From:   David Mosberger-Tang <davidm@egauge.net>
Subject: [PATCH v2 1/2] hwmon: (sht4x) Fix EREMOTEIO errors
Date:   Sat, 20 Nov 2021 20:40:59 +0000 (UTC)
Message-Id: <20211120204053.2299501-1-davidm@egauge.net>
MIME-Version: 1.0
To:     Navin Sankar Velliangiri <navin@linumiz.com>
Cc:     Jean Delvare <jdelvare@suse.com>,
        Guenter Roeck <linux@roeck-us.net>,
        linux-hwmon@vger.kernel.org, linux-kernel@vger.kernel.org,
        David Mosberger-Tang <davidm@egauge.net>
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Precedence: bulk

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>
---
 drivers/hwmon/sht4x.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/sht4x.c b/drivers/hwmon/sht4x.c
index 09c2a0b06444..3415d7a0e0fc 100644
--- 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) {
-- 
2.25.1