clockevents: Implement unbind functionality

Provide a sysfs interface to allow unbinding of clockevent
devices. The device is unbound if it is unused or if there is a
replacement device available. Unbinding of broadcast devices is not
supported as we don't want to foster that nonsense. If no replacement
device is available the unbind returns -EBUSY. Unbind is available
from the kernel and through sysfs, which is necessary to drop the
module refcount.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Magnus Damm <magnus.damm@gmail.com>
Link: http://lkml.kernel.org/r/20130425143436.499216659@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 125 insertions, 0 deletions

diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index 0a23f4f29934..38959c866789 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -25,6 +25,13 @@ static LIST_HEAD(clockevent_devices);
 static LIST_HEAD(clockevents_released);
 /* Protection for the above */
 static DEFINE_RAW_SPINLOCK(clockevents_lock);
+/* Protection for unbind operations */
+static DEFINE_MUTEX(clockevents_mutex);
+
+struct ce_unbind {
+        struct clock_event_device *ce;
+        int res;
+};
 
 /**
  * clockevents_delta2ns - Convert a latch value (device ticks) to nanoseconds
@@ -245,6 +252,90 @@ static void clockevents_notify_released(void)
         }
 }
 
+/*
+ * Try to install a replacement clock event device
+ */
+static int clockevents_replace(struct clock_event_device *ced)
+{
+        struct clock_event_device *dev, *newdev = NULL;
+
+        list_for_each_entry(dev, &clockevent_devices, list) {
+                if (dev == ced || dev->mode != CLOCK_EVT_MODE_UNUSED)
+                        continue;
+
+                if (!tick_check_replacement(newdev, dev))
+                        continue;
+
+                if (!try_module_get(dev->owner))
+                        continue;
+
+                if (newdev)
+                        module_put(newdev->owner);
+                newdev = dev;
+        }
+        if (newdev) {
+                tick_install_replacement(newdev);
+                list_del_init(&ced->list);
+        }
+        return newdev ? 0 : -EBUSY;
+}
+
+/*
+ * Called with clockevents_mutex and clockevents_lock held
+ */
+static int __clockevents_try_unbind(struct clock_event_device *ced, int cpu)
+{
+        /* Fast track. Device is unused */
+        if (ced->mode == CLOCK_EVT_MODE_UNUSED) {
+                list_del_init(&ced->list);
+                return 0;
+        }
+
+        return ced == per_cpu(tick_cpu_device, cpu).evtdev ? -EAGAIN : -EBUSY;
+}
+
+/*
+ * SMP function call to unbind a device
+ */
+static void __clockevents_unbind(void *arg)
+{
+        struct ce_unbind *cu = arg;
+        int res;
+
+        raw_spin_lock(&clockevents_lock);
+        res = __clockevents_try_unbind(cu->ce, smp_processor_id());
+        if (res == -EAGAIN)
+                res = clockevents_replace(cu->ce);
+        cu->res = res;
+        raw_spin_unlock(&clockevents_lock);
+}
+
+/*
+ * Issues smp function call to unbind a per cpu device. Called with
+ * clockevents_mutex held.
+ */
+static int clockevents_unbind(struct clock_event_device *ced, int cpu)
+{
+        struct ce_unbind cu = { .ce = ced, .res = -ENODEV };
+
+        smp_call_function_single(cpu, __clockevents_unbind, &cu, 1);
+        return cu.res;
+}
+
+/*
+ * Unbind a clockevents device.
+ */
+int clockevents_unbind_device(struct clock_event_device *ced, int cpu)
+{
+        int ret;
+
+        mutex_lock(&clockevents_mutex);
+        ret = clockevents_unbind(ced, cpu);
+        mutex_unlock(&clockevents_mutex);
+        return ret;
+}
+EXPORT_SYMBOL_GPL(clockevents_unbind);
+
 /**
  * clockevents_register_device - register a clock event device
  * @dev:        device to register
@@ -487,6 +578,38 @@ static ssize_t sysfs_show_current_tick_dev(struct device *dev,
 }
 static DEVICE_ATTR(current_device, 0444, sysfs_show_current_tick_dev, NULL);
 
+/* We don't support the abomination of removable broadcast devices */
+static ssize_t sysfs_unbind_tick_dev(struct device *dev,
+                                     struct device_attribute *attr,
+                                     const char *buf, size_t count)
+{
+        char name[CS_NAME_LEN];
+        size_t ret = sysfs_get_uname(buf, name, count);
+        struct clock_event_device *ce;
+
+        if (ret < 0)
+                return ret;
+
+        ret = -ENODEV;
+        mutex_lock(&clockevents_mutex);
+        raw_spin_lock_irq(&clockevents_lock);
+        list_for_each_entry(ce, &clockevent_devices, list) {
+                if (!strcmp(ce->name, name)) {
+                        ret = __clockevents_try_unbind(ce, dev->id);
+                        break;
+                }
+        }
+        raw_spin_unlock_irq(&clockevents_lock);
+        /*
+         * We hold clockevents_mutex, so ce can't go away
+         */
+        if (ret == -EAGAIN)
+                ret = clockevents_unbind(ce, dev->id);
+        mutex_unlock(&clockevents_mutex);
+        return ret ? ret : count;
+}
+static DEVICE_ATTR(unbind_device, 0200, NULL, sysfs_unbind_tick_dev);
+
 #ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
 static struct device tick_bc_dev = {
         .init_name      = "broadcast",
@@ -529,6 +652,8 @@ static int __init tick_init_sysfs(void)
                 err = device_register(dev);
                 if (!err)
                         err = device_create_file(dev, &dev_attr_current_device);
+                if (!err)
+                        err = device_create_file(dev, &dev_attr_unbind_device);
                 if (err)
                         return err;
         }