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rtc: Add functions to set and read rtc offset

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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>
This commit is contained in:
Joshua Clayton
2016-02-05 12:41:11 -08:00
committed by Alexandre Belloni
parent 9d1fa4c373
commit b3967067c2
2 changed files with 58 additions and 0 deletions
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54
drivers/rtc/interface.c
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@@ -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;
}