/kernel/machine_kexec.c

@@ -46,7 +46,8 @@ int machine_kexec_prepare(struct kimage *image)
 	 * and implements CPU hotplug for the current HW. If not, we won't be
 	 * able to kexec reliably, so fail the prepare operation.
 	 */
-	if (num_possible_cpus() > 1 && !platform_can_cpu_hotplug())
+	if (num_possible_cpus() > 1 && platform_can_secondary_boot() &&
+	    !platform_can_cpu_hotplug())
 		return -EINVAL;
 
 	/*

arch/arm/kernel/smp.c

@@ -145,6 +145,11 @@ void __init smp_init_cpus(void)
 		smp_ops.smp_init_cpus();
 }
 
+int platform_can_secondary_boot(void)
+{
+	return !!smp_ops.smp_boot_secondary;
+}
+
 int platform_can_cpu_hotplug(void)
 {
 #ifdef CONFIG_HOTPLUG_CPU