rtc: rzn1: Add oscillator offset support

The RZN1 RTC can compensate the imprecision of the oscillator up to
approximately 190ppm.

Seconds can last slightly shorter or longer depending on the
configuration.

Below ~65ppm of correction, we can change the time spent in a second
every minute, which is the most accurate compensation that the RTC can
offer. Above, the compensation will be active every 20s.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220516082504.33913-5-miquel.raynal@bootlin.com

drivers/rtc/rtc-rzn1.c

@@ -245,12 +245,85 @@ static int rzn1_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
 	return 0;
 }
 
+static int rzn1_rtc_read_offset(struct device *dev, long *offset)
+{
+	struct rzn1_rtc *rtc = dev_get_drvdata(dev);
+	unsigned int ppb_per_step;
+	bool subtract;
+	u32 val;
+
+	val = readl(rtc->base + RZN1_RTC_SUBU);
+	ppb_per_step = val & RZN1_RTC_SUBU_DEV ? 1017 : 3051;
+	subtract = val & RZN1_RTC_SUBU_DECR;
+	val &= 0x3F;
+
+	if (!val)
+		*offset = 0;
+	else if (subtract)
+		*offset = -(((~val) & 0x3F) + 1) * ppb_per_step;
+	else
+		*offset = (val - 1) * ppb_per_step;
+
+	return 0;
+}
+
+static int rzn1_rtc_set_offset(struct device *dev, long offset)
+{
+	struct rzn1_rtc *rtc = dev_get_drvdata(dev);
+	unsigned int steps;
+	int stepsh, stepsl;
+	u32 val;
+	int ret;
+
+	/*
+	 * Check which resolution mode (every 20 or 60s) can be used.
+	 * Between 2 and 124 clock pulses can be added or substracted.
+	 *
+	 * In 20s mode, the minimum resolution is 2 / (32768 * 20) which is
+	 * close to 3051 ppb. In 60s mode, the resolution is closer to 1017.
+	 */
+	stepsh = DIV_ROUND_CLOSEST(offset, 1017);
+	stepsl = DIV_ROUND_CLOSEST(offset, 3051);
+
+	if (stepsh >= -0x3E && stepsh <= 0x3E) {
+		/* 1017 ppb per step */
+		steps = stepsh;
+		val |= RZN1_RTC_SUBU_DEV;
+	} else if (stepsl >= -0x3E && stepsl <= 0x3E) {
+		/* 3051 ppb per step */
+		steps = stepsl;
+	} else {
+		return -ERANGE;
+	}
+
+	if (!steps)
+		return 0;
+
+	if (steps > 0) {
+		val |= steps + 1;
+	} else {
+		val |= RZN1_RTC_SUBU_DECR;
+		val |= (~(-steps - 1)) & 0x3F;
+	}
+
+	ret = readl_poll_timeout(rtc->base + RZN1_RTC_CTL2, val,
+				 !(val & RZN1_RTC_CTL2_WUST), 100, 2000000);
+	if (ret)
+		return ret;
+
+	writel(val, rtc->base + RZN1_RTC_SUBU);
+
+	return 0;
+}
+
 static const struct rtc_class_ops rzn1_rtc_ops = {
 	.read_time = rzn1_rtc_read_time,
 	.set_time = rzn1_rtc_set_time,
 	.read_alarm = rzn1_rtc_read_alarm,
 	.set_alarm = rzn1_rtc_set_alarm,
 	.alarm_irq_enable = rzn1_rtc_alarm_irq_enable,
+	.read_offset = rzn1_rtc_read_offset,
+	.set_offset = rzn1_rtc_set_offset,
 };
 
 static int rzn1_rtc_probe(struct platform_device *pdev)