[issue #10] Fixed encoding of temperature value read from sensors

The value of temperature that is read from register of the T100
sensor is encoded with twos complement on 16 bits. The encoding
was not proper thus it worked fine but only for positive values
of temperature. Added function that properly interprets the
value of temp read from sensor's regsister.

userspace/libwr/fan.c

@@ -156,6 +156,30 @@ static uint32_t tmp100_read_reg(int dev_addr, uint8_t reg_addr, int n_bytes)
 	return rv;
 }
 
+/* Interpret the readout from registers of TMP100 as temperature
+ * According to TMP100, TMP101 datasheet, the temperature is encoded
+ * on 12 bits in binary twos completment format, e.g.:
+ * temp =  128 Celsius degree for reg_val=b'0111 1111 1111
+ * temp =   25 Celsius degree for reg_val=b'0001 1001 0000
+ * temp =    0 Celsius degree for reg_val=b'0000 0000 0000
+ * temp =  -25 Celsius degree for reg_val=b'1110 0111 0000
+ * temp = -128 Celsius degree for reg_val=b'1000 0000 0000
+ *
+ * We read 2 bytes (16 bits) and the four LSB bits are 0x0. Thus,
+ * the value is interpreted as 16 bit twos complement.
+ */
+static int tmp100_read_temp(int dev_addr)
+{
+	int temp;
+	uint32_t rv = 0x0000FFFF & tmp100_read_reg(dev_addr, 0, 2);
+
+	if(rv & 0x00008000) // negative
+		temp = -(int)(0x0000FFFF & (~rv+1));
+	else                // positive
+		temp =  (int)(0x00007FFF & rv);
+	return temp;
+}
+
 static void tmp100_write_reg(int dev_addr, uint8_t reg_addr, uint8_t value)
 {
 	uint8_t data[2];
@@ -261,10 +285,10 @@ void shw_update_fans(struct hal_temp_sensors *sensors)
 	//pr_info("t=%f,pwm=%f\n",t_cur , drive);
 
 	/* update sensor values */
-	sensors->fpga = tmp100_read_reg(TEMP_SENSOR_ADDR_FPGA, 0, 2);
-	sensors->pll = tmp100_read_reg(TEMP_SENSOR_ADDR_PLL, 0, 2);
-	sensors->psl = tmp100_read_reg(TEMP_SENSOR_ADDR_PSL, 0, 2);
-	sensors->psr = tmp100_read_reg(TEMP_SENSOR_ADDR_PSR, 0, 2);
+	sensors->fpga = tmp100_read_temp(TEMP_SENSOR_ADDR_FPGA);
+	sensors->pll  = tmp100_read_temp(TEMP_SENSOR_ADDR_PLL);
+	sensors->psl  = tmp100_read_temp(TEMP_SENSOR_ADDR_PSL);
+	sensors->psr  = tmp100_read_temp(TEMP_SENSOR_ADDR_PSR);
 
 	/* drive fan based on PLL temperature */
 	t_cur = ((float)(sensors->pll >> 4)) / 16.0;