linux-kernel - [PATCH v2] iio: dac: ad5592r: fix temperature channel scaling value

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Date: Wed,  1 May 2024 11:05:54 -0400
From: marc.ferland@...il.com
To: lars@...afoo.de
Cc: Michael.Hennerich@...log.com,
	jic23@...nel.org,
	linux-iio@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Marc Ferland <marc.ferland@...atest.com>
Subject: [PATCH v2] iio: dac: ad5592r: fix temperature channel scaling value

From: Marc Ferland <marc.ferland@...atest.com>

The scale value for the temperature channel is (assuming Vref=2.5 and
the datasheet):

    376.7897513

When calculating both val and val2 for the temperature scale we
use (3767897513/25) and multiply it by Vref (here I assume 2500mV) to
obtain:

  2500 * (3767897513/25) ==> 376789751300

Finally we divide with remainder by 10^9 to get:

    val = 376
    val2 = 789751300

However, we return IIO_VAL_INT_PLUS_MICRO (should have been NANO) as
the scale type. So when converting the raw temperature value to the
'processed' temperature value we will get (assuming raw=810,
offset=-753):

    processed = (raw + offset) * scale_val
              = (810 + -753) * 376
	      = 21432

    processed += div((raw + offset) * scale_val2, 10^6)
              += div((810 + -753) * 789751300, 10^6)
	      += 45015
    ==> 66447
    ==> 66.4 Celcius

instead of the expected 21.5 Celsius.

Fix this issue by changing IIO_VAL_INT_PLUS_MICRO to
IIO_VAL_INT_PLUS_NANO.

Signed-off-by: Marc Ferland <marc.ferland@...atest.com>
---
Change in v2:
 - Improve commit message as suggested by Jonathan.

 drivers/iio/dac/ad5592r-base.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iio/dac/ad5592r-base.c b/drivers/iio/dac/ad5592r-base.c
index 076bc9ecfb49..4763402dbcd6 100644
--- a/drivers/iio/dac/ad5592r-base.c
+++ b/drivers/iio/dac/ad5592r-base.c
@@ -415,7 +415,7 @@ static int ad5592r_read_raw(struct iio_dev *iio_dev,
 			s64 tmp = *val * (3767897513LL / 25LL);
 			*val = div_s64_rem(tmp, 1000000000LL, val2);
 
-			return IIO_VAL_INT_PLUS_MICRO;
+			return IIO_VAL_INT_PLUS_NANO;
 		}
 
 		mutex_lock(&st->lock);

base-commit: 98369dccd2f8e16bf4c6621053af7aa4821dcf8e
-- 
2.34.1