linux-kernel - [PATCH 1/3] iio: adc: hx711: optimize sampling of data

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Message-ID: <20190907101759.kft6xwsqc5lf4acq@arbad>
Date:   Sat, 7 Sep 2019 12:18:00 +0200
From:   Andreas Klinger <ak@...klinger.de>
To:     jic23@...nel.org, knaack.h@....de, lars@...afoo.de,
        pmeerw@...erw.net
Cc:     linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] iio: adc: hx711: optimize sampling of data

Fix bug in sampling function hx711_cycle() when interrupt occures while
PD_SCK is high. If PD_SCK is high for at least 60 us power down mode of
the sensor is entered which in turn leads to a wrong measurement.

Move query of DOUT at the latest point of time which is at the end of
PD_SCK low period.

Signed-off-by: Andreas Klinger <ak@...klinger.de>
---
 drivers/iio/adc/hx711.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/iio/adc/hx711.c b/drivers/iio/adc/hx711.c
index 88c7fe15003b..0678964dbd21 100644
--- a/drivers/iio/adc/hx711.c
+++ b/drivers/iio/adc/hx711.c
@@ -101,13 +101,14 @@ struct hx711_data {
 static int hx711_cycle(struct hx711_data *hx711_data)
 {
 	int val;
+	unsigned long flags;
 
 	/*
 	 * if preempted for more then 60us while PD_SCK is high:
 	 * hx711 is going in reset
 	 * ==> measuring is false
 	 */
-	preempt_disable();
+	local_irq_save(flags);
 	gpiod_set_value(hx711_data->gpiod_pd_sck, 1);
 
 	/*
@@ -117,7 +118,6 @@ static int hx711_cycle(struct hx711_data *hx711_data)
 	 */
 	ndelay(hx711_data->data_ready_delay_ns);
 
-	val = gpiod_get_value(hx711_data->gpiod_dout);
 	/*
 	 * here we are not waiting for 0.2 us as suggested by the datasheet,
 	 * because the oscilloscope showed in a test scenario
@@ -125,7 +125,7 @@ static int hx711_cycle(struct hx711_data *hx711_data)
 	 * and 0.56 us for PD_SCK low on TI Sitara with 800 MHz
 	 */
 	gpiod_set_value(hx711_data->gpiod_pd_sck, 0);
-	preempt_enable();
+	local_irq_restore(flags);
 
 	/*
 	 * make it a square wave for addressing cases with capacitance on
@@ -133,7 +133,8 @@ static int hx711_cycle(struct hx711_data *hx711_data)
 	 */
 	ndelay(hx711_data->data_ready_delay_ns);
 
-	return val;
+	/* sample as late as possible */
+	return gpiod_get_value(hx711_data->gpiod_dout);
 }
 
 static int hx711_read(struct hx711_data *hx711_data)
-- 
2.13.3