iio: dht11: Simplify decoding algorithm

The new algorithm uses a 'one size fits em all' threshold, which should
be easier to understand and debug. I believe there are no regressions
compared to the old adaptive threshold algorithm. I don't remember why
I chose the old algorithm when I initially wrote the driver.
Signed-off-by: Harald Geyer <harald@ccbib.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/humidity/dht11.c

@@ -50,12 +50,32 @@
 #define DHT11_EDGES_PER_READ (2 * DHT11_BITS_PER_READ + \
 			      DHT11_EDGES_PREAMBLE + 1)
 
-/* Data transmission timing (nano seconds) */
+/*
+ * Data transmission timing:
+ * Data bits are encoded as pulse length (high time) on the data line.
+ * 0-bit: 22-30uS -- typically 26uS (AM2302)
+ * 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.
+ *
+ * Our decoding depends on the time resolution of the system:
+ * timeres > 34uS ... don't know what a 1-tick pulse is
+ * 34uS > timeres > 30uS ... no problem (30kHz and 32kHz clocks)
+ * 30uS > timeres > 23uS ... don't know what a 2-tick pulse is
+ * timeres < 23uS ... no problem
+ *
+ * Luckily clocks in the 33-44kHz range are quite uncommon, so we can
+ * support most systems if the threshold for decoding a pulse as 1-bit
+ * is chosen carefully. If somebody really wants to support clocks around
+ * 40kHz, where this driver is most unreliable, there are two options.
+ * 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_SENSOR_RESPONSE	80000
-#define DHT11_START_BIT		50000
-#define DHT11_DATA_BIT_LOW	27000
-#define DHT11_DATA_BIT_HIGH	70000
+#define DHT11_MIN_TIMERES	34000  /* ns */
+#define DHT11_THRESHOLD		49000  /* ns */
+#define DHT11_AMBIG_LOW		23000  /* ns */
+#define DHT11_AMBIG_HIGH	30000  /* ns */
 
 struct dht11 {
 	struct device			*dev;
@@ -76,43 +96,39 @@ struct dht11 {
 	struct {s64 ts; int value; }	edges[DHT11_EDGES_PER_READ];
 };
 
-static unsigned char dht11_decode_byte(int *timing, int threshold)
+static unsigned char dht11_decode_byte(char *bits)
 {
 	unsigned char ret = 0;
 	int i;
 
 	for (i = 0; i < 8; ++i) {
 		ret <<= 1;
-		if (timing[i] >= threshold)
+		if (bits[i])
 			++ret;
 	}
 
 	return ret;
 }
 
-static int dht11_decode(struct dht11 *dht11, int offset, int timeres)
+static int dht11_decode(struct dht11 *dht11, int offset)
 {
-	int i, t, timing[DHT11_BITS_PER_READ], threshold;
+	int i, t;
+	char bits[DHT11_BITS_PER_READ];
 	unsigned char temp_int, temp_dec, hum_int, hum_dec, checksum;
 
-	threshold = DHT11_DATA_BIT_HIGH / timeres;
-	if (DHT11_DATA_BIT_LOW / timeres + 1 >= threshold)
-		pr_err("dht11: WARNING: decoding ambiguous\n");
-
-	/* scale down with timeres and check validity */
 	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)
 			return -EIO;  /* lost synchronisation */
-		timing[i] = t / timeres;
+		bits[i] = t > DHT11_THRESHOLD;
 	}
 
-	hum_int = dht11_decode_byte(timing, threshold);
-	hum_dec = dht11_decode_byte(&timing[8], threshold);
-	temp_int = dht11_decode_byte(&timing[16], threshold);
-	temp_dec = dht11_decode_byte(&timing[24], threshold);
-	checksum = dht11_decode_byte(&timing[32], threshold);
+	hum_int = dht11_decode_byte(bits);
+	hum_dec = dht11_decode_byte(&bits[8]);
+	temp_int = dht11_decode_byte(&bits[16]);
+	temp_dec = dht11_decode_byte(&bits[24]);
+	checksum = dht11_decode_byte(&bits[32]);
 
 	if (((hum_int + hum_dec + temp_int + temp_dec) & 0xff) != checksum)
 		return -EIO;
@@ -166,7 +182,7 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
 	mutex_lock(&dht11->lock);
 	if (dht11->timestamp + DHT11_DATA_VALID_TIME < ktime_get_real_ns()) {
 		timeres = ktime_get_resolution_ns();
-		if (DHT11_DATA_BIT_HIGH < 2 * timeres) {
+		if (timeres > DHT11_MIN_TIMERES) {
 			dev_err(dht11->dev, "timeresolution %dns too low\n",
 				timeres);
 			/* In theory a better clock could become available
@@ -176,6 +192,10 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
 			ret = -EAGAIN;
 			goto err;
 		}
+		if (timeres > DHT11_AMBIG_LOW && timeres < DHT11_AMBIG_HIGH)
+			dev_warn(dht11->dev,
+				 "timeresolution: %dns - decoding ambiguous\n",
+				 timeres);
 
 		reinit_completion(&dht11->completion);
 
@@ -211,7 +231,7 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
 		offset = DHT11_EDGES_PREAMBLE +
 				dht11->num_edges - DHT11_EDGES_PER_READ;
 		for (; offset >= 0; --offset) {
-			ret = dht11_decode(dht11, offset, timeres);
+			ret = dht11_decode(dht11, offset);
 			if (!ret)
 				break;
 		}