linux-kernel - [PATCH] hwmon: (htu21) Use the nohold mode to read out values

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Date:	Tue,  9 Sep 2014 10:57:24 +0200
From:	Bernhard Walle <bernhard@...lle.de>
To:	william.markezana@...s-spec.com, linux@...ck-us.net
Cc:	jdelvare@...e.de, lm-sensors@...sensors.org,
	linux-kernel@...r.kernel.org, Bernhard Walle <bernhard@...lle.de>
Subject: [PATCH] hwmon: (htu21) Use the nohold mode to read out values

On my Raspberry Pi, the driver doesn't work. Every read fails with -EIO.
Reading the data sheet and experimenting a bit made me finding out that
using the nohold mode works. The Raspberry Pi I²C chip seems to have
problems with slaves blocking the SCK line.

In any case, freeing the bus while performing the measurement seems to
be the better way, I think.

Signed-off-by: Bernhard Walle <bernhard@...lle.de>
---
 drivers/hwmon/htu21.c | 52 +++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 46 insertions(+), 6 deletions(-)

diff --git a/drivers/hwmon/htu21.c b/drivers/hwmon/htu21.c
index 839086e..7defae2 100644
--- a/drivers/hwmon/htu21.c
+++ b/drivers/hwmon/htu21.c
@@ -25,10 +25,11 @@
 #include <linux/mutex.h>
 #include <linux/device.h>
 #include <linux/jiffies.h>
+#include <linux/delay.h>
 
 /* HTU21 Commands */
-#define HTU21_T_MEASUREMENT_HM	0xE3
-#define HTU21_RH_MEASUREMENT_HM	0xE5
+#define HTU21_T_MEASUREMENT_HM_NOHOLD	0xF3
+#define HTU21_RH_MEASUREMENT_HM_NOHOLD	0xF5
 
 struct htu21 {
 	struct device *hwmon_dev;
@@ -59,6 +60,47 @@ static inline int htu21_rh_ticks_to_per_cent_mille(int ticks)
 	return ((15625 * ticks) >> 13) - 6000;
 }
 
+/* retry for one second, then give up */
+#define MAX_RETRIES 20
+
+static int htu21_read_word(struct i2c_client *client, u8 command)
+{
+	char data[2];
+	int ret, i;
+
+	/* start the measurement */
+	ret = i2c_master_send(client, &command, sizeof(command));
+	if (ret < 0) {
+		dev_err(&client->dev, "Unable to send command 0x%hhx: %d\n",
+				command, ret);
+		return ret;
+	}
+
+	/*
+	 * Now poll until we get the data. On I2C level the device sends a NAK
+	 * until it is ready
+	 */
+
+	msleep(50);
+
+	for (i = 0; i < MAX_RETRIES; i++) {
+		ret = i2c_master_recv(client, data, sizeof(data));
+		if (ret == sizeof(data))
+			break;
+
+		msleep(50);
+	}
+
+	if (ret < 0) {
+		dev_err(&client->dev, "Unable to receive result from command 0x%hhx: %d\n",
+				command, ret);
+		return ret;
+	}
+
+	return (data[0] << 8) | data[1];
+}
+
+
 static int htu21_update_measurements(struct i2c_client *client)
 {
 	int ret = 0;
@@ -68,13 +110,11 @@ static int htu21_update_measurements(struct i2c_client *client)
 
 	if (time_after(jiffies, htu21->last_update + HZ / 2) ||
 	    !htu21->valid) {
-		ret = i2c_smbus_read_word_swapped(client,
-						  HTU21_T_MEASUREMENT_HM);
+		ret = htu21_read_word(client, HTU21_T_MEASUREMENT_HM_NOHOLD);
 		if (ret < 0)
 			goto out;
 		htu21->temperature = htu21_temp_ticks_to_millicelsius(ret);
-		ret = i2c_smbus_read_word_swapped(client,
-						  HTU21_RH_MEASUREMENT_HM);
+		ret = htu21_read_word(client, HTU21_RH_MEASUREMENT_HM_NOHOLD);
 		if (ret < 0)
 			goto out;
 		htu21->humidity = htu21_rh_ticks_to_per_cent_mille(ret);
-- 
2.1.0

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