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

drivers/rtc/class.c

@@ -389,6 +389,9 @@ 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

@@ -131,7 +131,8 @@ struct rtc_param {
 #define RTC_FEATURE_NEED_WEEK_DAY	2
 #define RTC_FEATURE_ALARM_RES_2S	3
 #define RTC_FEATURE_UPDATE_INTERRUPT	4
-#define RTC_FEATURE_CNT			5
+#define RTC_FEATURE_CORRECTION		5
+#define RTC_FEATURE_CNT			6
 
 /* parameter list */
 #define RTC_PARAM_FEATURES		0