cpufreq: intel_pstate: Use HWP if enabled by platform firmware

It turns out that there are systems where HWP is enabled during
initialization by the platform firmware (BIOS), but HWP EPP support
is not advertised.

After commit 7aa1031 ("cpufreq: intel_pstate: Avoid enabling HWP
if EPP is not supported") intel_pstate refuses to use HWP on those
systems, but the fallback PERF_CTL interface does not work on them
either because of enabled HWP, and once enabled, HWP cannot be
disabled.  Consequently, the users of those systems cannot control
CPU performance scaling.

Address this issue by making intel_pstate use HWP unconditionally if
it is enabled already when the driver starts.

Fixes: 7aa1031 ("cpufreq: intel_pstate: Avoid enabling HWP if EPP is not supported")
Reported-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Tested-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: 5.9+ <stable@vger.kernel.org> # 5.9+

drivers/cpufreq/intel_pstate.c

@@ -3033,6 +3033,14 @@ static const struct x86_cpu_id hwp_support_ids[] __initconst = {
 	{}
 };
 
+static bool intel_pstate_hwp_is_enabled(void)
+{
+	u64 value;
+
+	rdmsrl(MSR_PM_ENABLE, value);
+	return !!(value & 0x1);
+}
+
 static int __init intel_pstate_init(void)
 {
 	const struct x86_cpu_id *id;
@@ -3051,8 +3059,12 @@ static int __init intel_pstate_init(void)
 		 * Avoid enabling HWP for processors without EPP support,
 		 * because that means incomplete HWP implementation which is a
 		 * corner case and supporting it is generally problematic.
+		 *
+		 * If HWP is enabled already, though, there is no choice but to
+		 * deal with it.
 		 */
-		if (!no_hwp && boot_cpu_has(X86_FEATURE_HWP_EPP)) {
+		if ((!no_hwp && boot_cpu_has(X86_FEATURE_HWP_EPP)) ||
+		    intel_pstate_hwp_is_enabled()) {
 			hwp_active++;
 			hwp_mode_bdw = id->driver_data;
 			intel_pstate.attr = hwp_cpufreq_attrs;