linux-kernel - [PATCH] iio: adc: ad7124: Disable all channels at probe time

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Message-ID: <20241104101905.845737-2-u.kleine-koenig@baylibre.com>
Date: Mon,  4 Nov 2024 11:19:04 +0100
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Lars-Peter Clausen <lars@...afoo.de>,
	Michael Hennerich <Michael.Hennerich@...log.com>,
	Dumitru Ceclan <dumitru.ceclan@...log.com>,
	Nuno Sa <nuno.sa@...log.com>,
	Guillaume Ranquet <granquet@...libre.com>,
	linux-iio@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] iio: adc: ad7124: Disable all channels at probe time

When during a measurement two channels are enabled, two measurements are
done that are reported sequencially in the DATA register. As the code
triggered by reading one of the sysfs properties expects that only one
channel is enabled it only reads the first data set which might or might
not belong to the intended channel.

To prevent this situation disable all channels during probe. This fixes
a problem in practise because the reset default for channel 0 is
enabled. So all measurements before the first measurement on channel 0
(which disables channel 0 at the end) might report wrong values.

Fixes: 7b8d045e497a ("iio: adc: ad7124: allow more than 8 channels")
Reviewed-by: Nuno Sa <nuno.sa@...log.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...libre.com>
---
Hello,

this patch was part of a series before. The remaining patches are still
under discussion. As this is a fix orthogonal to the other patches of
the series (apart from the other relevant change there also being
necessary to make the ad7124 work for me) it IMHO makes sense to apply
this one already now. There are machines that don't suffer from the
other issue (i.e. the device irq becoming pending by spi traffic), so
this fix is also valuable stand alone. It's IMHO good enough to go in
before v6.12.

The previous submission is available at
https://lore.kernel.org/linux-iio/20241028160748.489596-10-u.kleine-koenig@baylibre.com/

b4 ignored Nuno's Reviewed-by tag with

	NOTE: some trailers ignored due to from/email mismatches:
	    ! Trailer: Reviewed-by: Nuno Sa <nuno.sa@...log.com>
	     Msg From: Nuno Sá <noname.nuno@...il.com>

I wonder if other maintainers use b4 apply's -S by default, because I
often run into this issue but don't see others mentioning that.
I added the tag here anyhow.

 drivers/iio/adc/ad7124.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/iio/adc/ad7124.c b/drivers/iio/adc/ad7124.c
index a5d91933f505..749304d38415 100644
--- a/drivers/iio/adc/ad7124.c
+++ b/drivers/iio/adc/ad7124.c
@@ -917,6 +917,9 @@ static int ad7124_setup(struct ad7124_state *st)
 		 * set all channels to this default value.
 		 */
 		ad7124_set_channel_odr(st, i, 10);
+
+		/* Disable all channels to prevent unintended conversions. */
+		ad_sd_write_reg(&st->sd, AD7124_CHANNEL(i), 2, 0);
 	}

 	ret = ad_sd_write_reg(&st->sd, AD7124_ADC_CONTROL, 2, st->adc_control);

base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
-- 
2.45.2