hwmon: (mcp3021) Fix broken output scaling

commit 347d7e45 upstream.

The mcp3021 scaling code is dividing the VDD (full-scale) value in
millivolts by the A2D resolution to obtain the scaling factor. When VDD
is 3300mV (the standard value) and the resolution is 12-bit (4096
divisions), the result is a scale factor of 3300/4096, which is always
one.  Effectively, the raw A2D reading is always being returned because
no scaling is applied.

This patch fixes the issue and simplifies the register-to-volts
calculation, removing the unneeded "output_scale" struct member.
Signed-off-by: Nick Stevens <Nick.Stevens@digi.com>
[Guenter Roeck: Dropped unnecessary value check]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/hwmon/mcp3021.c

@@ -31,14 +31,11 @@
 /* output format */
 #define MCP3021_SAR_SHIFT	2
 #define MCP3021_SAR_MASK	0x3ff
-
 #define MCP3021_OUTPUT_RES	10	/* 10-bit resolution */
-#define MCP3021_OUTPUT_SCALE	4
 
 #define MCP3221_SAR_SHIFT	0
 #define MCP3221_SAR_MASK	0xfff
 #define MCP3221_OUTPUT_RES	12	/* 12-bit resolution */
-#define MCP3221_OUTPUT_SCALE	1
 
 enum chips {
 	mcp3021,
@@ -54,7 +51,6 @@ struct mcp3021_data {
 	u16 sar_shift;
 	u16 sar_mask;
 	u8 output_res;
-	u8 output_scale;
 };
 
 static int mcp3021_read16(struct i2c_client *client)
@@ -84,13 +80,7 @@ static int mcp3021_read16(struct i2c_client *client)
 
 static inline u16 volts_from_reg(struct mcp3021_data *data, u16 val)
 {
-	if (val == 0)
-		return 0;
-
-	val = val * data->output_scale - data->output_scale / 2;
-
-	return val * DIV_ROUND_CLOSEST(data->vdd,
-			(1 << data->output_res) * data->output_scale);
+	return DIV_ROUND_CLOSEST(data->vdd * val, 1 << data->output_res);
 }
 
 static ssize_t show_in_input(struct device *dev, struct device_attribute *attr,
@@ -132,14 +122,12 @@ static int mcp3021_probe(struct i2c_client *client,
 		data->sar_shift = MCP3021_SAR_SHIFT;
 		data->sar_mask = MCP3021_SAR_MASK;
 		data->output_res = MCP3021_OUTPUT_RES;
-		data->output_scale = MCP3021_OUTPUT_SCALE;
 		break;
 
 	case mcp3221:
 		data->sar_shift = MCP3221_SAR_SHIFT;
 		data->sar_mask = MCP3221_SAR_MASK;
 		data->output_res = MCP3221_OUTPUT_RES;
-		data->output_scale = MCP3221_OUTPUT_SCALE;
 		break;
 	}