[PATCH 4/6] cpufreq: amd-pstate: add amd-pstate driver parameter for mode selection

[Paolo Pisati]

From: Perry Yuan <Perry.Yuan at amd.com>

BugLink: https://bugs.launchpad.net/bugs/1998106

When the amd_pstate driver is built-in users still need a method to be
able enable or disable it depending upon their circumstance.
Add support for an early parameter to do this.

There is some performance degradation on a number of ASICs in the
passive mode. This performance issue was originally discovered in
shared memory systems but it has been proven that certain workloads
on MSR systems also suffer performance issues.
Set the amd-pstate driver as disabled by default to temporarily
mitigate the performance problem.

 1) with `amd_pstate=disable`, pstate driver will be disabled to load at
    kernel booting.

 2) with `amd_pstate=passive`, pstate driver will be enabled and loaded
    as non-autonomous working mode supported in the low-level power
    management firmware.

 3) If neither parameter is specified, the driver will be disabled by
    default to avoid triggering performance regressions in certain ASICs

Acked-by: Huang Rui <ray.huang at amd.com>
Reviewed-by: Gautham R. Shenoy <gautham.shenoy at amd.com>
Tested-by: Wyes Karny <wyes.karny at amd.com>
Signed-off-by: Perry Yuan <Perry.Yuan at amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki at intel.com>
(cherry picked from commit 202e683df37cdf4c38e06e56ac91cc170ef49058)
Signed-off-by: Paolo Pisati <paolo.pisati at canonical.com>
---
 drivers/cpufreq/amd-pstate.c | 36 +++++++++++++++++++++++++++---------
 1 file changed, 27 insertions(+), 9 deletions(-)

diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
index 5b788492ce5e..3101a0c47a76 100644
--- a/drivers/cpufreq/amd-pstate.c
+++ b/drivers/cpufreq/amd-pstate.c
@@ -58,12 +58,8 @@
  * we disable it by default to go acpi-cpufreq on these processors and add a
  * module parameter to be able to enable it manually for debugging.
  */
-static bool shared_mem = false;
-module_param(shared_mem, bool, 0444);
-MODULE_PARM_DESC(shared_mem,
-		 "enable amd-pstate on processors with shared memory solution (false = disabled (default), true = enabled)");
-
 static struct cpufreq_driver amd_pstate_driver;
+static int cppc_load __initdata;
 
 /**
  * struct  amd_aperf_mperf
@@ -691,6 +687,15 @@ static int __init amd_pstate_init(void)
 
 	if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
 		return -ENODEV;
+	/*
+	 * by default the pstate driver is disabled to load
+	 * enable the amd_pstate passive mode driver explicitly
+	 * with amd_pstate=passive in kernel command line
+	 */
+	if (!cppc_load) {
+		pr_debug("driver load is disabled, boot with amd_pstate=passive to enable this\n");
+		return -ENODEV;
+	}
 
 	if (!acpi_cpc_valid()) {
 		pr_debug("the _CPC object is not present in SBIOS\n");
@@ -705,13 +710,11 @@ static int __init amd_pstate_init(void)
 	if (boot_cpu_has(X86_FEATURE_CPPC)) {
 		pr_debug("AMD CPPC MSR based functionality is supported\n");
 		amd_pstate_driver.adjust_perf = amd_pstate_adjust_perf;
-	} else if (shared_mem) {
+	} else {
+		pr_debug("AMD CPPC shared memory based functionality is supported\n");
 		static_call_update(amd_pstate_enable, cppc_enable);
 		static_call_update(amd_pstate_init_perf, cppc_init_perf);
 		static_call_update(amd_pstate_update_perf, cppc_update_perf);
-	} else {
-		pr_info("This processor supports shared memory solution, you can enable it with amd_pstate.shared_mem=1\n");
-		return -ENODEV;
 	}
 
 	/* enable amd pstate feature */
@@ -730,6 +733,21 @@ static int __init amd_pstate_init(void)
 }
 device_initcall(amd_pstate_init);
 
+static int __init amd_pstate_param(char *str)
+{
+	if (!str)
+		return -EINVAL;
+
+	if (!strcmp(str, "disable")) {
+		cppc_load = 0;
+		pr_info("driver is explicitly disabled\n");
+	} else if (!strcmp(str, "passive"))
+		cppc_load = 1;
+
+	return 0;
+}
+early_param("amd_pstate", amd_pstate_param);
+
 MODULE_AUTHOR("Huang Rui <ray.huang at amd.com>");
 MODULE_DESCRIPTION("AMD Processor P-state Frequency Driver");
 MODULE_LICENSE("GPL");
-- 
2.34.1