[CPUFREQ] Disable sysfs ui for p4-clockmod.

p4-clockmod has a long history of abuse.   It pretends to be a CPU
frequency scaling driver, even though it doesn't actually change
the CPU frequency, but instead just modulates the frequency with
wait-states.
The biggest misconception is that when running at the lower 'frequency'
p4-clockmod is saving power.  This isn't the case, as workloads running
slower take longer to complete, preventing the CPU from entering deep C states.

However p4-clockmod does have a purpose.  It can prevent overheating.
Having it hooked up to the cpufreq interfaces is the wrong way to achieve
cooling however. It should instead be hooked up to ACPI.

This diff introduces a means for a cpufreq driver to register with the
cpufreq core, but not present a sysfs interface.
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Dave Jones <davej@redhat.com>

arch/x86/kernel/cpu/cpufreq/p4-clockmod.c

@@ -276,6 +276,7 @@ static struct cpufreq_driver p4clockmod_driver = {
 	.name		= "p4-clockmod",
 	.owner		= THIS_MODULE,
 	.attr		= p4clockmod_attr,
+	.hide_interface	= 1,
 };
 
 

drivers/cpufreq/cpufreq.c

@@ -754,6 +754,11 @@ static struct kobj_type ktype_cpufreq = {
 	.release	= cpufreq_sysfs_release,
 };
 
+static struct kobj_type ktype_empty_cpufreq = {
+	.sysfs_ops	= &sysfs_ops,
+	.release	= cpufreq_sysfs_release,
+};
+
 
 /**
  * cpufreq_add_dev - add a CPU device
@@ -876,26 +881,36 @@ static int cpufreq_add_dev(struct sys_device *sys_dev)
 	memcpy(&new_policy, policy, sizeof(struct cpufreq_policy));
 
 	/* prepare interface data */
-	ret = kobject_init_and_add(&policy->kobj, &ktype_cpufreq, &sys_dev->kobj,
-				   "cpufreq");
-	if (ret)
-		goto err_out_driver_exit;
-
-	/* set up files for this cpu device */
-	drv_attr = cpufreq_driver->attr;
-	while ((drv_attr) && (*drv_attr)) {
-		ret = sysfs_create_file(&policy->kobj, &((*drv_attr)->attr));
+	if (!cpufreq_driver->hide_interface) {
+		ret = kobject_init_and_add(&policy->kobj, &ktype_cpufreq,
+					   &sys_dev->kobj, "cpufreq");
 		if (ret)
 			goto err_out_driver_exit;
-		drv_attr++;
-	}
-	if (cpufreq_driver->get) {
-		ret = sysfs_create_file(&policy->kobj, &cpuinfo_cur_freq.attr);
-		if (ret)
-			goto err_out_driver_exit;
-	}
-	if (cpufreq_driver->target) {
-		ret = sysfs_create_file(&policy->kobj, &scaling_cur_freq.attr);
+
+		/* set up files for this cpu device */
+		drv_attr = cpufreq_driver->attr;
+		while ((drv_attr) && (*drv_attr)) {
+			ret = sysfs_create_file(&policy->kobj,
+						&((*drv_attr)->attr));
+			if (ret)
+				goto err_out_driver_exit;
+			drv_attr++;
+		}
+		if (cpufreq_driver->get) {
+			ret = sysfs_create_file(&policy->kobj,
+						&cpuinfo_cur_freq.attr);
+			if (ret)
+				goto err_out_driver_exit;
+		}
+		if (cpufreq_driver->target) {
+			ret = sysfs_create_file(&policy->kobj,
+						&scaling_cur_freq.attr);
+			if (ret)
+				goto err_out_driver_exit;
+		}
+	} else {
+		ret = kobject_init_and_add(&policy->kobj, &ktype_empty_cpufreq,
+					   &sys_dev->kobj, "cpufreq");
 		if (ret)
 			goto err_out_driver_exit;
 	}

include/linux/cpufreq.h

@@ -234,6 +234,7 @@ struct cpufreq_driver {
 	int	(*suspend)	(struct cpufreq_policy *policy, pm_message_t pmsg);
 	int	(*resume)	(struct cpufreq_policy *policy);
 	struct freq_attr	**attr;
+	bool			hide_interface;
 };
 
 /* flags */