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rtc: pcf2123: implement read_offset and set_offset

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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@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
This commit is contained in:
Joshua Clayton 2016-02-05 12:41:13 -08:00 committed by Alexandre Belloni
parent 5495a4159f
commit bae2f64702
1 changed files with 57 additions and 0 deletions
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57
drivers/rtc/rtc-pcf2123.c
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@ -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, &reg, 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)