cpufreq: intel_pstate: Fix fast-switch fallback path

From: Rafael J. Wysocki <[email protected]>

When sugov_update_single_perf() falls back to the "frequency" path due to the missing scale-invariance, it will call cpufreq_driver_fast_switch() via sugov_fast_switch() and the driver's ->fast_switch() callback will be invoked, so it must not be NULL.

However, after commit a365ab6b9dfb ("cpufreq: intel_pstate: Implement the ->adjust_perf() callback") intel_pstate sets ->fast_switch() to NULL when it is going to use intel_cpufreq_adjust_perf(), which is a mistake, because on x86 the scale-invariance may be turned off dynamically, so modify it to retain the original ->adjust_perf() callback pointer.

---
 drivers/cpufreq/intel_pstate.c |    1 -
 1 file changed, 1 deletion(-)

Index: linux-pm/drivers/cpufreq/intel_pstate.c
===================================================================
@@ -3088,7 +3088,6 @@ static int __init intel_pstate_init(void
 			intel_pstate.attr = hwp_cpufreq_attrs;
 			intel_cpufreq.attr = hwp_cpufreq_attrs;
 			intel_cpufreq.flags |= CPUFREQ_NEED_UPDATE_LIMITS;
-			intel_cpufreq.fast_switch = NULL;
 			intel_cpufreq.adjust_perf = intel_cpufreq_adjust_perf;
 			if (!default_driver)
 				default_driver = &intel_pstate;