cpuidle,intel_idle: Fix CPUIDLE_FLAG_IRQ_ENABLE

Commit c227233 ("intel_idle: enable interrupts before C1 on
Xeons") wrecked intel_idle in two ways:

 - must not have tracing in idle functions
 - must return with IRQs disabled

Additionally, it added a branch for no good reason.

Fixes: c227233 ("intel_idle: enable interrupts before C1 on Xeons")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
[ rjw: Moved the intel_idle() kerneldoc comment next to the function ]
Cc: 5.16+ <stable@vger.kernel.org> # 5.16+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: Peter Zijlstra

drivers/idle/intel_idle.c

@@ -115,6 +115,18 @@ static unsigned int mwait_substates __initdata;
 #define flg2MWAIT(flags) (((flags) >> 24) & 0xFF)
 #define MWAIT2flg(eax) ((eax & 0xFF) << 24)
 
+static __always_inline int __intel_idle(struct cpuidle_device *dev,
+					struct cpuidle_driver *drv, int index)
+{
+	struct cpuidle_state *state = &drv->states[index];
+	unsigned long eax = flg2MWAIT(state->flags);
+	unsigned long ecx = 1; /* break on interrupt flag */
+
+	mwait_idle_with_hints(eax, ecx);
+
+	return index;
+}
+
 /**
  * intel_idle - Ask the processor to enter the given idle state.
  * @dev: cpuidle device of the target CPU.
@@ -132,16 +144,19 @@ static unsigned int mwait_substates __initdata;
 static __cpuidle int intel_idle(struct cpuidle_device *dev,
 				struct cpuidle_driver *drv, int index)
 {
-	struct cpuidle_state *state = &drv->states[index];
-	unsigned long eax = flg2MWAIT(state->flags);
-	unsigned long ecx = 1; /* break on interrupt flag */
+	return __intel_idle(dev, drv, index);
+}
 
-	if (state->flags & CPUIDLE_FLAG_IRQ_ENABLE)
-		local_irq_enable();
+static __cpuidle int intel_idle_irq(struct cpuidle_device *dev,
+				    struct cpuidle_driver *drv, int index)
+{
+	int ret;
 
-	mwait_idle_with_hints(eax, ecx);
+	raw_local_irq_enable();
+	ret = __intel_idle(dev, drv, index);
+	raw_local_irq_disable();
 
-	return index;
+	return ret;
 }
 
 /**
@@ -1801,6 +1816,9 @@ static void __init intel_idle_init_cstates_icpu(struct cpuidle_driver *drv)
 		/* Structure copy. */
 		drv->states[drv->state_count] = cpuidle_state_table[cstate];
 
+		if (cpuidle_state_table[cstate].flags & CPUIDLE_FLAG_IRQ_ENABLE)
+			drv->states[drv->state_count].enter = intel_idle_irq;
+
 		if ((disabled_states_mask & BIT(drv->state_count)) ||
 		    ((icpu->use_acpi || force_use_acpi) &&
 		     intel_idle_off_by_default(mwait_hint) &&