amd-pstate: Fix amd_pstate mode switch

amd_pstate mode can be changed by writing the mode name to the `status`
sysfs. But some combinations are not working. Fix this issue by taking
care of the edge cases.

Before the fix the mode change combination test fails:

 #./pst_test.sh
Test passed: from: disable, to
Test passed: from: disable, to disable
Test failed: 1, From mode: disable, to mode: passive
Test failed: 1, From mode: disable, to mode: active
Test failed: 1, From mode: passive, to mode: active
Test passed: from: passive, to disable
Test failed: 1, From mode: passive, to mode: passive
Test failed: 1, From mode: passive, to mode: active
Test failed: 1, From mode: active, to mode: active
Test passed: from: active, to disable
Test failed: 1, From mode: active, to mode: passive
Test failed: 1, From mode: active, to mode: active

After the fix test passes:

 #./pst_test.sh
Test passed: from: disable, to
Test passed: from: disable, to disable
Test passed: from: disable, to passive
Test passed: from: disable, to active
Test passed: from: passive, to active
Test passed: from: passive, to disable
Test passed: from: passive, to passive
Test passed: from: passive, to active
Test passed: from: active, to active
Test passed: from: active, to disable
Test passed: from: active, to passive
Test passed: from: active, to active

Fixes: abd61c0 ("cpufreq: amd-pstate: add driver working mode switch support")
Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alexey Kardashevskiy <aik@amd.com>
Signed-off-by: Wyes Karny <wyes.karny@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpufreq/amd-pstate.c

@@ -840,33 +840,31 @@ static int amd_pstate_update_status(const char *buf, size_t size)
 
 	switch(mode_idx) {
 	case AMD_PSTATE_DISABLE:
-		if (!current_pstate_driver)
-			return -EINVAL;
-		if (cppc_state == AMD_PSTATE_ACTIVE)
-			return -EBUSY;
-		cpufreq_unregister_driver(current_pstate_driver);
-		amd_pstate_driver_cleanup();
+		if (current_pstate_driver) {
+			cpufreq_unregister_driver(current_pstate_driver);
+			amd_pstate_driver_cleanup();
+		}
 		break;
 	case AMD_PSTATE_PASSIVE:
 		if (current_pstate_driver) {
 			if (current_pstate_driver == &amd_pstate_driver)
 				return 0;
 			cpufreq_unregister_driver(current_pstate_driver);
-			cppc_state = AMD_PSTATE_PASSIVE;
-			current_pstate_driver = &amd_pstate_driver;
 		}
 
+		current_pstate_driver = &amd_pstate_driver;
+		cppc_state = AMD_PSTATE_PASSIVE;
 		ret = cpufreq_register_driver(current_pstate_driver);
 		break;
 	case AMD_PSTATE_ACTIVE:
 		if (current_pstate_driver) {
 			if (current_pstate_driver == &amd_pstate_epp_driver)
 				return 0;
 			cpufreq_unregister_driver(current_pstate_driver);
-			current_pstate_driver = &amd_pstate_epp_driver;
-			cppc_state = AMD_PSTATE_ACTIVE;
 		}
 
+		current_pstate_driver = &amd_pstate_epp_driver;
+		cppc_state = AMD_PSTATE_ACTIVE;
 		ret = cpufreq_register_driver(current_pstate_driver);
 		break;
 	default: