hwmon: (max31827) Add PEC support

Add PEC support using the hwmon core infrastructure.
Signed-off-by: Radu Sabau <radu.sabau@analog.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240531084645.12935-2-radu.sabau@analog.com
[groeck: Adjusted subject; simplified description]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/max31827.rst

@@ -131,7 +131,14 @@ The Fault Queue bits select how many consecutive temperature faults must occur
 before overtemperature or undertemperature faults are indicated in the
 corresponding status bits.
 
-Notes
------
+PEC Support
+-----------
+
+When reading a register value, the PEC byte is computed and sent by the chip.
+
+PEC on word data transaction respresents a signifcant increase in bandwitdh
+usage (+33% for both write and reads) in normal conditions.
 
-PEC is not implemented.
+Since this operation implies there will be an extra delay to each
+transaction, PEC can be disabled or enabled through sysfs.
+Just write 1  to the "pec" file for enabling PEC and 0 for disabling it.

drivers/hwmon/max31827.c

@@ -24,6 +24,7 @@
 
 #define MAX31827_CONFIGURATION_1SHOT_MASK	BIT(0)
 #define MAX31827_CONFIGURATION_CNV_RATE_MASK	GENMASK(3, 1)
+#define MAX31827_CONFIGURATION_PEC_EN_MASK	BIT(4)
 #define MAX31827_CONFIGURATION_TIMEOUT_MASK	BIT(5)
 #define MAX31827_CONFIGURATION_RESOLUTION_MASK	GENMASK(7, 6)
 #define MAX31827_CONFIGURATION_ALRM_POL_MASK	BIT(8)
@@ -382,7 +383,8 @@ static int max31827_write(struct device *dev, enum hwmon_sensor_types type,
 		}
 
 	case hwmon_chip:
-		if (attr == hwmon_chip_update_interval) {
+		switch (attr) {
+		case hwmon_chip_update_interval:
 			if (!st->enable)
 				return -EINVAL;
 
@@ -410,14 +412,18 @@ static int max31827_write(struct device *dev, enum hwmon_sensor_types type,
 				return ret;
 
 			st->update_interval = val;
-		}
-		break;
 
+			return 0;
+		case hwmon_chip_pec:
+			return regmap_update_bits(st->regmap, MAX31827_CONFIGURATION_REG,
+						  MAX31827_CONFIGURATION_PEC_EN_MASK,
+						  val ? MAX31827_CONFIGURATION_PEC_EN_MASK : 0);
+		default:
+			return -EOPNOTSUPP;
+		}
 	default:
 		return -EOPNOTSUPP;
 	}
-
-	return 0;
 }
 
 static ssize_t temp1_resolution_show(struct device *dev,
@@ -583,7 +589,7 @@ static const struct hwmon_channel_info *max31827_info[] = {
 					 HWMON_T_MIN_HYST | HWMON_T_MIN_ALARM |
 					 HWMON_T_MAX | HWMON_T_MAX_HYST |
 					 HWMON_T_MAX_ALARM),
-	HWMON_CHANNEL_INFO(chip, HWMON_C_UPDATE_INTERVAL),
+	HWMON_CHANNEL_INFO(chip, HWMON_C_UPDATE_INTERVAL | HWMON_C_PEC),
 	NULL,
 };