rtc: suspend()/resume() restores system clock

RTC class suspend/resume support, re-initializing the system clock on resume
from the clock used to initialize it at boot time.

 - The reinit-on-resume is hooked to the existing RTC_HCTOSYS config
   option, on the grounds that a clock good enough for init must also
   be good enough for re-init.

 - Inlining a version of the code used by ARM, to save and restore the
   delta between a selected RTC and the current system wall-clock time.

 - Removes calls to that ARM code from AT91, OMAP1, and S3C RTCs.  This
   means that systems using those RTCs across suspend/resume will likely
   want to change their kernel configs to enable RTC_HCTOSYS.

   If HCTOSYS isn't using a second RTC (with battery?), this changes the
   system's initial date from Jan 1970 to the epoch this hardware uses:
   1998 for AT91, 2000 for OMAP1 (assuming no split power mode), etc.

This goes on top of the patch series removing "struct class_device" usage
from the RTC framework.  That's all needed for class suspend()/resume().

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Acked-By: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 17 insertions, 7 deletions

diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index ec33ee8..a53ef4d 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -21,21 +21,31 @@ config RTC_CLASS
 	  will be called rtc-class.
 
 config RTC_HCTOSYS
-	bool "Set system time from RTC on startup"
+	bool "Set system time from RTC on startup and resume"
 	depends on RTC_CLASS = y
 	default y
 	help
-	  If you say yes here, the system time will be set using
-	  the value read from the specified RTC device. This is useful
-	  in order to avoid unnecessary fsck runs.
+	  If you say yes here, the system time (wall clock) will be set using
+	  the value read from a specified RTC device. This is useful to avoid
+	  unnecessary fsck runs at boot time, and to network better.
 
 config RTC_HCTOSYS_DEVICE
-	string "The RTC to read the time from"
+	string "RTC used to set the system time"
 	depends on RTC_HCTOSYS = y
 	default "rtc0"
 	help
-	  The RTC device that will be used as the source for
-	  the system time, usually rtc0.
+	  The RTC device that will be used to (re)initialize the system
+	  clock, usually rtc0.  Initialization is done when the system
+	  starts up, and when it resumes from a low power state.
+
+	  This clock should be battery-backed, so that it reads the correct
+	  time when the system boots from a power-off state.  Otherwise, your
+	  system will need an external clock source (like an NTP server).
+
+	  If the clock you specify here is not battery backed, it may still
+	  be useful to reinitialize system time when resuming from system
+	  sleep states.  Do not specify an RTC here unless it stays powered
+	  during all this system's supported sleep states.
 
 config RTC_DEBUG
 	bool "RTC debug support"