[PATCH] ARM: kexec: Fix validating CPU hotplug support

[HuKeping]

Due to commit:2103f6cba61a8b8bea3fc1b63661d830a2125e76, there is
a hotplug checking in machine_kexec_prepare(), but it will lead
a failure when loading the crash-kernel in some cases.

Kexec utility can load the crash kernel by two ways:
1. kexec -l kernel-image --append=command-line-options --initrd=initrd-image
2. kexec -p kernel-image --append=command-line-options --initrd=initrd-image

In case #1, it's ok to do the hotplug checking things, for it will cut off
cpus in _cpu_down() later when command "kexec -e" be sended, function _cpu_down()
needs the support of cpu hotplug.

In case #2, it is no need to do the checking, because we actually do not
shut the cpus down, but just keep them invoking cpu_relax(), the whole routing
has no business with cpu cutting off. That means, even if the CONFIG_HOTPLUG_CPU
was not selected, it can also load and reboot the crash-kernel successfully if
we use "kexec -p ..."  and then a crash came to trigger the kexec.

Prior to this patch, if the first kernel is not support the CPU hotplug, we can not
use kexec utility.

Signed-off-by: Hu Keping <hukeping at huawei.com>
---
 arch/arm/kernel/machine_kexec.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/arch/arm/kernel/machine_kexec.c b/arch/arm/kernel/machine_kexec.c
index 8cf0996..1e5620f 100644
--- a/arch/arm/kernel/machine_kexec.c
+++ b/arch/arm/kernel/machine_kexec.c
@@ -41,12 +41,20 @@ int machine_kexec_prepare(struct kimage *image)
 	int i, err;
 
 	/*
-	 * Validate that if the current HW supports SMP, then the SW supports
-	 * and implements CPU hotplug for the current HW. If not, we won't be
-	 * able to kexec reliably, so fail the prepare operation.
+	 * When it is a * crash * triggered the kexec, we do not really shut the
+	 * cpus down but just let them keep invoking cpu_relax(), so there is no need
+	 * to do the hotplug checking things.
 	 */
-	if (num_possible_cpus() > 1 && !platform_can_cpu_hotplug())
-		return -EINVAL;
+	if (image->type != KEXEC_TYPE_CRASH)
+	{
+		/*
+		 * Validate that if the current HW supports SMP, then the SW supports
+		 * 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())
+			return -EINVAL;
+	}
 
 	/*
 	 * No segment at default ATAGs address. try to locate
-- 
1.8.5.5