linux-kernel - [PATCH AUTOSEL 4.19 089/100] iio: adc: hx711: fix bug in sampling of data

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Message-Id: <20191018220525.9042-89-sashal@kernel.org>
Date:   Fri, 18 Oct 2019 18:05:14 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Andreas Klinger <ak@...klinger.de>, Stable@...r.kernel.org,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Sasha Levin <sashal@...nel.org>, linux-iio@...r.kernel.org
Subject: [PATCH AUTOSEL 4.19 089/100] iio: adc: hx711: fix bug in sampling of data

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

[ Upstream commit 4043ecfb5fc4355a090111e14faf7945ff0fdbd5 ]

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: Sasha Levin <sashal@...nel.org>
---
 drivers/iio/adc/hx711.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/iio/adc/hx711.c b/drivers/iio/adc/hx711.c
index 36b59d8957fb8..6c5d81a89aec9 100644
--- a/drivers/iio/adc/hx711.c
+++ b/drivers/iio/adc/hx711.c
@@ -109,14 +109,14 @@ 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);
 
 	/*
@@ -126,7 +126,6 @@ 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
@@ -134,7 +133,7 @@ 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
@@ -142,7 +141,8 @@ 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)
-- 
2.20.1