[PATCH 2/3] iio:humidity:si7020: added No Hold read mode

[Nicola Corna <nicola@corna.info>]

The Si7013/20/21 modules support 2 read modes:
 * Hold mode, where the device stretches the clock until the end of the
measurement
 * No Hold mode, where the device replies NACK for every I2C call during
the measurement
Here the No Hold mode is implemented, selectable with the boolean
parameter holdmode=N. The No Hold mode is less efficient, since it
requires multiple calls to the device, but it can be used as a fallback if
the clock stretching is not supported.

Signed-off-by: Nicola Corna <nicola@corna.info>
---
I've tested this on a Raspberry Pi 2, where the clock stretching is
currently bugged.
 drivers/iio/humidity/si7020.c | 55 +++++++++++++++++++++++++++++++++++++------
 1 file changed, 48 insertions(+), 7 deletions(-)

diff --git a/drivers/iio/humidity/si7020.c b/drivers/iio/humidity/si7020.c
index 62fdbcf..a8bad04 100644
--- a/drivers/iio/humidity/si7020.c
+++ b/drivers/iio/humidity/si7020.c
@@ -2,6 +2,7 @@
  * si7020.c - Silicon Labs Si7013/20/21 Relative Humidity and Temp Sensors
  * Copyright (c) 2013,2014  Uplogix, Inc.
  * David Barksdale <dbarksdale@uplogix.com>
+ * Copyright (c) 2015 Nicola Corna <nicola@corna.info>
  *
  * This program is free software; you can redistribute it and/or modify it
  * under the terms and conditions of the GNU General Public License,
@@ -30,16 +31,33 @@
 #include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/sysfs.h>
+#include <linux/jiffies.h>
 
 #include <linux/iio/iio.h>
 #include <linux/iio/sysfs.h>
 
 /* Measure Relative Humidity, Hold Master Mode */
 #define SI7020CMD_RH_HOLD	0xE5
+/* Measure Relative Humidity, No Hold Master Mode */
+#define SI7020CMD_RH_NO_HOLD	0xF5
 /* Measure Temperature, Hold Master Mode */
 #define SI7020CMD_TEMP_HOLD	0xE3
+/* Measure Temperature, No Hold Master Mode */
+#define SI7020CMD_TEMP_NO_HOLD	0xF3
 /* Software Reset */
 #define SI7020CMD_RESET		0xFE
+/* Relative humidity measurement timeout (us) */
+#define SI7020_RH_TIMEOUT	22800
+/* Temperature measurement timeout (us) */
+#define SI7020_TEMP_TIMEOUT	10800
+/* Minimum delay between retries (No Hold Mode) in us */
+#define SI7020_NOHOLD_SLEEP_MIN	2000
+/* Maximum delay between retries (No Hold Mode) in us */
+#define SI7020_NOHOLD_SLEEP_MAX	6000
+
+static bool holdmode = 1;
+module_param(holdmode, bool, 0644);
+MODULE_PARM_DESC(holdmode, "Select whether the measurement has to be done with Hold Mode (clock stretching) or No Hold Mode (repeated calls)");
 
 static int si7020_read_raw(struct iio_dev *indio_dev,
 			   struct iio_chan_spec const *chan, int *val,
@@ -47,16 +65,39 @@ static int si7020_read_raw(struct iio_dev *indio_dev,
 {
 	struct i2c_client **client = iio_priv(indio_dev);
 	int ret;
+	unsigned char buf[2];
+	unsigned long start;
 
 	switch (mask) {
 	case IIO_CHAN_INFO_RAW:
-		ret = i2c_smbus_read_word_data(*client,
-					       chan->type == IIO_TEMP ?
-					       SI7020CMD_TEMP_HOLD :
-					       SI7020CMD_RH_HOLD);
-		if (ret < 0)
-			return ret;
-		*val = ret >> 2;
+		if (holdmode) {
+			ret = i2c_smbus_read_word_data(*client,
+						       chan->type == IIO_TEMP ?
+						       SI7020CMD_TEMP_HOLD :
+						       SI7020CMD_RH_HOLD);
+			if (ret < 0)
+				return ret;
+			*val = ret >> 2;
+		} else {
+			ret = i2c_smbus_write_byte(*client,
+						   chan->type == IIO_TEMP ?
+						   SI7020CMD_TEMP_NO_HOLD :
+						   SI7020CMD_RH_NO_HOLD);
+			if (ret < 0)
+				return ret;
+			start = jiffies;
+			while ((ret = i2c_master_recv(*client, buf, 2)) < 0) {
+				if (time_after(jiffies, start +
+					       usecs_to_jiffies(
+							chan->type == IIO_TEMP ?
+							SI7020_TEMP_TIMEOUT :
+							SI7020_RH_TIMEOUT)))
+					return ret;
+				usleep_range(SI7020_NOHOLD_SLEEP_MIN,
+					     SI7020_NOHOLD_SLEEP_MAX);
+			}
+			*val = ((buf[0] << 8) | buf[1]) >> 2;
+		}
 		/*
 		 * Humidity values can sligthly exceed the 0-100%RH
 		 * range and should be corrected by software
-- 
2.5.0