linux-kernel - [PATCH] iio: adc: ad7606: fix raw read for 18-bit chips

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Message-Id: <20250502-iio-adc-ad7606-fix-raw-read-for-18-bit-chips-v1-1-06caa92d8f11@baylibre.com>
Date: Fri, 02 May 2025 10:04:30 -0500
From: David Lechner <dlechner@...libre.com>
To: Michael Hennerich <Michael.Hennerich@...log.com>, 
 Jonathan Cameron <jic23@...nel.org>, 
 Alexandru Ardelean <aardelean@...libre.com>
Cc: Jonathan Cameron <Jonathan.Cameron@...wei.com>, 
 linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org, 
 David Lechner <dlechner@...libre.com>
Subject: [PATCH] iio: adc: ad7606: fix raw read for 18-bit chips

Fix 18-bit raw read for 18-bit chips by applying a mask to the value
we receive from the SPI controller.

SPI controllers either return 1, 2 or 4 bytes per word depending on the
bits_per_word. For 16-bit chips, there was no problem since they raw
data fit exactly in the 2 bytes received from the SPI controller. But
now that we have 18-bit chips and we are using bits_per_word = 18, we
cannot assume that the extra bits in the 32-bit word are always zero.
In fact, with the AXI SPI Engine controller, these bits are not always
zero which caused the raw values to read 10s of 1000s of volts instead
of the correct value. Therefore, we need to mask the value we receive
from the SPI controller to ensure that only the 18 bits of real data
are used.

Fixes: f3838e934dff ("iio: adc: ad7606: add support for AD7606C-{16,18} parts")
Signed-off-by: David Lechner <dlechner@...libre.com>
---
Jonathan, I based this on the fixes-togreg branch since I assume it will
go that route, but beware that there is a trivial merge conflict with
https://git.kernel.org/pub/scm/linux/kernel/git/jic23/iio.git/commit/?h=togreg&id=3b5b55ca940733b7b533fe3137a307ceacb4fb90
as that patch modifies an adjacent line.
---
 drivers/iio/adc/ad7606.c | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/drivers/iio/adc/ad7606.c b/drivers/iio/adc/ad7606.c
index 703556eb7257ea0647135c4b268a8ead93115c6f..8ed65a35b4862315431a4852453639b4e056178e 100644
--- a/drivers/iio/adc/ad7606.c
+++ b/drivers/iio/adc/ad7606.c
@@ -727,17 +727,16 @@ static int ad7606_scan_direct(struct iio_dev *indio_dev, unsigned int ch,
 		goto error_ret;
 
 	chan = &indio_dev->channels[ch + 1];
-	if (chan->scan_type.sign == 'u') {
-		if (realbits > 16)
-			*val = st->data.buf32[ch];
-		else
-			*val = st->data.buf16[ch];
-	} else {
-		if (realbits > 16)
-			*val = sign_extend32(st->data.buf32[ch], realbits - 1);
-		else
-			*val = sign_extend32(st->data.buf16[ch], realbits - 1);
-	}
+
+	if (realbits > 16)
+		*val = st->data.buf32[ch];
+	else
+		*val = st->data.buf16[ch];
+
+	*val &= GENMASK(realbits - 1, 0);
+
+	if (chan->scan_type.sign == 's')
+		*val = sign_extend32(*val, realbits - 1);
 
 error_ret:
 	if (!st->gpio_convst) {

---
base-commit: 609bc31eca06c7408e6860d8b46311ebe45c1fef
change-id: 20250502-iio-adc-ad7606-fix-raw-read-for-18-bit-chips-ed035c488eca

Best regards,
-- 
David Lechner <dlechner@...libre.com>