cpufreq: intel_pstate: Refuse to turn off with HWP enabled

After commit f6ebbcf0 ("cpufreq: intel_pstate: Implement passive
mode with HWP enabled") it is possible to change the driver status
to "off" via sysfs with HWP enabled, which effectively causes the
driver to unregister itself, but HWP remains active and it forces the
minimum performance, so even if another cpufreq driver is loaded,
it will not be able to control the CPU frequency.

For this reason, make the driver refuse to change the status to
"off" with HWP enabled.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>

drivers/cpufreq/intel_pstate.c

@@ -2716,9 +2716,15 @@ static int intel_pstate_update_status(const char *buf, size_t size)
 {
 	int ret;
 
-	if (size == 3 && !strncmp(buf, "off", size))
-		return intel_pstate_driver ?
-			intel_pstate_unregister_driver() : -EINVAL;
+	if (size == 3 && !strncmp(buf, "off", size)) {
+		if (!intel_pstate_driver)
+			return -EINVAL;
+
+		if (hwp_active)
+			return -EBUSY;
+
+		return intel_pstate_unregister_driver();
+	}
 
 	if (size == 6 && !strncmp(buf, "active", size)) {
 		if (intel_pstate_driver) {