[PATCH v2 2/3] cpufreq: amd-pstate: Allow replacing acpi-cpufreq when loaded

[Mario Limonciello <mario.limonciello@amd.com>]

`amd-pstate` can be compiled as a module.  This however can be a
deficiency because `acpi-cpufreq` will be loaded earlier when compiled
into the kernel meaning `amd-pstate` doesn't get a chance.
`acpi-cpufreq` is also unable to be unloaded in this circumstance.

To better improve the usability of `amd-pstate` when compiled as a module,
add an optional module parameter that will force it to replace other
cpufreq drivers at startup.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
v1->v2:
 * Update to changes from v1.
 * Verify the driver being matched is acpi-cpufreq.
 * Show a message letting users know they can use amd-pstate.

 drivers/cpufreq/amd-pstate.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
index 7be38bc6a673..7dc2e344f222 100644
--- a/drivers/cpufreq/amd-pstate.c
+++ b/drivers/cpufreq/amd-pstate.c
@@ -63,6 +63,12 @@ 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 bool replace = false;
+module_param(replace, bool, 0444);
+MODULE_PARM_DESC(replace,
+		  "replace acpi-cpufreq driver upon init if necessary");
+extern void acpi_cpufreq_exit(void);
+
 static struct cpufreq_driver amd_pstate_driver;
 
 /**
@@ -643,6 +649,7 @@ static struct cpufreq_driver amd_pstate_driver = {
 
 static int __init amd_pstate_init(void)
 {
+	const char *current_driver;
 	int ret;
 
 	if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
@@ -653,9 +660,15 @@ static int __init amd_pstate_init(void)
 		return -ENODEV;
 	}
 
-	/* don't keep reloading if cpufreq_driver exists */
-	if (cpufreq_get_current_driver())
-		return -EEXIST;
+	current_driver = cpufreq_get_current_driver();
+	if (current_driver) {
+		if (replace && strcmp(current_driver, "acpi-cpufreq") == 0) {
+			acpi_cpufreq_exit();
+		} else {
+			pr_info("This processor supports amd-pstate, you can enable it with amd_pstate.replace=1\n");
+			return -EEXIST;
+		}
+	}
 
 	/* capability check */
 	if (boot_cpu_has(X86_FEATURE_CPPC)) {
-- 
2.34.1