cpufreq: intel_pstate: Fix active mode offline/online EPP handling

After commit 4adcf2e582 ("cpufreq: intel_pstate: Add ->offline and
->online callbacks") the EPP value set by the "performance" scaling
algorithm in the active mode is not restored after an offline/online
cycle which replaces it with the saved EPP value coming from user
space.

Address this issue by forcing intel_pstate_hwp_set() to set a new
EPP value when it runs first time after online.

Fixes: 4adcf2e582 ("cpufreq: intel_pstate: Add ->offline and ->online callbacks")
Link: https://lore.kernel.org/linux-pm/[email protected]/
Reported-by: Srinivas Pandruvada <[email protected]>
Cc: 5.9+ <[email protected]> # 5.9+
Signed-off-by: Rafael J. Wysocki <[email protected]>

drivers/cpufreq/intel_pstate.c

@@ -1006,6 +1006,12 @@ static void intel_pstate_hwp_offline(struct cpudata *cpu)
 		 */
 		value &= ~GENMASK_ULL(31, 24);
 		value |= HWP_ENERGY_PERF_PREFERENCE(cpu->epp_cached);
+		/*
+		 * However, make sure that EPP will be set to "performance" when
+		 * the CPU is brought back online again and the "performance"
+		 * scaling algorithm is still in effect.
+		 */
+		cpu->epp_policy = CPUFREQ_POLICY_UNKNOWN;
 	}
 
 	/*