[PATCH] iio: dht11: Read bit stream from IRQ on falling edges only

[Andreas Feldner <pelzi@xxxxxxxxxxxxxxx>]

Currently, IRQs for both falling and raising edges of the GPIO
line connected to the DHT11 device are requested. However, the
low states do not carry information, it is possible to determine
0 and 1 bits from the timing of two adjacent falling edges as
well.

Doing so does no longer requires to read the GPIO line value
within the IRQ handler, plus halves the number of IRQs to be
handled at all.

Signed-off-by: Andreas Feldner <pelzi@xxxxxxxxxxxxxxx>
---
 drivers/iio/humidity/dht11.c | 28 +++++++++++-----------------
 1 file changed, 11 insertions(+), 17 deletions(-)

diff --git a/drivers/iio/humidity/dht11.c b/drivers/iio/humidity/dht11.c
index c97e25448772..d1cd053c5dd4 100644
--- a/drivers/iio/humidity/dht11.c
+++ b/drivers/iio/humidity/dht11.c
@@ -30,13 +30,13 @@
 
 #define DHT11_DATA_VALID_TIME	2000000000  /* 2s in ns */
 
-#define DHT11_EDGES_PREAMBLE 2
+#define DHT11_EDGES_PREAMBLE 1
 #define DHT11_BITS_PER_READ 40
 /*
  * Note that when reading the sensor actually 84 edges are detected, but
  * since the last edge is not significant, we only store 83:
  */
-#define DHT11_EDGES_PER_READ (2 * DHT11_BITS_PER_READ + \
+#define DHT11_EDGES_PER_READ (DHT11_BITS_PER_READ + \
 			      DHT11_EDGES_PREAMBLE + 1)
 
 /*
@@ -46,6 +46,7 @@
  * 1-bit: 68-75uS -- typically 70uS (AM2302)
  * The acutal timings also depend on the properties of the cable, with
  * longer cables typically making pulses shorter.
+ * Low time is constant 50uS.
  *
  * Our decoding depends on the time resolution of the system:
  * timeres > 34uS ... don't know what a 1-tick pulse is
@@ -63,7 +64,8 @@
 #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_LOW		50000  /* ns */
+#define DHT11_THRESHOLD		(49000 + DHT11_LOW)  /* ns */
 #define DHT11_AMBIG_LOW		23000  /* ns */
 #define DHT11_AMBIG_HIGH	30000  /* ns */
 
@@ -83,7 +85,7 @@ struct dht11 {
 
 	/* num_edges: -1 means "no transmission in progress" */
 	int				num_edges;
-	struct {s64 ts; int value; }	edges[DHT11_EDGES_PER_READ];
+	struct {s64 ts; }	edges[DHT11_EDGES_PER_READ];
 };
 
 #ifdef CONFIG_DYNAMIC_DEBUG
@@ -99,7 +101,7 @@ static void dht11_edges_print(struct dht11 *dht11)
 	for (i = 1; i < dht11->num_edges; ++i) {
 		dev_dbg(dht11->dev, "%d: %lld ns %s\n", i,
 			dht11->edges[i].ts - dht11->edges[i - 1].ts,
-			dht11->edges[i - 1].value ? "high" : "low");
+			"falling");
 	}
 }
 #endif /* CONFIG_DYNAMIC_DEBUG */
@@ -125,14 +127,8 @@ static int dht11_decode(struct dht11 *dht11, int offset)
 	unsigned char temp_int, temp_dec, hum_int, hum_dec, checksum;
 
 	for (i = 0; i < DHT11_BITS_PER_READ; ++i) {
-		t = dht11->edges[offset + 2 * i + 2].ts -
-			dht11->edges[offset + 2 * i + 1].ts;
-		if (!dht11->edges[offset + 2 * i + 1].value) {
-			dev_dbg(dht11->dev,
-				"lost synchronisation at edge %d\n",
-				offset + 2 * i + 1);
-			return -EIO;
-		}
+		t = dht11->edges[offset + i + 1].ts -
+		    dht11->edges[offset + i].ts;
 		bits[i] = t > DHT11_THRESHOLD;
 	}
 
@@ -174,9 +170,7 @@ static irqreturn_t dht11_handle_irq(int irq, void *data)
 	struct dht11 *dht11 = iio_priv(iio);
 
 	if (dht11->num_edges < DHT11_EDGES_PER_READ && dht11->num_edges >= 0) {
-		dht11->edges[dht11->num_edges].ts = ktime_get_boottime_ns();
-		dht11->edges[dht11->num_edges++].value =
-						gpiod_get_value(dht11->gpiod);
+		dht11->edges[dht11->num_edges++].ts = ktime_get_boottime_ns();
 
 		if (dht11->num_edges >= DHT11_EDGES_PER_READ)
 			complete(&dht11->completion);
@@ -224,7 +218,7 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
 			goto err;
 
 		ret = request_irq(dht11->irq, dht11_handle_irq,
-				  IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING,
+				  IRQF_TRIGGER_FALLING,
 				  iio_dev->name, iio_dev);
 		if (ret)
 			goto err;
-- 
2.30.2