hwmon: (mcp3021) Fix broken output scaling

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>
Cc: stable@vger.kernel.org # v3.10+
[Guenter Roeck: Dropped unnecessary value check]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

diff --git a/drivers/hwmon/mcp3021.c b/drivers/hwmon/mcp3021.c
index d219c06a857bb5795a176bb88d9e186fd6a4b286..972444a14cca5feb333c5244fd438fbb435f1e7e 100644 (file)
--- a/drivers/hwmon/mcp3021.c
+++ b/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;
        }