[PATCH v5 4/5] rtc: pcf85363: add oscillator offset calibration support

[Lakshay Piplani <lakshay.piplani@nxp.com>]

Expose the oscillator offset register of PCF85263/PCF85363 through the
read_offset and set_offset rtc_class_ops callbacks, so userspace can
apply frequency correction for drift compensation.

The offset is in parts per billion (ppb). CTRL_OFFSET is a signed 8-bit
step count whose size depends on the OFFM bit: 2170 ppb/step normal and
2034.5 ppb/step fast, the latter computed with an x10-scaled constant to
keep the arithmetic integer. Values are range-checked before that scaling
so the multiply cannot overflow, and out-of-range values are rejected
with -ERANGE rather than clamped.

Signed-off-by: Lakshay Piplani <lakshay.piplani@nxp.com>

---
V4 -> V5:
- Report and accept the offset in ppb (per the RTC offset ABI) using the
  step selected by OFFM (2170 ppb, or 2034.5 ppb fast via x10 scaling).
- Range-check before the x10 scaling to avoid overflow, and reject
  out-of-range values with -ERANGE instead of clamping.

V3 -> V4:
- No changes in v4.

V2 -> V3:
- Split into separate patches as suggested:
  - Battery switch-over detection.
  - Timestamp recording for TS pin and battery switch-over events.
  - Offset calibration.
  - Watchdog timer (to be reviewed by watchdog maintainers).
- Dropped Alarm2 support
- Switched to rtc_add_group() for sysfs attributes

V1 -> V2:
- Watchdog related changes due to removal of vendor specific properties
  from device tree
  * remove vendor DT knobs (enable/timeout/stepsize/repeat)
  * use watchdog_init_timeout (with 10s default)
  * derive clock_sel from final timeout
  * default, repeat=true (repeat mode)
- Fixed uninitalised warning on 'ret' (reported by kernel test robot)
- Use dev_dbg instead of dev_info for debug related print messages
- Minor cleanup and comments.
---
---
 drivers/rtc/rtc-pcf85363.c | 67 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)

diff --git a/drivers/rtc/rtc-pcf85363.c b/drivers/rtc/rtc-pcf85363.c
index 38bff8954e7f..9da4b1617f82 100644
--- a/drivers/rtc/rtc-pcf85363.c
+++ b/drivers/rtc/rtc-pcf85363.c
@@ -115,6 +115,7 @@
 #define OSC_CAP_SEL	GENMASK(1, 0)
 #define OSC_CAP_6000	0x01
 #define OSC_CAP_12500	0x02
+#define OSC_OFFM	BIT(6)
 
 #define STOP_EN_STOP	BIT(0)
 
@@ -131,6 +132,14 @@
 #define TSR2_SHIFT      2
 #define TSR3_SHIFT      6
 
+#define OFFSET_SIGN_BIT 7
+#define OFFSET_MASK     0xFF
+/* Offset step in ppb; fast mode (OFFM=1) is scaled x10 for integer maths. */
+#define OFFSET_STEP_PPB		2170
+#define OFFSET_STEP_PPB_FAST_X10	20345
+#define OFFSET_PPB_MAX		(127 * OFFSET_STEP_PPB)
+#define OFFSET_PPB_MIN		(-128 * OFFSET_STEP_PPB)
+
 #define PCF85363_NUM_TS		3
 /* Bytes latched per timestamp register (sec, min, hour, day, mon, year). */
 #define PCF85363_TS_LEN		6
@@ -515,6 +524,62 @@ static irqreturn_t pcf85363_rtc_handle_irq(int irq, void *dev_id)
 	return handled ? IRQ_HANDLED : IRQ_NONE;
 }
 
+/*
+ * CTRL_OFFSET is a signed step count; the step is 2170 ppb (normal) or
+ * 2034.5 ppb (fast/OFFM, scaled x10 to keep the arithmetic integer).
+ */
+static int pcf85363_read_offset(struct device *dev, long *offset)
+{
+	struct pcf85363 *pcf85363 = dev_get_drvdata(dev);
+	unsigned int val, osc;
+	long steps;
+	int ret;
+
+	ret = regmap_read(pcf85363->regmap, CTRL_OFFSET, &val);
+	if (ret)
+		return ret;
+
+	ret = regmap_read(pcf85363->regmap, CTRL_OSCILLATOR, &osc);
+	if (ret)
+		return ret;
+
+	steps = sign_extend32(val, OFFSET_SIGN_BIT);
+
+	if (osc & OSC_OFFM)
+		*offset = steps * OFFSET_STEP_PPB_FAST_X10 / 10;
+	else
+		*offset = steps * OFFSET_STEP_PPB;
+
+	return 0;
+}
+
+static int pcf85363_set_offset(struct device *dev, long offset)
+{
+	struct pcf85363 *pcf85363 = dev_get_drvdata(dev);
+	unsigned int osc;
+	long steps;
+	int ret;
+
+	ret = regmap_read(pcf85363->regmap, CTRL_OSCILLATOR, &osc);
+	if (ret)
+		return ret;
+
+	/* Range-check before the x10 scaling so the multiply cannot overflow. */
+	if (offset > OFFSET_PPB_MAX || offset < OFFSET_PPB_MIN)
+		return -ERANGE;
+
+	if (osc & OSC_OFFM)
+		steps = DIV_ROUND_CLOSEST(offset * 10, OFFSET_STEP_PPB_FAST_X10);
+	else
+		steps = DIV_ROUND_CLOSEST(offset, OFFSET_STEP_PPB);
+
+	if (steps < -128 || steps > 127)
+		return -ERANGE;
+
+	return regmap_write(pcf85363->regmap, CTRL_OFFSET,
+			    steps & OFFSET_MASK);
+}
+
 static int pcf85363_rtc_ioctl(struct device *dev,
 			      unsigned int cmd, unsigned long arg)
 {
@@ -562,6 +627,8 @@ static const struct rtc_class_ops rtc_ops = {
 	.read_alarm	= pcf85363_rtc_read_alarm,
 	.set_alarm	= pcf85363_rtc_set_alarm,
 	.alarm_irq_enable = pcf85363_rtc_alarm_irq_enable,
+	.read_offset = pcf85363_read_offset,
+	.set_offset = pcf85363_set_offset,
 };
 
 static int pcf85363_nvram_read(void *priv, unsigned int offset, void *val,
-- 
2.25.1