[PATCH 2/2 V9] intel_pstate: add kernel parameter to force loading.

[Ethan Zhao <ethan.zhao@oracle.com>]

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

  intel_pstate = force

For those who be 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>
---
 v2: change to hardware vendor specific naming parameter.
 v4: refine code and doc.
 v5&v6: fix a typo in doc.
 v7: change enum PCC to PPC.
 v8: change the name of kernel command line parameter to generic one.
 v9: refine doc

 Documentation/kernel-parameters.txt | 5 +++++
 drivers/cpufreq/intel_pstate.c      | 6 +++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 479f332..7d0983e 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1446,6 +1446,11 @@ 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.
 
 	intremap=	[X86-64, Intel-IOMMU]
 			on	enable Interrupt Remapping (default)
diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index 1bb62ca..2654e13 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -866,6 +866,7 @@ static struct cpufreq_driver intel_pstate_driver = {
 };
 
 static int __initdata no_load;
+static unsigned int  force_load;
 
 static int intel_pstate_msrs_not_valid(void)
 {
@@ -1003,7 +1004,8 @@ 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);
 			}
 	}
 
@@ -1078,6 +1080,8 @@ static int __init intel_pstate_setup(char *str)
 
 	if (!strcmp(str, "disable"))
 		no_load = 1;
+	if (!strcmp(str, "force"))
+		force_load = 1;
 	return 0;
 }
 early_param("intel_pstate", intel_pstate_setup);
-- 
1.8.3.1