[PATCH] alarmtimer: Expose information about next alarm to userspace via sysfs

[Pranav Prasad]

The alarmtimer driver currently fails suspend attempts when there is an
alarm pending within the next 2 seconds, since the system is expected to wake
up soon anyway. The entire suspend process is initiated even though the
system will immediately awaken. This process includes substantial work before
the suspend fails and additional work afterwards to undo the failed suspend
that was attempted. Therefore on battery-powered devices that initiate suspend
attempts from userspace, it may be advantageous to be able to skip the entire
suspend attempt to avoid power consumption instead of unnecessarily trying and
failing. As one data point, an analysis of a subset of Android devices showed that
imminent alarms account for roughly 40% of all suspend failures on average leading
to unnecessary power wastage.

To facilitate this, create a sysfs node for the alarmtimer subsystem to
provide information about the next coming alarm to userspace. When userspace code
reads the sysfs node, the same code flow already used in suspend will run to
provide a value for the time until the next alarm. Userspace code may then opt to
avoid suspend attempts if the next alarm pending is considered "too soon" for
the suspend to be worthwhile.

This mechanism has some limitations that the value readers must take into
account. First, due to any latencies between the calculation of the value and the
reader actually acting upon it (copying the data out to userspace, scheduling
delays, etc), the value will already be "obsolete" by the time the reader can act
upon it. Second, since alarms can be scheduled and canceled at any time, there
is no guarantee that the specific alarm that was "next" during the read
still exists, or that an earlier alarm hasn't been scheduled since the read.
Since no guarantee is provided about the value that was read remaining correct
by the time the reader uses it, consumers should only treat this value as a
hint to influence power optimization decisions rather than as a reliable
prediction of future events.

Signed-off-by: Pranav Prasad <pranavpp@xxxxxxxxxx>
Signed-off-by: Kelly Rossmoyer <krossmo@xxxxxxxxxx>
---
 kernel/time/alarmtimer.c | 131 +++++++++++++++++++++++++++++++--------
 1 file changed, 105 insertions(+), 26 deletions(-)

diff --git a/kernel/time/alarmtimer.c b/kernel/time/alarmtimer.c
index 4657cb8e8b1f..80d576b8c2d7 100644
--- a/kernel/time/alarmtimer.c
+++ b/kernel/time/alarmtimer.c
@@ -27,12 +27,15 @@
 #include <linux/compat.h>
 #include <linux/module.h>
 #include <linux/time_namespace.h>
+#include <linux/sysfs.h>
 
 #include "posix-timers.h"
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/alarmtimer.h>
 
+static const char alarmtimer_group_name[] = "alarmtimer";
+
 /**
  * struct alarm_base - Alarm timer bases
  * @lock:		Lock for syncrhonized access to the base
@@ -63,6 +66,99 @@ static struct rtc_timer		rtctimer;
 static struct rtc_device	*rtcdev;
 static DEFINE_SPINLOCK(rtcdev_lock);
 
