[PATCH v2] iio: si1133: read 24-bit signed integer for measurement

["Maxime Roussin-Bélanger" <maxime.roussinbelanger@gmail.com>]

The chip is configured in 24 bit mode. The values read from
it must always be treated as is. This fixes the issue by
replacing the previous 16 bits value by a 24 bits buffer.

This changes affects the value output by previous version of
the driver, since the least significant byte was missing.
The upper half of 16 bit values previously output are now
the upper half of a 24 bit value.

Fixes: e01e7eaf37d8 ("iio: light: introduce si1133")

Reported-by: Simon Goyette <simon.goyette@gmail.com>
Co-authored-by: Guillaume Champagne <champagne.guillaume.c@gmail.com>
Signed-off-by: Maxime Roussin-Bélanger <maxime.roussinbelanger@gmail.com>
Signed-off-by: Guillaume Champagne <champagne.guillaume.c@gmail.com>
---
Changes in v2:
	* Add missing `Fixes` tag
	* Add Reported-by
	* Add missing signed-off-by from co-author.

drivers/iio/light/si1133.c | 37 ++++++++++++++++++++++++-------------
 1 file changed, 24 insertions(+), 13 deletions(-)

diff --git a/drivers/iio/light/si1133.c b/drivers/iio/light/si1133.c
index 777b1a0848c9..509af982e185 100644
--- a/drivers/iio/light/si1133.c
+++ b/drivers/iio/light/si1133.c
@@ -102,6 +102,9 @@
 #define SI1133_INPUT_FRACTION_LOW	15
 #define SI1133_LUX_OUTPUT_FRACTION	12
 #define SI1133_LUX_BUFFER_SIZE		9
+#define SI1133_MEASURE_BUFFER_SIZE	3
+
+#define SI1133_SIGN_BIT_INDEX 23
 
 static const int si1133_scale_available[] = {
 	1, 2, 4, 8, 16, 32, 64, 128};
@@ -234,13 +237,13 @@ static const struct si1133_lux_coeff lux_coeff = {
 	}
 };
 
-static int si1133_calculate_polynomial_inner(u32 input, u8 fraction, u16 mag,
+static int si1133_calculate_polynomial_inner(s32 input, u8 fraction, u16 mag,
 					     s8 shift)
 {
 	return ((input << fraction) / mag) << shift;
 }
 
-static int si1133_calculate_output(u32 x, u32 y, u8 x_order, u8 y_order,
+static int si1133_calculate_output(s32 x, s32 y, u8 x_order, u8 y_order,
 				   u8 input_fraction, s8 sign,
 				   const struct si1133_coeff *coeffs)
 {
@@ -276,7 +279,7 @@ static int si1133_calculate_output(u32 x, u32 y, u8 x_order, u8 y_order,
  * The algorithm is from:
  * https://siliconlabs.github.io/Gecko_SDK_Doc/efm32zg/html/si1133_8c_source.html#l00716
  */
-static int si1133_calc_polynomial(u32 x, u32 y, u8 input_fraction, u8 num_coeff,
+static int si1133_calc_polynomial(s32 x, s32 y, u8 input_fraction, u8 num_coeff,
 				  const struct si1133_coeff *coeffs)
 {
 	u8 x_order, y_order;
@@ -614,23 +617,24 @@ static int si1133_measure(struct si1133_data *data,
 {
 	int err;
 
-	__be16 resp;
+	u8 buffer[SI1133_MEASURE_BUFFER_SIZE];
 
 	err = si1133_set_adcmux(data, 0, chan->channel);
 	if (err)
 		return err;
 
 	/* Deactivate lux measurements if they were active */
 	err = si1133_set_chlist(data, BIT(0));
 	if (err)
 		return err;
 
-	err = si1133_bulk_read(data, SI1133_REG_HOSTOUT(0), sizeof(resp),
-			       (u8 *)&resp);
+	err = si1133_bulk_read(data, SI1133_REG_HOSTOUT(0), sizeof(buffer),
+			       buffer);
 	if (err)
 		return err;
 
-	*val = be16_to_cpu(resp);
+	*val = sign_extend32((buffer[0] << 16) | (buffer[1] << 8) | buffer[2],
+			     SI1133_SIGN_BIT_INDEX);
 
 	return err;
 }
@@ -704,9 +708,9 @@ static int si1133_get_lux(struct si1133_data *data, int *val)
 {
 	int err;
 	int lux;
-	u32 high_vis;
-	u32 low_vis;
-	u32 ir;
+	s32 high_vis;
+	s32 low_vis;
+	s32 ir;
 	u8 buffer[SI1133_LUX_BUFFER_SIZE];
 
 	/* Activate lux channels */
@@ -719,9 +723,16 @@ static int si1133_get_lux(struct si1133_data *data, int *val)
 	if (err)
 		return err;
 
-	high_vis = (buffer[0] << 16) | (buffer[1] << 8) | buffer[2];
-	low_vis = (buffer[3] << 16) | (buffer[4] << 8) | buffer[5];
-	ir = (buffer[6] << 16) | (buffer[7] << 8) | buffer[8];
+	high_vis =
+		sign_extend32((buffer[0] << 16) | (buffer[1] << 8) | buffer[2],
+			      SI1133_SIGN_BIT_INDEX);
+
+	low_vis =
+		sign_extend32((buffer[3] << 16) | (buffer[4] << 8) | buffer[5],
+			      SI1133_SIGN_BIT_INDEX);
+
+	ir = sign_extend32((buffer[6] << 16) | (buffer[7] << 8) | buffer[8],
+			   SI1133_SIGN_BIT_INDEX);
 
 	if (high_vis > SI1133_ADC_THRESHOLD || ir > SI1133_ADC_THRESHOLD)
 		lux = si1133_calc_polynomial(high_vis, ir,
-- 
2.20.1