linux-kernel - [PATCH 5.15 040/107] iio: adc: mcp3911: use correct formula for AD conversion

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Message-Id: <20220906132823.508070326@linuxfoundation.org>
Date:   Tue,  6 Sep 2022 15:30:21 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Marcus Folkesson <marcus.folkesson@...il.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Stable@...r.kernel.org,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>
Subject: [PATCH 5.15 040/107] iio: adc: mcp3911: use correct formula for AD conversion

From: Marcus Folkesson <marcus.folkesson@...il.com>

commit 9e2238e3ae40d371a1130226e0e740aa1601efa6 upstream.

The ADC conversion is actually not rail-to-rail but with a factor 1.5.
Make use of this factor when calculating actual voltage.

Fixes: 3a89b289df5d ("iio: adc: add support for mcp3911")
Signed-off-by: Marcus Folkesson <marcus.folkesson@...il.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@...il.com>
Link: https://lore.kernel.org/r/20220722130726.7627-4-marcus.folkesson@gmail.com
Cc: <Stable@...r.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/iio/adc/mcp3911.c |   17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

--- a/drivers/iio/adc/mcp3911.c
+++ b/drivers/iio/adc/mcp3911.c
@@ -38,8 +38,8 @@
 #define MCP3911_CHANNEL(x)		(MCP3911_REG_CHANNEL0 + x * 3)
 #define MCP3911_OFFCAL(x)		(MCP3911_REG_OFFCAL_CH0 + x * 6)
 
-/* Internal voltage reference in uV */
-#define MCP3911_INT_VREF_UV		1200000
+/* Internal voltage reference in mV */
+#define MCP3911_INT_VREF_MV		1200
 
 #define MCP3911_REG_READ(reg, id)	((((reg) << 1) | ((id) << 5) | (1 << 0)) & 0xff)
 #define MCP3911_REG_WRITE(reg, id)	((((reg) << 1) | ((id) << 5) | (0 << 0)) & 0xff)
@@ -137,11 +137,18 @@ static int mcp3911_read_raw(struct iio_d
 
 			*val = ret / 1000;
 		} else {
-			*val = MCP3911_INT_VREF_UV;
+			*val = MCP3911_INT_VREF_MV;
 		}
 
-		*val2 = 24;
-		ret = IIO_VAL_FRACTIONAL_LOG2;
+		/*
+		 * For 24bit Conversion
+		 * Raw = ((Voltage)/(Vref) * 2^23 * Gain * 1.5
+		 * Voltage = Raw * (Vref)/(2^23 * Gain * 1.5)
+		 */
+
+		/* val2 = (2^23 * 1.5) */
+		*val2 = 12582912;
+		ret = IIO_VAL_FRACTIONAL;
 		break;
 	}