linux-kernel - [PATCH 5.3 052/112] iio: adc: hx711: fix bug in sampling of data

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Date:   Wed, 16 Oct 2019 14:50:44 -0700
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Andreas Klinger <ak@...klinger.de>,
        Stable@...r.kernel.org,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>
Subject: [PATCH 5.3 052/112] iio: adc: hx711: fix bug in sampling of data

From: Andreas Klinger <ak@...klinger.de>

commit 4043ecfb5fc4355a090111e14faf7945ff0fdbd5 upstream.

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.

Switch off interrupts during a PD_SCK high period and move query of DOUT
to the latest point of time which is at the end of PD_SCK low period.

This bug exists in the driver since it's initial addition. The more
interrupts on the system the higher is the probability that it happens.

Fixes: c3b2fdd0ea7e ("iio: adc: hx711: Add IIO driver for AVIA HX711")
Signed-off-by: Andreas Klinger <ak@...klinger.de>
Cc: <Stable@...r.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/iio/adc/hx711.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

--- a/drivers/iio/adc/hx711.c
+++ b/drivers/iio/adc/hx711.c
@@ -100,14 +100,14 @@ 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 +117,6 @@ static int hx711_cycle(struct 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 +124,7 @@ static int hx711_cycle(struct 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 +132,8 @@ static int hx711_cycle(struct 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)