From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.fastquake.com ([45.33.83.177]:47576 "EHLO mail.fastquake.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750817AbdARWd3 (ORCPT ); Wed, 18 Jan 2017 17:33:29 -0500 Date: Wed, 18 Jan 2017 21:50:39 +0000 From: John Brooks To: linux-iio@vger.kernel.org, Harald Geyer Cc: Jonathan Cameron Subject: [PATCH] iio: dht11: Use usleep_range instead of msleep for start signal Message-ID: <20170118215008.GA12173@oldkitsune.fastquake.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-iio-owner@vger.kernel.org List-Id: linux-iio@vger.kernel.org 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 --- 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 9c47bc9..2a22ad9 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; -- 2.7.4