intel_pstate: add kernel parameter to force loading

To force loading on Oracle Sun X86 servers, provide one kernel command line
parameter

  intel_pstate = force

For those who are aware of the risk of no power capping capabily working
and try to get better performance with this driver.

Signed-off-by: Ethan Zhao <ethan.zhao@oracle.com>
Tested-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Reviewed-by: Linda Knippers <linda.knippers@hp.com>
Acked-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/kernel-parameters.txt

@@ -1446,6 +1446,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 		       disable
 		         Do not enable intel_pstate as the default
 		         scaling driver for the supported processors
+		       force
+			 Enable intel_pstate on systems that prohibit it by default
+			 in favor of acpi-cpufreq. Forcing the intel_pstate driver
+			 instead of acpi-cpufreq may disable platform features, such
+			 as thermal controls and power capping, that rely on ACPI
+			 P-States information being indicated to OSPM and therefore
+			 should be used with caution. This option does not work with
+			 processors that aren't supported by the intel_pstate driver
+			 or on platforms that use pcc-cpufreq instead of acpi-cpufreq.
 		       no_hwp
 		         Do not enable hardware P state control (HWP)
 			 if available.

drivers/cpufreq/intel_pstate.c

@@ -948,6 +948,7 @@ static struct cpufreq_driver intel_pstate_driver = {
 
 static int __initdata no_load;
 static int __initdata no_hwp;
+static unsigned int force_load;
 
 static int intel_pstate_msrs_not_valid(void)
 {
@@ -1094,7 +1095,8 @@ static bool intel_pstate_platform_pwr_mgmt_exists(void)
 			case PSS:
 				return intel_pstate_no_acpi_pss();
 			case PPC:
-				return intel_pstate_has_acpi_ppc();
+				return intel_pstate_has_acpi_ppc() &&
+					(!force_load);
 			}
 	}
 
@@ -1175,6 +1177,8 @@ static int __init intel_pstate_setup(char *str)
 		no_load = 1;
 	if (!strcmp(str, "no_hwp"))
 		no_hwp = 1;
+	if (!strcmp(str, "force"))
+		force_load = 1;
 	return 0;
 }
 early_param("intel_pstate", intel_pstate_setup);