From 5c113b5e0082e90d2e1c7b12e96a7b8cf0623e27 Mon Sep 17 00:00:00 2001 From: John Brooks Date: Wed, 18 Jan 2017 21:50:39 +0000 Subject: [PATCH] iio: dht11: Use usleep_range instead of msleep for start signal 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% Cc: stable@vger.kernel.org Signed-off-by: John Brooks Reviewed-by: Harald Geyer Signed-off-by: Jonathan Cameron --- drivers/iio/humidity/dht11.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/drivers/iio/humidity/dht11.c b/drivers/iio/humidity/dht11.c index 9c47bc98f3ac..2a22ad920333 100644 --- a/drivers/iio/humidity/dht11.c +++ b/drivers/iio/humidity/dht11.c @@ -71,7 +71,8 @@ * a) select an implementation using busy loop polling on those systems * b) use the checksum to do some probabilistic decoding */ -#define DHT11_START_TRANSMISSION 18 /* ms */ +#define DHT11_START_TRANSMISSION_MIN 18000 /* us */ +#define DHT11_START_TRANSMISSION_MAX 20000 /* us */ #define DHT11_MIN_TIMERES 34000 /* ns */ #define DHT11_THRESHOLD 49000 /* ns */ #define DHT11_AMBIG_LOW 23000 /* ns */ @@ -228,7 +229,8 @@ static int dht11_read_raw(struct iio_dev *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;