iio: adc: hx711: optimize performance in read cycle

Set gain in hx711_reset() to its default value after a reset cycle. This
omits one precautionary read cycle, because the read is performed in
hx711_set_gain_for_channel() anyway if gain has changed.

Check for DOUT low and if its high wait some time if it goes down
instead of doing a blind reset cycle when DOUT is not down.

This is a performance optimization which allows to query the sensor with
a higher frequency.
Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/hx711.c

@@ -23,6 +23,7 @@
 
 /* gain to pulse and scale conversion */
 #define HX711_GAIN_MAX		3
+#define HX711_RESET_GAIN	128
 
 struct hx711_gain_to_scale {
 	int			gain;
@@ -185,8 +186,7 @@ static int hx711_wait_for_ready(struct hx711_data *hx711_data)
 
 static int hx711_reset(struct hx711_data *hx711_data)
 {
-	int ret;
-	int val = gpiod_get_value(hx711_data->gpiod_dout);
+	int val = hx711_wait_for_ready(hx711_data);
 
 	if (val) {
 		/*
@@ -202,22 +202,10 @@ static int hx711_reset(struct hx711_data *hx711_data)
 		msleep(10);
 		gpiod_set_value(hx711_data->gpiod_pd_sck, 0);
 
-		ret = hx711_wait_for_ready(hx711_data);
-		if (ret)
-			return ret;
-		/*
-		 * after a reset the gain is 128 so we do a dummy read
-		 * to set the gain for the next read
-		 */
-		ret = hx711_read(hx711_data);
-		if (ret < 0)
-			return ret;
-
-		/*
-		 * after a dummy read we need to wait vor readiness
-		 * for not mixing gain pulses with the clock
-		 */
 		val = hx711_wait_for_ready(hx711_data);
+
+		/* after a reset the gain is 128 */
+		hx711_data->gain_set = HX711_RESET_GAIN;
 	}
 
 	return val;