intel_pstate: provide option to only use intel_pstate with HWP

Allow users the option to disable the driver for any hardware
which does not support HWP.
Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/kernel-parameters.txt

@@ -1469,6 +1469,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 		       no_hwp
 		         Do not enable hardware P state control (HWP)
 			 if available.
+		hwp_only
+			Only load intel_pstate on systems which support
+			hardware P state control (HWP) if available.
 
 	intremap=	[X86-64, Intel-IOMMU]
 			on	enable Interrupt Remapping (default)

drivers/cpufreq/intel_pstate.c

@@ -1019,6 +1019,7 @@ static struct cpufreq_driver intel_pstate_driver = {
 
 static int __initdata no_load;
 static int __initdata no_hwp;
+static int __initdata hwp_only;
 static unsigned int force_load;
 
 static int intel_pstate_msrs_not_valid(void)
@@ -1216,6 +1217,9 @@ static int __init intel_pstate_init(void)
 	if (cpu_has(c,X86_FEATURE_HWP) && !no_hwp)
 		intel_pstate_hwp_enable();
 
+	if (!hwp_active && hwp_only)
+		goto out;
+
 	rc = cpufreq_register_driver(&intel_pstate_driver);
 	if (rc)
 		goto out;
@@ -1250,6 +1254,8 @@ static int __init intel_pstate_setup(char *str)
 		no_hwp = 1;
 	if (!strcmp(str, "force"))
 		force_load = 1;
+	if (!strcmp(str, "hwp_only"))
+		hwp_only = 1;
 	return 0;
 }
 early_param("intel_pstate", intel_pstate_setup);