rtc: expose correction feature

Add a new feature for RTCs able to correct the oscillator imprecision. This
is also called offset or trimming. Such drivers have a .set_offset callback,
use that to set the feature bit from the core.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211018151933.76865-4-alexandre.belloni@bootlin.com
(cherry picked from commit 2268551)
[ta: handle conflicts]
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Isaac True <isaac.true@canonical.com>

drivers/rtc/class.c

@@ -395,6 +395,9 @@ int __devm_rtc_register_device(struct module *owner, struct rtc_device *rtc)
 	if (!rtc->ops->set_alarm)
 		clear_bit(RTC_FEATURE_ALARM, rtc->features);
 
+	if (rtc->ops->set_offset)
+		set_bit(RTC_FEATURE_CORRECTION, rtc->features);
+
 	rtc->owner = owner;
 	rtc_device_get_offset(rtc);
 

include/uapi/linux/rtc.h

@@ -114,7 +114,8 @@ struct rtc_pll_info {
 #define RTC_FEATURE_ALARM		0
 #define RTC_FEATURE_ALARM_RES_MINUTE	1
 #define RTC_FEATURE_NEED_WEEK_DAY	2
-#define RTC_FEATURE_CNT			3
+#define RTC_FEATURE_CORRECTION		3
+#define RTC_FEATURE_CNT			4
 
 #define RTC_MAX_FREQ	8192