• John Brooks's avatar
    iio: dht11: Use usleep_range instead of msleep for start signal · 12c2fcf6
    John Brooks authored
    commit 5c113b5e 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: default avatarJohn Brooks <john@fastquake.com>
    Reviewed-by: default avatarHarald Geyer <harald@ccbib.org>
    Signed-off-by: default avatarJonathan Cameron <jic23@kernel.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    12c2fcf6
dht11.c 9.58 KB