linux-kernel - [PATCH] iio: light: veml6030: fix microlux value calculation

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Message-Id: <20241016-veml6030-fix-processed-micro-v1-1-4a5644796437@gmail.com>
Date: Wed, 16 Oct 2024 19:04:31 +0200
From: Javier Carrasco <javier.carrasco.cruz@...il.com>
To: Jonathan Cameron <jic23@...nel.org>, 
 Lars-Peter Clausen <lars@...afoo.de>, Rishi Gupta <gupt21@...il.com>
Cc: Jonathan Cameron <Jonathan.Cameron@...wei.com>, 
 linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org, 
 stable@...r.kernel.org, Javier Carrasco <javier.carrasco.cruz@...il.com>
Subject: [PATCH] iio: light: veml6030: fix microlux value calculation

The raw value conversion to obtain a measurement in lux as
INT_PLUS_MICRO does not calculate the decimal part properly to display
it as micro (in this case microlux). It only calculates the module to
obtain the decimal part from a resolution that is 10000 times the
provided in the datasheet (0.5376 lux/cnt for the veml6030). The
resulting value must still be multiplied by 100 to make it micro.

This bug was introduced with the original implementation of the driver.

Cc: stable@...r.kernel.org
Fixes: 7b779f573c48 ("iio: light: add driver for veml6030 ambient light sensor")
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@...il.com>
---
I found this almost by chance while testing new supported devices. The
decimal part was always suspiciously small, and when I compared samples
to the expected value according to the datasheet, it became clear what was
going on.

Example with a veml7700 (same resolution as the veml6030):

Resolution for gain = 1/8, IT = 100 ms: 0.5736 lux/cnt.

cat in_illuminance_raw in_illuminance_input
40
21.005040 -> wrong! 40 * 0.5736 is 21.504.

Tested with a veml6035 and a veml7700, the same will happen with the
original veml6030, as the operation is identical for all devices.
---
 drivers/iio/light/veml6030.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iio/light/veml6030.c b/drivers/iio/light/veml6030.c
index d6f3b104b0e6..a0bf03e37df7 100644
--- a/drivers/iio/light/veml6030.c
+++ b/drivers/iio/light/veml6030.c
@@ -691,7 +691,7 @@ static int veml6030_read_raw(struct iio_dev *indio_dev,
 			}
 			if (mask == IIO_CHAN_INFO_PROCESSED) {
 				*val = (reg * data->cur_resolution) / 10000;
-				*val2 = (reg * data->cur_resolution) % 10000;
+				*val2 = (reg * data->cur_resolution) % 10000 * 100;
 				return IIO_VAL_INT_PLUS_MICRO;
 			}
 			*val = reg;

---
base-commit: 15e7d45e786a62a211dd0098fee7c57f84f8c681
change-id: 20241016-veml6030-fix-processed-micro-616d00d555dc

Best regards,
-- 
Javier Carrasco <javier.carrasco.cruz@...il.com>