[IA64] Count disabled cpus as potential hot-pluggable CPUs

Minor updates to earlier patch.
- Added to documentation to add ia64 as well.
- Minor clarification on how to use disabled cpus
- used plain max instead of max_t per Andew Morton.
Signed-off-by: NAshok Raj <ashok.raj@intel.com>
Signed-off-by: NTony Luck <tony.luck@intel.com>

Documentation/cpu-hotplug.txt

@@ -44,10 +44,20 @@ maxcpus=n    Restrict boot time cpus to n. Say if you have 4 cpus, using
              maxcpus=2 will only boot 2. You can choose to bring the
              other cpus later online, read FAQ's for more info.
 
-additional_cpus=n	[x86_64 only] use this to limit hotpluggable cpus.
-                        This option sets
+additional_cpus*=n	Use this to limit hotpluggable cpus. This option sets
 			cpu_possible_map = cpu_present_map + additional_cpus
 
+(*) Option valid only for following architectures
+- x86_64, ia64
+
+ia64 and x86_64 use the number of disabled local apics in ACPI tables MADT
+to determine the number of potentially hot-pluggable cpus. The implementation
+should only rely on this to count the #of cpus, but *MUST* not rely on the
+apicid values in those tables for disabled apics. In the event BIOS doesnt
+mark such hot-pluggable cpus as disabled entries, one could use this
+parameter "additional_cpus=x" to represent those cpus in the cpu_possible_map.
+
+
 CPU maps and such
 -----------------
 [More on cpumaps and primitive to manipulate, please check

arch/ia64/kernel/acpi.c

@@ -794,24 +794,21 @@ __init void prefill_possible_map(void)
 	int possible, disabled_cpus;
 
 	disabled_cpus = total_cpus - available_cpus;
+
  	if (additional_cpus == -1) {
- 		if (disabled_cpus > 0) {
- 			possible = total_cpus;
+ 		if (disabled_cpus > 0)
 			additional_cpus = disabled_cpus;
-		}
- 		else {
-			possible = available_cpus;
+ 		else
 			additional_cpus = 0;
-		}
- 	} else {
-		possible = available_cpus + additional_cpus;
-	}
+ 	}
+
+	possible = available_cpus + additional_cpus;
+
 	if (possible > NR_CPUS)
 		possible = NR_CPUS;
 
 	printk(KERN_INFO "SMP: Allowing %d CPUs, %d hotplug CPUs\n",
-		possible,
-	        max_t(int, additional_cpus, 0));
+		possible, max((possible - available_cpus), 0));
 
 	for (i = 0; i < possible; i++)
 		cpu_set(i, cpu_possible_map);