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SubjectRE: [PATCH] hwmon: (htu21) Use the nohold mode to read out values
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Hi Bernhard,

This is a well-known issue on the Raspberry Pi whose i2c stretching timeout is too short to allow proper HTU21 conversion.
See this discussion over here which provides a patch to allow hold master mode on the Raspberry Pi: https://github.com/raspberrypi/linux/issues/211

We will soon publish a new version of the HTU21 driver which provides a lot of new features including both hold master and no hold measurement modes support.

Best regards,

William MARKEZANA
Advanced Development
MEAS France
Impasse Jeanne Benozzi
CS 83163
31027 Toulouse Cedex 3
Direct : + 33 (0) 582 082 286
Fax : + 33 (0) 582 082 151
william.markezana@meas-spec.com
http://www.meas-spec.com

-----Message d'origine-----
De : Bernhard Walle [mailto:bernhard@bwalle.de]
Envoyé : mardi 9 septembre 2014 10:57
À : Markezana, William; linux@roeck-us.net
Cc : jdelvare@suse.de; lm-sensors@lm-sensors.org; linux-kernel@vger.kernel.org; Bernhard Walle
Objet : [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@bwalle.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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