rtc: sun6i: Switch to the external oscillator

The RTC is clocked from either an internal, imprecise, oscillator or an
external one, which is usually much more accurate.

The difference perceived between the time elapsed and the time reported by
the RTC is in a 10% scale, which prevents the RTC from being useful at all.

Fortunately, the external oscillator is reported to be mandatory in the
Allwinner datasheet, so we can just switch to it.

Cc: stable@vger.kernel.org
Fixes: 9765d2d94309 ("rtc: sun6i: Add sun6i RTC driver")
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-sun6i.c

@@ -37,9 +37,11 @@
 
 /* Control register */
 #define SUN6I_LOSC_CTRL				0x0000
+#define SUN6I_LOSC_CTRL_KEY			(0x16aa << 16)
 #define SUN6I_LOSC_CTRL_ALM_DHMS_ACC		BIT(9)
 #define SUN6I_LOSC_CTRL_RTC_HMS_ACC		BIT(8)
 #define SUN6I_LOSC_CTRL_RTC_YMD_ACC		BIT(7)
+#define SUN6I_LOSC_CTRL_EXT_OSC			BIT(0)
 #define SUN6I_LOSC_CTRL_ACC_MASK		GENMASK(9, 7)
 
 /* RTC */
@@ -417,6 +419,10 @@ static int sun6i_rtc_probe(struct platform_device *pdev)
 	/* disable alarm wakeup */
 	writel(0, chip->base + SUN6I_ALARM_CONFIG);
 
+	/* switch to the external, more precise, oscillator */
+	writel(SUN6I_LOSC_CTRL_KEY | SUN6I_LOSC_CTRL_EXT_OSC,
+	       chip->base + SUN6I_LOSC_CTRL);
+
 	chip->rtc = rtc_device_register("rtc-sun6i", &pdev->dev,
 					&sun6i_rtc_ops, THIS_MODULE);
 	if (IS_ERR(chip->rtc)) {