pcf2123 has an offset register, which can be used to make minor
adjustments to the clock rate to compensate for temperature or
a crystal that is not exactly right.
Signed-off-by: Joshua Clayton <stillcompiling@xxxxxxxxx>
---
drivers/rtc/rtc-pcf2123.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 57 insertions(+)
diff --git a/drivers/rtc/rtc-pcf2123.c b/drivers/rtc/rtc-pcf2123.c
index d9a44ad..da27738 100644
--- a/drivers/rtc/rtc-pcf2123.c
+++ b/drivers/rtc/rtc-pcf2123.c
@@ -100,6 +100,7 @@
/* PCF2123_REG_OFFSET BITS */
#define OFFSET_SIGN_BIT BIT(6) /* 2's complement sign bit */
#define OFFSET_COARSE BIT(7) /* Coarse mode offset */
+#define OFFSET_STEP (2170) /* Offset step in parts per billion */
/* READ/WRITE ADDRESS BITS */
#define PCF2123_WRITE BIT(4)
@@ -206,6 +207,59 @@ static ssize_t pcf2123_store(struct device *dev, struct device_attribute *attr,
return count;
}
+static int pcf2123_read_offset(struct device *dev, long *offset)
+{
+ int ret;
+ s8 reg;
+
+ ret = pcf2123_read(dev, PCF2123_REG_OFFSET, ®, 1);
+ if (ret < 0)
+ return ret;
+
+ if (reg & OFFSET_COARSE)
+ reg <<= 1; /* multiply by 2 and sign extend */
+ else
+ reg |= (reg & OFFSET_SIGN_BIT) << 1; /* sign extend only */
+
+ *offset = ((long)reg) * OFFSET_STEP;
+
+ return 0;
+}
+
+/*
+ * The offset register is a 7 bit signed value with a coarse bit in bit 7.
+ * The main difference between the two is normal offset adjusts the first
+ * second of n minutes every other hour, with 61, 62 and 63 being shoved
+ * into the 60th minute.
+ * The coarse adjustment does the same, but every hour.
+ * the two overlap, with every even normal offset value corresponding
+ * to a coarse offset. Based on this algorithm, it seems that despite the
+ * name, coarse offset is a better fit for overlapping values.
+ */
+static int pcf2123_set_offset(struct device *dev, long offset)
+{
+ s8 reg;
+
+ if (offset > OFFSET_STEP * 127)
+ reg = 127;
+ else if (offset < OFFSET_STEP * -128)
+ reg = -128;
+ else
+ reg = (s8)((offset + (OFFSET_STEP >> 1)) / OFFSET_STEP);
+
+ /* choose fine offset only for odd values in the normal range */
+ if (reg & 1 && reg <= 63 && reg >= -64) {
+ /* Normal offset. Clear the coarse bit */
+ reg &= ~OFFSET_COARSE;
+ } else {
+ /* Coarse offset. Divide by 2 and set the coarse bit */
+ reg >>= 1;
+ reg |= OFFSET_COARSE;
+ }
+
+ return pcf2123_write_reg(dev, PCF2123_REG_OFFSET, reg);
+}
+
static int pcf2123_rtc_read_time(struct device *dev, struct rtc_time *tm)
{
u8 rxbuf[7];
@@ -314,6 +368,9 @@ static int pcf2123_reset(struct device *dev)
static const struct rtc_class_ops pcf2123_rtc_ops = {
.read_time = pcf2123_rtc_read_time,
.set_time = pcf2123_rtc_set_time,
+ .read_offset = pcf2123_read_offset,
+ .set_offset = pcf2123_set_offset,
+
};
static int pcf2123_probe(struct spi_device *spi)
--
2.5.0
--
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