ACPI: CPPC: Introduce cppc_get_nominal_perf()

On some systems the nominal_perf value retrieved via CPPC is just
a constant and fetching it doesn't require accessing any registers,
so if it is the only CPPC capability that's needed, it is wasteful
to run cppc_get_perf_caps() in order to get just that value alone,
especially when this is done for CPUs other than the one running
the code.

For this reason, introduce cppc_get_nominal_perf() allowing
nominal_perf to be obtained individually, by generalizing the
existing cppc_get_desired_perf() (and renaming it) so it can be
used to retrieve any specific CPPC capability value.

While at it, clean up the cppc_get_desired_perf() kerneldoc comment
a bit.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/cppc_acpi.c

@@ -1008,23 +1008,14 @@ static int cpc_write(int cpu, struct cpc_register_resource *reg_res, u64 val)
 	return ret_val;
 }
 
-/**
- * cppc_get_desired_perf - Get the value of desired performance register.
- * @cpunum: CPU from which to get desired performance.
- * @desired_perf: address of a variable to store the returned desired performance
- *
- * Return: 0 for success, -EIO otherwise.
- */
-int cppc_get_desired_perf(int cpunum, u64 *desired_perf)
+static int cppc_get_perf(int cpunum, enum cppc_regs reg_idx, u64 *perf)
 {
 	struct cpc_desc *cpc_desc = per_cpu(cpc_desc_ptr, cpunum);
-	int pcc_ss_id = per_cpu(cpu_pcc_subspace_idx, cpunum);
-	struct cpc_register_resource *desired_reg;
-	struct cppc_pcc_data *pcc_ss_data = NULL;
-
-	desired_reg = &cpc_desc->cpc_regs[DESIRED_PERF];
+	struct cpc_register_resource *reg = &cpc_desc->cpc_regs[reg_idx];
 
-	if (CPC_IN_PCC(desired_reg)) {
+	if (CPC_IN_PCC(reg)) {
+		int pcc_ss_id = per_cpu(cpu_pcc_subspace_idx, cpunum);
+		struct cppc_pcc_data *pcc_ss_data = NULL;
 		int ret = 0;
 
 		if (pcc_ss_id < 0)
@@ -1035,7 +1026,7 @@ int cppc_get_desired_perf(int cpunum, u64 *desired_perf)
 		down_write(&pcc_ss_data->pcc_lock);
 
 		if (send_pcc_cmd(pcc_ss_id, CMD_READ) >= 0)
-			cpc_read(cpunum, desired_reg, desired_perf);
+			cpc_read(cpunum, reg, perf);
 		else
 			ret = -EIO;
 
@@ -1044,12 +1035,36 @@ int cppc_get_desired_perf(int cpunum, u64 *desired_perf)
 		return ret;
 	}
 
-	cpc_read(cpunum, desired_reg, desired_perf);
+	cpc_read(cpunum, reg, perf);
 
 	return 0;
 }
+
+/**
+ * cppc_get_desired_perf - Get the desired performance register value.
+ * @cpunum: CPU from which to get desired performance.
+ * @desired_perf: Return address.
+ *
+ * Return: 0 for success, -EIO otherwise.
+ */
+int cppc_get_desired_perf(int cpunum, u64 *desired_perf)
+{
+	return cppc_get_perf(cpunum, DESIRED_PERF, desired_perf);
+}
 EXPORT_SYMBOL_GPL(cppc_get_desired_perf);
 
+/**
+ * cppc_get_nominal_perf - Get the nominal performance register value.
+ * @cpunum: CPU from which to get nominal performance.
+ * @nominal_perf: Return address.
+ *
+ * Return: 0 for success, -EIO otherwise.
+ */
+int cppc_get_nominal_perf(int cpunum, u64 *nominal_perf)
+{
+	return cppc_get_perf(cpunum, NOMINAL_PERF, nominal_perf);
+}
+
 /**
  * cppc_get_perf_caps - Get a CPU's performance capabilities.
  * @cpunum: CPU from which to get capabilities info.

include/acpi/cppc_acpi.h

@@ -135,6 +135,7 @@ struct cppc_cpudata {
 
 #ifdef CONFIG_ACPI_CPPC_LIB
 extern int cppc_get_desired_perf(int cpunum, u64 *desired_perf);
+extern int cppc_get_nominal_perf(int cpunum, u64 *nominal_perf);
 extern int cppc_get_perf_ctrs(int cpu, struct cppc_perf_fb_ctrs *perf_fb_ctrs);
 extern int cppc_set_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls);
 extern int cppc_get_perf_caps(int cpu, struct cppc_perf_caps *caps);
@@ -149,6 +150,10 @@ static inline int cppc_get_desired_perf(int cpunum, u64 *desired_perf)
 {
 	return -ENOTSUPP;
 }
+static inline int cppc_get_nominal_perf(int cpunum, u64 *nominal_perf)
+{
+	return -ENOTSUPP;
+}
 static inline int cppc_get_perf_ctrs(int cpu, struct cppc_perf_fb_ctrs *perf_fb_ctrs)
 {
 	return -ENOTSUPP;