rtc: NTP: Add CONFIG_RTC_SYSTOHC_DEVICE for NTP synchronization

Currently, CONFIG_RTC_SYSTOHC uses CONFIG_RTC_HCTOSYS_DEVICE which
is originally used by CONFIG_RTC_HCTOSYS, but this rtc device has
some limiations, for example, it must be battery-backed, be able
to work with irq off and through system suspension, etc.

So add CONFIG_RTC_SYSTOHC_DEVICE used exclusively for CONFIG_RTC_SYSTOHC,
it is more lenient compared to CONFIG_RTC_HCTOSYS_DEVICE, and could
be assigned any available RTC in the system.

Default value is CONFIG_RTC_HCTOSYS_DEVICE which is "rtc0" by default.
After this patch, NTP will sync up "rtc0" by default.

Cc: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/Kconfig

@@ -25,17 +25,9 @@ config RTC_HCTOSYS
 	  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_SYSTOHC
-	bool "Set the RTC time based on NTP synchronization"
-	default y
-	help
-	  If you say yes here, the system time (wall clock) will be stored
-	  in the RTC specified by RTC_HCTOSYS_DEVICE approximately every 11
-	  minutes if userspace reports synchronized NTP status.
-
 config RTC_HCTOSYS_DEVICE
 	string "RTC used to set the system time"
-	depends on RTC_HCTOSYS = y || RTC_SYSTOHC = y
+	depends on RTC_HCTOSYS
 	default "rtc0"
 	help
 	  The RTC device that will be used to (re)initialize the system
@@ -56,6 +48,25 @@ config RTC_HCTOSYS_DEVICE
 	  sleep states. Do not specify an RTC here unless it stays powered
 	  during all this system's supported sleep states.
 
+config RTC_SYSTOHC
+	bool "Set the RTC time based on NTP synchronization"
+	default y
+	help
+	  If you say yes here, the system time (wall clock) will be stored
+	  in the RTC specified by RTC_HCTOSYS_DEVICE approximately every 11
+	  minutes if userspace reports synchronized NTP status.
+
+config RTC_SYSTOHC_DEVICE
+	string "RTC used to synchronize NTP adjustment"
+	depends on RTC_SYSTOHC
+	default RTC_HCTOSYS_DEVICE if RTC_HCTOSYS
+	default "rtc0"
+	help
+	  The RTC device used for NTP synchronization. The main difference
+	  between RTC_HCTOSYS_DEVICE and RTC_SYSTOHC_DEVICE is that this
+	  one can sleep when setting time, because it runs in the workqueue
+	  context.
+
 config RTC_DEBUG
 	bool "RTC debug support"
 	help

drivers/rtc/systohc.c

@@ -31,7 +31,7 @@ int rtc_set_ntp_time(struct timespec64 now)
 	else
 		rtc_time64_to_tm(now.tv_sec + 1, &tm);
 
-	rtc = rtc_class_open(CONFIG_RTC_HCTOSYS_DEVICE);
+	rtc = rtc_class_open(CONFIG_RTC_SYSTOHC_DEVICE);
 	if (rtc) {
 		/* rtc_hctosys exclusively uses UTC, so we call set_time here,
 		 * not set_mmss. */