+/**
+ * alarmtimer_init_soonest - Initializes parameters to find soonest alarm.
+ * @min: ptr to relative time to the soonest alarm to expire
+ * @expires: ptr to absolute time of the soonest alarm to expire
+ * @type: ptr to alarm type
+ * @reset_freezer_delta: boolean to indicate if freezer_delta should be reset to 0
+ *
+ */
+static void alarmtimer_init_soonest(ktime_t *min, ktime_t *expires, int *type,
+			bool reset_freezer_delta)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&freezer_delta_lock, flags);
+	*min = freezer_delta;
+	*expires = freezer_expires;
+	*type = freezer_alarmtype;
+	if (reset_freezer_delta)
+		freezer_delta = 0;
+	spin_unlock_irqrestore(&freezer_delta_lock, flags);
+}
+
+/**
+ * alarmtimer_get_soonest - Finds the soonest alarm to expire among the alarm bases.
+ * @min: ptr to relative time to the soonest alarm to expire
+ * @expires: ptr to absolute time of the soonest alarm to expire
+ * @type: ptr to alarm type
+ *
+ */
+static void alarmtimer_get_soonest(ktime_t *min, ktime_t *expires, int *type)
+{
+	int i;
+	unsigned long flags;
+
+	/* Find the soonest timer to expire */
+	for (i = 0; i < ALARM_NUMTYPE; i++) {
+		struct alarm_base *base = &alarm_bases[i];
+		struct timerqueue_node *next;
+		ktime_t delta;
+
+		spin_lock_irqsave(&base->lock, flags);
+		next = timerqueue_getnext(&base->timerqueue);
+		spin_unlock_irqrestore(&base->lock, flags);
+		if (!next)
+			continue;
+		delta = ktime_sub(next->expires, base->get_ktime());
+		if (!(*min) || (delta < *min)) {
+			*expires = next->expires;
+			*min = delta;
+			*type = i;
+		}
+	}
+}
+
+static ssize_t next_alarm_show(struct kobject *kobj,
+			struct kobj_attribute *attr, char *buf)
+{
+	ktime_t min, expires;
+	int type;
+
+	/* Init and find the soonest timer to expire */
+	alarmtimer_init_soonest(&min, &expires, &type, false);
+	alarmtimer_get_soonest(&min, &expires, &type);
+
+	return sysfs_emit(buf, "%lld\n", ktime_to_ns(min));
+}
+static struct kobj_attribute next_alarm = __ATTR_RO(next_alarm);
+
+static struct attribute *alarmtimer_attrs[] = {
+	&next_alarm.attr,
+	NULL,
+};
+
+static const struct attribute_group alarmtimer_attr_group = {
+	.name	= alarmtimer_group_name,
+	.attrs	= alarmtimer_attrs,
+};
+
+/**
+ * alarmtimer_sysfs_add - Adds sysfs attributes for alarmtimer
+ *
+ * Returns 0 if successful, non-zero value for error.
+ */
+static int alarmtimer_sysfs_add(void)
+{
+	int ret = sysfs_create_group(kernel_kobj, &alarmtimer_attr_group);
+
+	if (ret)
+		pr_warn("[%s] failed to create a sysfs group\n", __func__);
+
+	return ret;
+}
+
 /**
  * alarmtimer_get_rtcdev - Return selected rtcdevice
  *
@@ -98,8 +194,11 @@ static int alarmtimer_rtc_add_device(struct device *dev)
 
 	pdev = platform_device_register_data(dev, "alarmtimer",
 					     PLATFORM_DEVID_AUTO, NULL, 0);
-	if (!IS_ERR(pdev))
+	if (!IS_ERR(pdev)) {
 		device_init_wakeup(&pdev->dev, true);
+		if (alarmtimer_sysfs_add())
+			pr_warn("[%s] failed to add alarmtimer sysfs attributes\n", __func__);
+	}
 
 	spin_lock_irqsave(&rtcdev_lock, flags);
 	if (!IS_ERR(pdev) && !rtcdev) {
@@ -241,41 +340,21 @@ EXPORT_SYMBOL_GPL(alarm_expires_remaining);
 static int alarmtimer_suspend(struct device *dev)
 {
 	ktime_t min, now, expires;
-	int i, ret, type;
+	int ret, type;
 	struct rtc_device *rtc;
-	unsigned long flags;
 	struct rtc_time tm;
 
-	spin_lock_irqsave(&freezer_delta_lock, flags);
-	min = freezer_delta;
-	expires = freezer_expires;
-	type = freezer_alarmtype;
-	freezer_delta = 0;
-	spin_unlock_irqrestore(&freezer_delta_lock, flags);
+	/* Initialize parameters to find soonest timer */
+	alarmtimer_init_soonest(&min, &expires, &type, true);
 
 	rtc = alarmtimer_get_rtcdev();
 	/* If we have no rtcdev, just return */
 	if (!rtc)
 		return 0;
 
-	/* Find the soonest timer to expire*/
-	for (i = 0; i < ALARM_NUMTYPE; i++) {
-		struct alarm_base *base = &alarm_bases[i];
-		struct timerqueue_node *next;
-		ktime_t delta;
+	/* Find the soonest timer to expire */
+	alarmtimer_get_soonest(&min, &expires, &type);
 
-		spin_lock_irqsave(&base->lock, flags);
-		next = timerqueue_getnext(&base->timerqueue);
-		spin_unlock_irqrestore(&base->lock, flags);
-		if (!next)
-			continue;
-		delta = ktime_sub(next->expires, base->get_ktime());
-		if (!min || (delta < min)) {
-			expires = next->expires;
-			min = delta;
-			type = i;
-		}
-	}
 	if (min == 0)
 		return 0;
 
-- 
2.43.0.381.gb435a96ce8-goog