[PATCH v4 2/3] rtc: implement a sysfs interface for clock offset

[Joshua Clayton]

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 <stillcompil...@gmail.com>
---
 Documentation/rtc.txt   |  6 ++++++
 drivers/rtc/rtc-sysfs.c | 35 ++++++++++++++++++++++++++++++++++-
 2 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/Documentation/rtc.txt b/Documentation/rtc.txt
index 8446f1e..ddc3660 100644
--- a/Documentation/rtc.txt
+++ b/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
 ---------------
diff --git a/drivers/rtc/rtc-sysfs.c b/drivers/rtc/rtc-sysfs.c
index 463e286..63b9fb1 100644
--- a/drivers/rtc/rtc-sysfs.c
+++ b/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;
 }
-- 
2.5.0

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