rtc: add correction parameter

Add a new parameter allowing the get and set the correction using ioctls
instead of just sysfs.
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211018151933.76865-5-alexandre.belloni@bootlin.com

drivers/rtc/dev.c

@@ -398,6 +398,16 @@ static long rtc_dev_ioctl(struct file *file,
 			param.uvalue = rtc->features[0];
 			break;
 
+		case RTC_PARAM_CORRECTION:
+			mutex_unlock(&rtc->ops_lock);
+			if (param.index != 0)
+				return -EINVAL;
+			err = rtc_read_offset(rtc, &offset);
+			mutex_lock(&rtc->ops_lock);
+			if (err == 0)
+				param.svalue = offset;
+			break;
+
 		default:
 			err = -EINVAL;
 		}
@@ -416,6 +426,15 @@ static long rtc_dev_ioctl(struct file *file,
 
 		switch(param.param) {
 		case RTC_PARAM_FEATURES:
+			err = -EINVAL;
+			break;
+
+		case RTC_PARAM_CORRECTION:
+			mutex_unlock(&rtc->ops_lock);
+			if (param.index != 0)
+				return -EINVAL;
+			return rtc_set_offset(rtc, param.svalue);
+
 		default:
 			err = -EINVAL;
 		}

include/uapi/linux/rtc.h

@@ -136,6 +136,7 @@ struct rtc_param {
 
 /* parameter list */
 #define RTC_PARAM_FEATURES		0
+#define RTC_PARAM_CORRECTION		1
 
 #define RTC_MAX_FREQ	8192