[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>

3 files changed, 35 insertions, 18 deletions

diff --git a/arch/x86/kernel/cpu/cpufreq/p4-clockmod.c b/arch/x86/kernel/cpu/cpufreq/p4-clockmod.c
index ba3a94a997c3..0c43b2240517 100644
--- a/arch/x86/kernel/cpu/cpufreq/p4-clockmod.c
+++ b/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,
 };
 
 
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 31d6f535a79d..9044b911d8ae 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/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;
         }
diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index 1ee608fd7b77..484b3abf61bb 100644
--- a/include/linux/cpufreq.h
+++ b/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 */