Patch "iio: dht11: Use usleep_range instead of msleep for start signal" has been added to the 4.9-stable tree

Patch "iio: dht11: Use usleep_range instead of msleep for start signal" has been added to the 4.9-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    iio: dht11: Use usleep_range instead of msleep for start signal

to the 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     iio-dht11-use-usleep_range-instead-of-msleep-for-start-signal.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 5c113b5e0082e90d2e1c7b12e96a7b8cf0623e27 Mon Sep 17 00:00:00 2001
From: John Brooks <john@fastquake.com>
Date: Wed, 18 Jan 2017 21:50:39 +0000
Subject: iio: dht11: Use usleep_range instead of msleep for start signal

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 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
@@ -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
 		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;


Patches currently in stable-queue which might be from john@fastquake.com are

queue-4.9/iio-dht11-use-usleep_range-instead-of-msleep-for-start-signal.patch