rtc: add stand-alone driver for RX8025 chip

Add support for the Epson RX-8025SA/NB RTC chips.  It includes support for
alarms, periodic interrupts (1 Hz) and clock precision adjustment.

For clock precision adjustment, the SYSFS file "clock_adjust_ppb" gets
created in "/sys/class/rtc/rtcX/device".  It permits to set and get the
clock adjustment in ppb (parts per billion), e.g.:

  # echo -183000 > /sys/class/rtc/rtc0/device/clock_adjust_ppb
  # cat /sys/class/rtc/rtc0/device/clock_adjust_ppb
  -183000

This allows to compensate temperature dependent clock drifts.  According
to the RX8025 SA/NB application manual the frequency and temperature
characteristics can be approximated using the following equation:

  df = a * (ut - t)**2

  df: Frequency deviation in any temperature
  a : Coefficient = (-35 +-5) * 10**-9
  ut: Ultimate temperature in degree = +25 +-5 degree
  t : Any temperature in degree

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Sergei Poselenov <sposelenov@emcraft.com>
Signed-off-by: Yuri Tikhonov <yur@emcraft.com>
Signed-off-by: Dmitry Rakhchev <rda@emcraft.com>
Signed-off-by: Matthias Fuchs <matthias.fuchs@esd.eu>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index 277d35d232f..81adbdbd504 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -296,6 +296,15 @@ config RTC_DRV_RX8581
           This driver can also be built as a module. If so the module
           will be called rtc-rx8581.
 
+config RTC_DRV_RX8025
+        tristate "Epson RX-8025SA/NB"
+        help
+          If you say yes here you get support for the Epson
+          RX-8025SA/NB RTC chips.
+
+          This driver can also be built as a module. If so, the module
+          will be called rtc-rx8025.
+
 endif # I2C
 
 comment "SPI RTC drivers"