hwmon: (sht4x) Fix EREMOTEIO errors

Per datasheet, SHT4x may need up to 8.2ms for a "high repeatability"
measurement to complete.  Attempting to read the result too early
triggers a NAK which then causes an EREMOTEIO error.

This behavior has been confirmed with a logic analyzer while running
the I2C bus at only 40kHz.  The low frequency precludes any
signal-integrity issues, which was also confirmed by the absence of
any CRC8 errors.  In this configuration, a NAK occurred on any read
that followed the measurement command within less than 8.2ms.

Signed-off-by: David Mosberger-Tang <davidm@egauge.net>
Link: https://lore.kernel.org/r/20211120212849.2300854-2-davidm@egauge.net
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

diff --git a/drivers/hwmon/sht4x.c b/drivers/hwmon/sht4x.c
index 09c2a0b..3415d7a 100644 (file)
--- a/drivers/hwmon/sht4x.c
+++ b/drivers/hwmon/sht4x.c
@@ -23,7 +23,7 @@
 /*
  * I2C command delays (in microseconds)
  */
-#define SHT4X_MEAS_DELAY       1000
+#define SHT4X_MEAS_DELAY_HPM   8200    /* see t_MEAS,h in datasheet */
 #define SHT4X_DELAY_EXTRA      10000
 
 /*
@@ -90,7 +90,7 @@ static int sht4x_read_values(struct sht4x_data *data)
        if (ret < 0)
                goto unlock;
 
-       usleep_range(SHT4X_MEAS_DELAY, SHT4X_MEAS_DELAY + SHT4X_DELAY_EXTRA);
+       usleep_range(SHT4X_MEAS_DELAY_HPM, SHT4X_MEAS_DELAY_HPM + SHT4X_DELAY_EXTRA);
 
        ret = i2c_master_recv(client, raw_data, SHT4X_RESPONSE_LENGTH);
        if (ret != SHT4X_RESPONSE_LENGTH) {