i2c: add api call to read byte from a specified register

Signed-off-by: Anton Krasovsky <anton@no-mad.net>
Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>

api/mraa/i2c.h

@@ -95,6 +95,15 @@ int mraa_i2c_read(mraa_i2c_context dev, uint8_t *data, int length);
  */
 uint8_t mraa_i2c_read_byte(mraa_i2c_context dev);
 
+/**
+ * Read a single byte from i2c context, from designated register
+ *
+ * @param dev The i2c context
+ * @param command The register
+ * @return The result of the read or -1 if failed
+ */
+uint8_t mraa_i2c_read_byte_data(mraa_i2c_context dev, uint8_t command);
+
 /**
  * Write to an i2c context
  *

api/mraa/i2c.hpp

@@ -143,6 +143,18 @@ class I2c {
             return mraa_i2c_write(m_i2c, buf, 2);
         }
 
+        /**
+         * Read an i2c register
+         *
+         * @param reg Register to read from
+
+         * @return char read from the bus
+         * @return char read from register
+         */
+        int readReg(char reg) {
+            return mraa_i2c_read_byte_data(m_i2c, reg);
+        }
+
         /**
          * Write multiple bytes to the bus
          *

src/i2c/i2c.c

@@ -101,6 +101,13 @@ mraa_i2c_read_byte(mraa_i2c_context dev)
     return byte;
 }
 
+uint8_t
+mraa_i2c_read_byte_data(mraa_i2c_context dev, uint8_t command)
+{
+    uint8_t byte = i2c_smbus_read_byte_data(dev->fh, command);
+    return byte;
+}
+
 mraa_result_t
 mraa_i2c_write(mraa_i2c_context dev, const uint8_t* data, int length)
 {