3 files changed, 35 insertions, 7 deletions

diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c
index ab455ddb16cf..ff7539a4dbea 100644
--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c
@@ -472,10 +472,22 @@ static struct cmos_rtc	cmos_rtc;
 static irqreturn_t cmos_interrupt(int irq, void *p)
 {
         u8              irqstat;
+        u8              rtc_control;
 
         spin_lock(&rtc_lock);
         irqstat = CMOS_READ(RTC_INTR_FLAGS);
-        irqstat &= (CMOS_READ(RTC_CONTROL) & RTC_IRQMASK) | RTC_IRQF;
+        rtc_control = CMOS_READ(RTC_CONTROL);
+        irqstat &= (rtc_control & RTC_IRQMASK) | RTC_IRQF;
+
+        /* All Linux RTC alarms should be treated as if they were oneshot.
+         * Similar code may be needed in system wakeup paths, in case the
+         * alarm woke the system.
+         */
+        if (irqstat & RTC_AIE) {
+                rtc_control &= ~RTC_AIE;
+                CMOS_WRITE(rtc_control, RTC_CONTROL);
+                CMOS_READ(RTC_INTR_FLAGS);
+        }
         spin_unlock(&rtc_lock);
 
         if (is_intr(irqstat)) {
diff --git a/drivers/rtc/rtc-dev.c b/drivers/rtc/rtc-dev.c
index 025c60a17a4a..90dfa0df747a 100644
--- a/drivers/rtc/rtc-dev.c
+++ b/drivers/rtc/rtc-dev.c
@@ -246,6 +246,15 @@ static int rtc_dev_ioctl(struct inode *inode, struct file *file,
         /* if the driver does not provide the ioctl interface
          * or if that particular ioctl was not implemented
          * (-ENOIOCTLCMD), we will try to emulate here.
+         *
+         * Drivers *SHOULD NOT* provide ioctl implementations
+         * for these requests.  Instead, provide methods to
+         * support the following code, so that the RTC's main
+         * features are accessible without using ioctls.
+         *
+         * RTC and alarm times will be in UTC, by preference,
+         * but dual-booting with MS-Windows implies RTCs must
+         * use the local wall clock time.
          */
 
         switch (cmd) {
diff --git a/drivers/rtc/rtc-sysfs.c b/drivers/rtc/rtc-sysfs.c
index 2ae0e8304d3a..4d27ccc4fc06 100644
--- a/drivers/rtc/rtc-sysfs.c
+++ b/drivers/rtc/rtc-sysfs.c
@@ -17,6 +17,13 @@
 
 /* device attributes */
 
+/*
+ * NOTE:  RTC times displayed in sysfs use the RTC's timezone.  That's
+ * ideally UTC.  However, PCs that also boot to MS-Windows normally use
+ * the local time and change to match daylight savings time.  That affects
+ * attributes including date, time, since_epoch, and wakealarm.
+ */
+
 static ssize_t
 rtc_sysfs_show_name(struct device *dev, struct device_attribute *attr,
                 char *buf)
@@ -113,13 +120,13 @@ rtc_sysfs_show_wakealarm(struct device *dev, struct device_attribute *attr,
         unsigned long alarm;
         struct rtc_wkalrm alm;
 
-        /* Don't show disabled alarms; but the RTC could leave the
-         * alarm enabled after it's already triggered.  Alarms are
-         * conceptually one-shot, even though some common hardware
-         * (PCs) doesn't actually work that way.
+        /* Don't show disabled alarms.  For uniformity, RTC alarms are
+         * conceptually one-shot, even though some common RTCs (on PCs)
+         * don't actually work that way.
          *
-         * REVISIT maybe we should require RTC implementations to
-         * disable the RTC alarm after it triggers, for uniformity.
+         * NOTE: RTC implementations where the alarm doesn't match an
+         * exact YYYY-MM-DD HH:MM[:SS] date *must* disable their RTC
+         * alarms after they trigger, to ensure one-shot semantics.
          */
         retval = rtc_read_alarm(to_rtc_device(dev), &alm);
         if (retval == 0 && alm.enabled) {