[PATCH v4 1/2] hwmon: sy7636a: read the temperature only while the rails are on

[Pablo Mazzini <pmazzini@gmail.com>]

The driver enables VCOM at probe, which keeps the panel's high voltage
rails on for good. Read the temperature only while the rails are on, when
the chip samples it, and return -ENODATA otherwise. Read power good with
it in one transfer, so the value can not be from before the first sample
or after the rails went off.

Signed-off-by: Pablo Mazzini <pmazzini@gmail.com>
---
 drivers/hwmon/sy7636a-hwmon.c | 35 ++++++++++++++++++++++++-----------
 1 file changed, 24 insertions(+), 11 deletions(-)

diff --git a/drivers/hwmon/sy7636a-hwmon.c b/drivers/hwmon/sy7636a-hwmon.c
index a0d0b4119bb3..a8bfbf1716a1 100644
--- a/drivers/hwmon/sy7636a-hwmon.c
+++ b/drivers/hwmon/sy7636a-hwmon.c
@@ -9,26 +9,43 @@
  */
 
 #include <linux/auxiliary_bus.h>
+#include <linux/bits.h>
 #include <linux/err.h>
 #include <linux/hwmon.h>
 #include <linux/module.h>
 #include <linux/regmap.h>
-#include <linux/regulator/machine.h>
 
-#define SY7636A_REG_TERMISTOR_READOUT	0x08
+#define SY7636A_REG_FAULT_FLAG		0x07
+#define SY7636A_FAULT_FLAG_PG		BIT(0)
 
 static int sy7636a_read(struct device *dev, enum hwmon_sensor_types type,
 			u32 attr, int channel, long *temp)
 {
 	struct regmap *regmap = dev_get_drvdata(dev);
-	int ret, reg_val;
-
-	ret = regmap_read(regmap,
-			  SY7636A_REG_TERMISTOR_READOUT, &reg_val);
+	u8 buf[2];
+	int ret;
+
+	/*
+	 * The thermistor is sampled while ON_OFF is set, but the first sample
+	 * is only ready after the ADC sampling and conversion time (ADC_SPL +
+	 * ADC_DEL in the datasheet); until then it holds its reset value, 0.
+	 * PG is only set once all rails are up, after the first sample, so
+	 * read it in the same transfer and report the temperature only while
+	 * it is set.
+	 *
+	 * With ON_OFF set but PG low (rails still ramping up, or a fault), no
+	 * temperature is reported even if one may be available. That is
+	 * accepted: the display driver reads the temperature only while it
+	 * holds the rails on, after power good.
+	 */
+	ret = regmap_bulk_read(regmap, SY7636A_REG_FAULT_FLAG, buf, sizeof(buf));
 	if (ret)
 		return ret;
 
-	*temp = reg_val * 1000;
+	if (!(buf[0] & SY7636A_FAULT_FLAG_PG))
+		return -ENODATA;
+
+	*temp = (s8)buf[1] * 1000;
 
 	return 0;
 }
@@ -72,10 +89,6 @@ static int sy7636a_sensor_probe(struct auxiliary_device *adev,
 	if (!regmap)
 		return -ENODEV;
 
-	err = devm_regulator_get_enable(&adev->dev, "vcom");
-	if (err)
-		return err;
-
 	hwmon_dev = devm_hwmon_device_register_with_info(&adev->dev,
 							 "sy7636a_temperature", regmap,
 							 &sy7636a_chip_info, NULL);
-- 
2.52.0