iio: dht11: Improve reliability - be more tolerant about missing start bits

Instead of guessing where the data starts, we now just try to decode from
every possible start position. This causes no additional overhead if we
properly received the full preamble and only costs a few extra CPU cycles
in the case where the preamble is corrupted. This is much more efficient
than to return an error to userspace and start over again.

Signed-off-by: Harald Geyer <harald@ccbib.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

diff --git a/drivers/iio/humidity/dht11.c b/drivers/iio/humidity/dht11.c
index 1165b1c4f9d67bd93db24f784da5b13010d9fd99..1ca284aaacfcc578855c14c8f63c0bbd99c5d585 100644 (file)
--- a/drivers/iio/humidity/dht11.c
+++ b/drivers/iio/humidity/dht11.c
@@ -161,7 +161,7 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
                        int *val, int *val2, long m)
 {
        struct dht11 *dht11 = iio_priv(iio_dev);
-       int ret, timeres;
+       int ret, timeres, offset;
 
        mutex_lock(&dht11->lock);
        if (dht11->timestamp + DHT11_DATA_VALID_TIME < ktime_get_real_ns()) {
@@ -208,11 +208,14 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
                if (ret < 0)
                        goto err;
 
-               ret = dht11_decode(dht11,
-                                  dht11->num_edges == DHT11_EDGES_PER_READ ?
-                                       DHT11_EDGES_PREAMBLE :
-                                       DHT11_EDGES_PREAMBLE - 2,
-                               timeres);
+               offset = DHT11_EDGES_PREAMBLE +
+                               dht11->num_edges - DHT11_EDGES_PER_READ;
+               for (; offset >= 0; --offset) {
+                       ret = dht11_decode(dht11, offset, timeres);
+                       if (!ret)
+                               break;
+               }
+
                if (ret)
                        goto err;
        }