[RFC][PATCH v0.1 3/6] PM: EM: Add special case to em_dev_register_perf_domain()

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Allow em_dev_register_perf_domain() to register a cost-only stub
perf domain with one-element states table if the .active_power()
callback is not provided.

Subsequently, this will be used by the intel_pstate driver to register
stub perf domains for CPUs on hybrid systems.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 kernel/power/energy_model.c |   26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

Index: linux-pm/kernel/power/energy_model.c
===================================================================
--- linux-pm.orig/kernel/power/energy_model.c
+++ linux-pm/kernel/power/energy_model.c
@@ -426,9 +426,11 @@ static int em_create_pd(struct device *d
 	if (!em_table)
 		goto free_pd;
 
-	ret = em_create_perf_table(dev, pd, em_table->state, cb, flags);
-	if (ret)
-		goto free_pd_table;
+	if (cb->active_power) {
+		ret = em_create_perf_table(dev, pd, em_table->state, cb, flags);
+		if (ret)
+			goto free_pd_table;
+	}
 
 	ret = em_compute_costs(dev, em_table->state, cb, nr_states, flags);
 	if (ret)
@@ -561,11 +563,20 @@ int em_dev_register_perf_domain(struct d
 {
 	unsigned long cap, prev_cap = 0;
 	unsigned long flags = 0;
+	bool stub_pd = false;
 	int cpu, ret;
 
 	if (!dev || !nr_states || !cb)
 		return -EINVAL;
 
+	if (!cb->active_power) {
+		if (!cb->get_cost || nr_states > 1 || microwatts)
+			return -EINVAL;
+
+		/* Special case: a stub perf domain. */
+		stub_pd = true;
+	}
+
 	/*
 	 * Use a mutex to serialize the registration of performance domains and
 	 * let the driver-defined callback functions sleep.
@@ -590,6 +601,15 @@ int em_dev_register_perf_domain(struct d
 				ret = -EEXIST;
 				goto unlock;
 			}
+
+			/*
+			 * The capacity need not be the same for all CPUs in a
+			 * stub perf domain, so long as the average cost of
+			 * running on each of them is approximately the same.
+			 */
+			if (stub_pd)
+				continue;
+
 			/*
 			 * All CPUs of a domain must have the same
 			 * micro-architecture since they all share the same