rtc: implement a sysfs interface for clock offset

clock offset may be set and read in decimal parts per billion
attribute is /sys/class/rtc/rtcN/offset
The attribute is only visible for rtcs that have set_offset implemented.
Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/rtc.txt

@@ -157,6 +157,12 @@ wakealarm:	 The time at which the clock will generate a system wakeup
 		 the epoch by default, or if there's a leading +, seconds in the
 		 future, or if there is a leading +=, seconds ahead of the current
 		 alarm.
+offset:		 The amount which the rtc clock has been adjusted in firmware.
+		 Visible only if the driver supports clock offset adjustment.
+		 The unit is parts per billion, i.e. The number of clock ticks
+		 which are added to or removed from the rtc's base clock per
+		 billion ticks. A positive value makes a day pass more slowly,
+		 longer, and a negative value makes a day pass more quickly.
 
 IOCTL INTERFACE
 ---------------

drivers/rtc/rtc-sysfs.c

@@ -218,6 +218,34 @@ wakealarm_store(struct device *dev, struct device_attribute *attr,
 }
 static DEVICE_ATTR_RW(wakealarm);
 
+static ssize_t
+offset_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	ssize_t retval;
+	long offset;
+
+	retval = rtc_read_offset(to_rtc_device(dev), &offset);
+	if (retval == 0)
+		retval = sprintf(buf, "%ld\n", offset);
+
+	return retval;
+}
+
+static ssize_t
+offset_store(struct device *dev, struct device_attribute *attr,
+	     const char *buf, size_t n)
+{
+	ssize_t retval;
+	long offset;
+
+	retval = kstrtol(buf, 10, &offset);
+	if (retval == 0)
+		retval = rtc_set_offset(to_rtc_device(dev), offset);
+
+	return (retval < 0) ? retval : n;
+}
+static DEVICE_ATTR_RW(offset);
+
 static struct attribute *rtc_attrs[] = {
 	&dev_attr_name.attr,
 	&dev_attr_date.attr,
@@ -226,6 +254,7 @@ static struct attribute *rtc_attrs[] = {
 	&dev_attr_max_user_freq.attr,
 	&dev_attr_hctosys.attr,
 	&dev_attr_wakealarm.attr,
+	&dev_attr_offset.attr,
 	NULL,
 };
 
@@ -249,9 +278,13 @@ static umode_t rtc_attr_is_visible(struct kobject *kobj,
 	struct rtc_device *rtc = to_rtc_device(dev);
 	umode_t mode = attr->mode;
 
-	if (attr == &dev_attr_wakealarm.attr)
+	if (attr == &dev_attr_wakealarm.attr) {
 		if (!rtc_does_wakealarm(rtc))
 			mode = 0;
+	} else if (attr == &dev_attr_offset.attr) {
+		if (!rtc->ops->set_offset)
+			mode = 0;
+	}
 
 	return mode;
 }