rtc: Add functions to set and read rtc offset

A number of rtc devices, such as the NXP pcf2123 include a facility
to adjust the clock in order to compensate for temperature or a
crystal, capacitor, etc, that results in the rtc clock not running
at exactly 32.768 kHz.

Data sheets I have seen refer to this as a clock offset, and measure it
in parts per million, however they often reference ppm to 2 digits of
precision, which makes integer ppm less than ideal.

We use parts per billion, which more than covers the precision needed
and works nicely within 32 bits

Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

diff --git a/drivers/rtc/interface.c b/drivers/rtc/interface.c
index 5836751..9ef5f6f 100644 (file)
--- a/drivers/rtc/interface.c
+++ b/drivers/rtc/interface.c
@@ -939,4 +939,58 @@ void rtc_timer_cancel(struct rtc_device *rtc, struct rtc_timer *timer)
        mutex_unlock(&rtc->ops_lock);
 }
 
+/**
+ * rtc_read_offset - Read the amount of rtc offset in parts per billion
+ * @ rtc: rtc device to be used
+ * @ offset: the offset in parts per billion
+ *
+ * see below for details.
+ *
+ * Kernel interface to read rtc clock offset
+ * Returns 0 on success, or a negative number on error.
+ * If read_offset() is not implemented for the rtc, return -EINVAL
+ */
+int rtc_read_offset(struct rtc_device *rtc, long *offset)
+{
+       int ret;
+
+       if (!rtc->ops)
+               return -ENODEV;
+
+       if (!rtc->ops->read_offset)
+               return -EINVAL;
+
+       mutex_lock(&rtc->ops_lock);
+       ret = rtc->ops->read_offset(rtc->dev.parent, offset);
+       mutex_unlock(&rtc->ops_lock);
+       return ret;
+}
 
+/**
+ * rtc_set_offset - Adjusts the duration of the average second
+ * @ rtc: rtc device to be used
+ * @ offset: the offset in parts per billion
+ *
+ * Some rtc's allow an adjustment to the average duration of a second
+ * to compensate for differences in the actual clock rate due to temperature,
+ * the crystal, capacitor, etc.
+ *
+ * Kernel interface to adjust an rtc clock offset.
+ * Return 0 on success, or a negative number on error.
+ * If the rtc offset is not setable (or not implemented), return -EINVAL
+ */
+int rtc_set_offset(struct rtc_device *rtc, long offset)
+{
+       int ret;
+
+       if (!rtc->ops)
+               return -ENODEV;
+
+       if (!rtc->ops->set_offset)
+               return -EINVAL;
+
+       mutex_lock(&rtc->ops_lock);
+       ret = rtc->ops->set_offset(rtc->dev.parent, offset);
+       mutex_unlock(&rtc->ops_lock);
+       return ret;
+}

diff --git a/include/linux/rtc.h b/include/linux/rtc.h
index 3359f04..b693ada 100644 (file)
--- a/include/linux/rtc.h
+++ b/include/linux/rtc.h
@@ -89,6 +89,8 @@ struct rtc_class_ops {
        int (*set_mmss)(struct device *, unsigned long secs);
        int (*read_callback)(struct device *, int data);
        int (*alarm_irq_enable)(struct device *, unsigned int enabled);
+       int (*read_offset)(struct device *, long *offset);
+       int (*set_offset)(struct device *, long offset);
 };
 
 #define RTC_DEVICE_NAME_SIZE 20
@@ -208,6 +210,8 @@ void rtc_timer_init(struct rtc_timer *timer, void (*f)(void *p), void *data);
 int rtc_timer_start(struct rtc_device *rtc, struct rtc_timer *timer,
                    ktime_t expires, ktime_t period);
 void rtc_timer_cancel(struct rtc_device *rtc, struct rtc_timer *timer);
+int rtc_read_offset(struct rtc_device *rtc, long *offset);
+int rtc_set_offset(struct rtc_device *rtc, long offset);
 void rtc_timer_do_work(struct work_struct *work);
 
 static inline bool is_leap_year(unsigned int year)