rtc: rzn1: support input frequencies other than 32768Hz

When using the SCMP mode instead of SUBU, this RTC can also support
other input frequencies than 32768Hz. Also, upcoming SoCs will only
support SCMP.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20250526095801.35781-8-wsa+renesas@sang-engineering.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Author: Wolfram Sang

drivers/rtc/rtc-rzn1.c

@@ -12,6 +12,7 @@
  */
 
 #include <linux/bcd.h>
+#include <linux/clk.h>
 #include <linux/init.h>
 #include <linux/iopoll.h>
 #include <linux/module.h>
@@ -22,7 +23,6 @@
 #include <linux/spinlock.h>
 
 #define RZN1_RTC_CTL0 0x00
-#define   RZN1_RTC_CTL0_SLSB_SUBU 0
 #define   RZN1_RTC_CTL0_SLSB_SCMP BIT(4)
 #define   RZN1_RTC_CTL0_AMPM BIT(5)
 #define   RZN1_RTC_CTL0_CEST BIT(6)
@@ -50,6 +50,8 @@
 #define   RZN1_RTC_SUBU_DEV BIT(7)
 #define   RZN1_RTC_SUBU_DECR BIT(6)
 
+#define RZN1_RTC_SCMP 0x3c
+
 #define RZN1_RTC_ALM 0x40
 #define RZN1_RTC_ALH 0x44
 #define RZN1_RTC_ALW 0x48
@@ -357,7 +359,7 @@ static int rzn1_rtc_set_offset(struct device *dev, long offset)
 	return 0;
 }
 
-static const struct rtc_class_ops rzn1_rtc_ops = {
+static const struct rtc_class_ops rzn1_rtc_ops_subu = {
 	.read_time = rzn1_rtc_read_time,
 	.set_time = rzn1_rtc_set_time,
 	.read_alarm = rzn1_rtc_read_alarm,
@@ -367,12 +369,21 @@ static const struct rtc_class_ops rzn1_rtc_ops = {
 	.set_offset = rzn1_rtc_set_offset,
 };
 
+static const struct rtc_class_ops rzn1_rtc_ops_scmp = {
+	.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,
+};
+
 static int rzn1_rtc_probe(struct platform_device *pdev)
 {
 	struct rzn1_rtc *rtc;
-	u32 val;
-	int irq;
-	int ret;
+	u32 val, scmp_val = 0;
+	struct clk *xtal;
+	unsigned long rate;
+	int irq, ret;
 
 	rtc = devm_kzalloc(&pdev->dev, sizeof(*rtc), GFP_KERNEL);
 	if (!rtc)
@@ -395,7 +406,6 @@ static int rzn1_rtc_probe(struct platform_device *pdev)
 	rtc->rtcdev->range_min = RTC_TIMESTAMP_BEGIN_2000;
 	rtc->rtcdev->range_max = RTC_TIMESTAMP_END_2099;
 	rtc->rtcdev->alarm_offset_max = 7 * 86400;
-	rtc->rtcdev->ops = &rzn1_rtc_ops;
 
 	ret = devm_pm_runtime_enable(&pdev->dev);
 	if (ret < 0)
@@ -404,10 +414,24 @@ static int rzn1_rtc_probe(struct platform_device *pdev)
 	if (ret < 0)
 		return ret;
 
-	/*
-	 * Ensure the clock counter is enabled.
-	 * Set 24-hour mode and possible oscillator offset compensation in SUBU mode.
-	 */
+	/* Only switch to scmp if we have an xtal clock with a valid rate and != 32768 */
+	xtal = devm_clk_get_optional(&pdev->dev, "xtal");
+	if (IS_ERR(xtal)) {
+		ret = PTR_ERR(xtal);
+		goto dis_runtime_pm;
+	} else if (xtal) {
+		rate = clk_get_rate(xtal);
+
+		if (rate < 32000 || rate > BIT(22)) {
+			ret = -EOPNOTSUPP;
+			goto dis_runtime_pm;
+		}
+
+		if (rate != 32768)
+			scmp_val = RZN1_RTC_CTL0_SLSB_SCMP;
+	}
+
+	/* Disable controller during SUBU/SCMP setup */
 	val = readl(rtc->base + RZN1_RTC_CTL0) & ~RZN1_RTC_CTL0_CE;
 	writel(val, rtc->base + RZN1_RTC_CTL0);
 	/* Wait 2-4 32k clock cycles for the disabled controller */
@@ -416,8 +440,18 @@ static int rzn1_rtc_probe(struct platform_device *pdev)
 	if (ret)
 		goto dis_runtime_pm;
 
-	writel(RZN1_RTC_CTL0_CE | RZN1_RTC_CTL0_AMPM | RZN1_RTC_CTL0_SLSB_SUBU,
-	       rtc->base + RZN1_RTC_CTL0);
+	/* Set desired modes leaving the controller disabled */
+	writel(RZN1_RTC_CTL0_AMPM | scmp_val, rtc->base + RZN1_RTC_CTL0);
+
+	if (scmp_val) {
+		writel(rate - 1, rtc->base + RZN1_RTC_SCMP);
+		rtc->rtcdev->ops = &rzn1_rtc_ops_scmp;
+	} else {
+		rtc->rtcdev->ops = &rzn1_rtc_ops_subu;
+	}
+
+	/* Enable controller finally */
+	writel(RZN1_RTC_CTL0_CE | RZN1_RTC_CTL0_AMPM | scmp_val, rtc->base + RZN1_RTC_CTL0);
 
 	/* Disable all interrupts */
 	writel(0, rtc->base + RZN1_RTC_CTL1);