[PATCH 3.16 249/370] iio: dht11: Use usleep_range instead of msleep for start signal

Ben Hutchings
2017-03-10T13:49:44Z

From: John Brooks <john@fastquake.com>

commit 5c113b5e0082e90d2e1c7b12e96a7b8cf0623e27 upstream.

The DHT22 (AM2302) datasheet specifies that the LOW start pulse should not
exceed 20ms. However, observations with an oscilloscope of an RPi Model 2B
(rev 1.1) communicating with a DHT22 sensor showed that the driver was
consistently sending start pulses longer than 20ms:

Kernel 4.7.10-v7+ (n=132):
Minimum pulse length: 20.20ms
Maximum: 29.84ms
Mean: 24.96ms
StDev: 2.82ms
Sensor response rate: 100%
Read success rate: 76%

On kernel 4.8, the start pulse was so long that the sensor would not even
respond 97% of the time:

Kernel 4.8.16-v7+ (n=100):
Minimum pulse length: 30.4ms
Maximum: 74.4ms
Mean: 39.3ms
StDev: 10.2ms
Sensor response rate: 3%
Read success rate: 3%

The driver would return ETIMEDOUT and write log messages like this:

[ 51.430987] dht11 dht11@0: Only 1 signal edges detected
[ 66.311019] dht11 dht11@0: Only 0 signal edges detected

Replacing msleep(18) with usleep_range(18000, 20000) made the pulse length
sane again and restored responsiveness:

Kernel 4.8.16-v7+ with usleep_range (n=123):
Minimum pulse length: 18.16ms
Maximum: 20.20ms
Mean: 19.85ms
StDev: 0.51ms
Sensor response rate: 100%
Read success rate: 84%

Signed-off-by: John Brooks <john@fastquake.com>
Reviewed-by: Harald Geyer <harald@ccbib.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/iio/humidity/dht11.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

--- a/drivers/iio/humidity/dht11.c
+++ b/drivers/iio/humidity/dht11.c
@@ -44,7 +44,8 @@
#define DHT11_EDGES_PER_READ (2*DHT11_BITS_PER_READ + DHT11_EDGES_PREAMBLE + 1)

/* Data transmission timing (nano seconds) */
-#define DHT11_START_TRANSMISSION 18 /* ms */
+#define DHT11_START_TRANSMISSION_MIN 18000 /* us */
+#define DHT11_START_TRANSMISSION_MAX 20000 /* us */
#define DHT11_SENSOR_RESPONSE 80000
#define DHT11_START_BIT 50000
#define DHT11_DATA_BIT_LOW 27000
@@ -152,7 +153,8 @@ static int dht11_read_raw(struct iio_dev
ret = gpio_direction_output(dht11->gpio, 0);
if (ret)
goto err;
- msleep(DHT11_START_TRANSMISSION);
+ usleep_range(DHT11_START_TRANSMISSION_MIN,
+ DHT11_START_TRANSMISSION_MAX);
ret = gpio_direction_input(dht11->gpio);
if (ret)
goto err;