[PATCH v4] hwmon: (ads7828) Fix external VREF regulator handling

[Qingshuang Fu <fffsqian@163.com>]

From: Qingshuang Fu <fuqingshuang@kylinos.cn>

The driver currently has two issues with the external VREF regulator
handling in ads7828_probe():

1. All errors from devm_regulator_get_optional() are ignored, causing the
   driver to incorrectly fall back to internal VREF even for transient
   errors like -EPROBE_DEFER or genuine failures like -ENOMEM.

2. The external regulator is never enabled. The driver calls
   regulator_get_voltage() without first calling regulator_enable(),
   so the VREF pin may remain unpowered if the regulator is not
   configured as always-on.

Fix both issues by switching to devm_regulator_get_enable_read_voltage(),
which handles regulator get, enable, and voltage read in one call.
Only -ENODEV (no regulator specified in device tree) should trigger the
fallback to internal VREF. All other errors are propagated to the caller.

Fixes: a8ddfea09566 ("hwmon: (ads7828) Accept optional parameters from device tree")
Signed-off-by: Qingshuang Fu <fuqingshuang@kylinos.cn>
---
Changes in v4:
- Invert the control flow to check errors first (if (vref_uv < 0) { ... }
  else { ... }), as requested by Guenter Roeck.
Changes in v3:
- Switch to devm_regulator_get_enable_read_voltage() to also enable the
  external regulator, as suggested by Guenter Roeck.
Changes in v2:
- Broaden the error check to handle all errors except -ENODEV, instead of
  only checking for -EPROBE_DEFER. This addresses the Sashiko AI review
  concern about masking genuine errors like -ENOMEM and -EINVAL.

 drivers/hwmon/ads7828.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/hwmon/ads7828.c b/drivers/hwmon/ads7828.c
index 149cfcec78dc..f8f2f621dfea 100644
--- a/drivers/hwmon/ads7828.c
+++ b/drivers/hwmon/ads7828.c
@@ -106,12 +106,11 @@ static int ads7828_probe(struct i2c_client *client)
 	struct ads7828_data *data;
 	struct device *hwmon_dev;
 	unsigned int vref_mv = ADS7828_INT_VREF_MV;
-	unsigned int vref_uv;
+	int vref_uv;
 	bool diff_input = false;
 	bool ext_vref = false;
 	unsigned int regval;
 	enum ads7828_chips chip;
-	struct regulator *reg;
 
 	data = devm_kzalloc(dev, sizeof(struct ads7828_data), GFP_KERNEL);
 	if (!data)
@@ -125,9 +124,11 @@ static int ads7828_probe(struct i2c_client *client)
 	} else if (dev->of_node) {
 		diff_input = of_property_read_bool(dev->of_node,
 						   "ti,differential-input");
-		reg = devm_regulator_get_optional(dev, "vref");
-		if (!IS_ERR(reg)) {
-			vref_uv = regulator_get_voltage(reg);
+		vref_uv = devm_regulator_get_enable_read_voltage(dev, "vref");
+		if (vref_uv < 0) {
+			if (vref_uv != -ENODEV)
+				return vref_uv;
+		} else {
 			vref_mv = DIV_ROUND_CLOSEST(vref_uv, 1000);
 			if (vref_mv < ADS7828_EXT_VREF_MV_MIN ||
 			    vref_mv > ADS7828_EXT_VREF_MV_MAX)
-- 
2.25.1