rtc: gamecube: Report low battery as invalid data

I haven’t been able to test this patch as all of my consoles have a
working RTC battery, but according to the documentation it should work
like that.
Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211215175501.6761-3-linkmauve@linkmauve.fr

drivers/rtc/rtc-gamecube.c

@@ -83,6 +83,10 @@
 #define RTC_CONTROL0	0x21000c
 #define RTC_CONTROL1	0x21000d
 
+/* RTC flags */
+#define RTC_CONTROL0_UNSTABLE_POWER	0x00000800
+#define RTC_CONTROL0_LOW_BATTERY	0x00000200
+
 struct priv {
 	struct regmap *regmap;
 	void __iomem *iob;
@@ -182,9 +186,35 @@ static int gamecube_rtc_set_time(struct device *dev, struct rtc_time *t)
 	return regmap_write(d->regmap, RTC_COUNTER, timestamp - d->rtc_bias);
 }
 
+static int gamecube_rtc_ioctl(struct device *dev, unsigned int cmd, unsigned long arg)
+{
+	struct priv *d = dev_get_drvdata(dev);
+	int value;
+	int control0;
+	int ret;
+
+	switch (cmd) {
+	case RTC_VL_READ:
+		ret = regmap_read(d->regmap, RTC_CONTROL0, &control0);
+		if (ret)
+			return ret;
+
+		value = 0;
+		if (control0 & RTC_CONTROL0_UNSTABLE_POWER)
+			value |= RTC_VL_DATA_INVALID;
+		if (control0 & RTC_CONTROL0_LOW_BATTERY)
+			value |= RTC_VL_BACKUP_LOW;
+		return put_user(value, (unsigned int __user *)arg);
+
+	default:
+		return -ENOIOCTLCMD;
+	}
+}
+
 static const struct rtc_class_ops gamecube_rtc_ops = {
 	.read_time	= gamecube_rtc_read_time,
 	.set_time	= gamecube_rtc_set_time,
+	.ioctl		= gamecube_rtc_ioctl,
 };
 
 static int gamecube_rtc_read_offset_from_sram(struct priv *d)