cpufreq / intel_pstate: Add kernel command line option disable intel_pstate.

When intel_pstate is configured into the kernel it will become the
preferred scaling driver for processors that it supports.  Allow the
user to override this by adding:
   intel_pstate=disable
on the kernel command line.
Signed-off-by: Dirk Brandewie <dirk.j.brandewie@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/kernel-parameters.txt

@@ -1131,6 +1131,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			0	disables intel_idle and fall back on acpi_idle.
 			1 to 6	specify maximum depth of C-state.
 
+	intel_pstate=  [X86]
+		       disable
+		         Do not enable intel_pstate as the default
+		         scaling driver for the supported processors
+
 	intremap=	[X86-64, Intel-IOMMU]
 			on	enable Interrupt Remapping (default)
 			off	disable Interrupt Remapping

drivers/cpufreq/intel_pstate.c

@@ -773,11 +773,16 @@ static void intel_pstate_exit(void)
 }
 module_exit(intel_pstate_exit);
 
+static int __initdata no_load;
+
 static int __init intel_pstate_init(void)
 {
 	int rc = 0;
 	const struct x86_cpu_id *id;
 
+	if (no_load)
+		return -ENODEV;
+
 	id = x86_match_cpu(intel_pstate_cpu_ids);
 	if (!id)
 		return -ENODEV;
@@ -802,6 +807,17 @@ static int __init intel_pstate_init(void)
 }
 device_initcall(intel_pstate_init);
 
+static int __init intel_pstate_setup(char *str)
+{
+	if (!str)
+		return -EINVAL;
+
+	if (!strcmp(str, "disable"))
+		no_load = 1;
+	return 0;
+}
+early_param("intel_pstate", intel_pstate_setup);
+
 MODULE_AUTHOR("Dirk Brandewie <dirk.j.brandewie@intel.com>");
 MODULE_DESCRIPTION("'intel_pstate' - P state driver Intel Core processors");
 MODULE_LICENSE("GPL");