
This adds a sysfs 'clock_source' attribute which can be used to query
and set the clock source of the RTC, either 'internal' or 'external'.

Important note: Probing the pins of the 32kHz crystal with a scope will
not reliably tell you whether the (usually) more accurate external is
selected! On a 'Cubietech Cubietruck' board, the author saw a stable
32768Hz signal on the crystal, even though the internal oscillator was
selected and the clock wildly drifting.

Using adjtimex might help to figure out which oscillator is selected:

~# echo internal > /sys/devices/.../1c20d00.rtc/clock_source
~# adjtimex -n -c=3
                                      --- current ---   -- suggested --
cmos time     system-cmos  error_ppm   tick      freq    tick      freq
1469627826      31.224561
1469627835      31.858098    63353.6  10000         0
1469627844      32.490782    63268.4  10000         0    9367   2069425
~# echo external > /sys/devices/.../1c20d00.rtc/clock_source
~# adjtimex -n -c=3
                                      --- current ---   -- suggested --
cmos time     system-cmos  error_ppm   tick      freq    tick      freq
1469627851      32.883407
1469627861      32.883380       -2.7  10000         0
1469627871      32.883352       -2.8  10000         0   10000    185937

Signed-off-by: Onno Kortmann <on...@gmx.net>
---
 drivers/rtc/rtc-sunxi.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/drivers/rtc/rtc-sunxi.c b/drivers/rtc/rtc-sunxi.c
index abada60..3fcf571 100644
--- a/drivers/rtc/rtc-sunxi.c
+++ b/drivers/rtc/rtc-sunxi.c
@@ -36,6 +36,8 @@
 #define SUNXI_LOSC_CTRL				0x0000
 #define SUNXI_LOSC_CTRL_RTC_HMS_ACC		BIT(8)
 #define SUNXI_LOSC_CTRL_RTC_YMD_ACC		BIT(7)
+#define SUNXI_LOSC_OSC32K_SRC_SEL 		BIT(0)
+#define SUNXI_LOSC_KEY_VALUE			0x16aa0000
 
 #define SUNXI_RTC_YMD				0x0004
 
@@ -432,6 +434,50 @@ static const struct of_device_id sunxi_rtc_dt_ids[] = {
 };
 MODULE_DEVICE_TABLE(of, sunxi_rtc_dt_ids);
 
+/* As per page 126 of the A20 manual, the lowest bit in LOSC_CTRL_REG controls
+ * the 32.768KHz clock source to use for the RTC. Using the clock_source sysfs
+ * attribute, the clock can be selected between external (accurate 32kHz
+ * crystal) and internal (seems to be an inaccurate RC oscillator) mode.  It
+ * appears that this bit is non-volatile and will be kept in the RTC when the
+ * system is powered off.
+ */
+static ssize_t sunxi_rtc_show_clock_source(struct device *dev,
+					struct device_attribute *attr,
+					char *buf)
+{
+	struct sunxi_rtc_dev *chip = dev_get_drvdata(dev);
+	u32 val = readl(chip->base + SUNXI_LOSC_CTRL);
+	if (val & SUNXI_LOSC_OSC32K_SRC_SEL)
+		return sprintf(buf, "internal [external]\n");
+	else
+		return sprintf(buf, "[internal] external\n");
+}
+
+static ssize_t sunxi_rtc_store_clock_source(struct device *dev,
+					struct device_attribute *attr,
+					const char *buf, size_t count) {
+	struct sunxi_rtc_dev *chip = dev_get_drvdata(dev);
+	u32 val = readl(chip->base + SUNXI_LOSC_CTRL);
+
+	if (strncmp(buf, "external", 8) == 0)
+		val|=SUNXI_LOSC_OSC32K_SRC_SEL;
+	else if (strncmp(buf, "internal", 8) == 0)
+		val&=~SUNXI_LOSC_OSC32K_SRC_SEL;
+	else
+		return -EINVAL;
+
+        /* Writing this bit requires setting the upper 16 bit to 0x16aa (key
+	 * value). */
+	val |= SUNXI_LOSC_KEY_VALUE;
+
+	writel(val, chip->base + SUNXI_LOSC_CTRL);
+	return count;
+}
+
+static DEVICE_ATTR(clock_source, S_IRUGO | S_IWUSR,
+		sunxi_rtc_show_clock_source,
+		sunxi_rtc_store_clock_source);
+
 static int sunxi_rtc_probe(struct platform_device *pdev)
 {
 	struct sunxi_rtc_dev *chip;
@@ -490,6 +536,13 @@ static int sunxi_rtc_probe(struct platform_device *pdev)
 
 	dev_info(&pdev->dev, "RTC enabled\n");
 
+	ret = device_create_file(&pdev->dev, &dev_attr_clock_source);
+	if (ret) {
+		dev_err(&pdev->dev, "Unable to create sysfs entry: %s\n",
+			dev_attr_clock_source.attr.name);
+		return ret;
+	}
+
 	return 0;
 }
 
@@ -499,6 +552,7 @@ static int sunxi_rtc_remove(struct platform_device *pdev)
 
 	rtc_device_unregister(chip->rtc);
 
+	device_remove_file(&pdev->dev, &dev_attr_clock_source);
 	return 0;
 }
 
-- 
2.2.0.34.gb8f29bf

