smbus: fix issue rust-embedded#1 and expose smbus publically

Previously, there were some bugs with the SMBus implementation
and it was not actually exposed at all.  Now, the nunchuck
example uses the SMBus funtions where it can.

.gitignore

@@ -10,3 +10,7 @@
 # Generated by Cargo
 Cargo.lock
 /target/
+
+# TAGS
+GPATH
+*TAGS

examples/nunchuck.rs

@@ -12,11 +12,12 @@ extern crate i2cdev;
 extern crate docopt;
 
 use i2cdev::core::*;
+use i2cdev::smbus::*;
+use std::io;
 use std::io::prelude::*;
 use std::env::args;
 use docopt::Docopt;
 use std::thread;
-use std::io;
 use std::path::Path;
 
 const NUNCHUCK_SLAVE_ADDR: u16 = 0x52;
@@ -46,15 +47,19 @@ struct NunchuckReading {
 }
 
 impl NunchuckReading {
-    fn from_data(data: &[u8; 6]) -> NunchuckReading {
-        NunchuckReading {
-            joystick_x: data[0],
-            joystick_y: data[1],
-            accel_x: (data[2] as u16) << 2 | ((data[5] as u16 >> 6) & 0b11),
-            accel_y: (data[3] as u16) << 2 | ((data[5] as u16 >> 4) & 0b11),
-            accel_z: (data[4] as u16) << 2 | ((data[5] as u16 >> 2) & 0b11),
-            c_button_pressed: (data[5] & 0b10) == 0,
-            z_button_pressed: (data[5] & 0b01) == 0,
+    fn from_data(data: &[u8]) -> Option<NunchuckReading> {
+        if data.len() < 6 {
+            None
+        } else {
+            Some(NunchuckReading {
+                joystick_x: data[0],
+                joystick_y: data[1],
+                accel_x: (data[2] as u16) << 2 | ((data[5] as u16 >> 6) & 0b11),
+                accel_y: (data[3] as u16) << 2 | ((data[5] as u16 >> 4) & 0b11),
+                accel_z: (data[4] as u16) << 2 | ((data[5] as u16 >> 2) & 0b11),
+                c_button_pressed: (data[5] & 0b10) == 0,
+                z_button_pressed: (data[5] & 0b01) == 0,
+            })
         }
     }
 }
@@ -76,7 +81,9 @@ enum NunchuckInitError {
 
 #[derive(Debug)]
 enum NunchuckReadError {
+    I2CError(I2CError),
     IOError(io::Error),
+    ParsingError,
 }
 
 impl Nunchuck {
@@ -95,21 +102,30 @@ impl Nunchuck {
 
     /// Send the init sequence to the Wii Nunchuck
     pub fn init(&mut self) -> Result<(), NunchuckInitError> {
-        try!(self.i2cdev.write_all(&[0xF0, 0x55])
+        // These registers must be written; the documentation is a bit
+        // lacking but it appears this is some kind of handshake to
+        // perform unencrypted data tranfers
+        try!(self.i2cdev.smbus_write_byte_data(0xF0, 0x55)
              .or_else(|_| Err(NunchuckInitError::WriteFailed)));
-        try!(self.i2cdev.write_all(&[0xFB, 0x00])
+        try!(self.i2cdev.smbus_write_byte_data(0xFB, 0x00)
              .or_else(|_| Err(NunchuckInitError::WriteFailed)));
-        thread::sleep_ms(100);
         Ok(())
     }
 
     pub fn read(&mut self) -> Result<NunchuckReading, NunchuckReadError> {
         let mut buf: [u8; 6] = [0; 6];
-        try!(self.i2cdev.write_all(&[0x00])
-             .or_else(|e| Err(NunchuckReadError::IOError(e))));
+
+        // tell the nunchuck to prepare a sample
+        try!(self.i2cdev.smbus_write_byte(0x00)
+             .or_else(|e| Err(NunchuckReadError::I2CError(e))));
+
+        // now, read it!
         thread::sleep_ms(10);
         match self.i2cdev.read(&mut buf) {
-            Ok(_len) => Ok(NunchuckReading::from_data(&buf)),
+            Ok(_len) => match NunchuckReading::from_data(&buf) {
+                Some(reading) => Ok(reading),
+                None => Err(NunchuckReadError::ParsingError),
+            },
             Err(err) => Err(NunchuckReadError::IOError(err)),
         }
     }

src/lib.rs

@@ -23,3 +23,4 @@ extern crate byteorder;
 
 mod ffi;
 pub mod core;
+pub mod smbus;

src/smbus.rs

@@ -1,5 +1,6 @@
 use ffi;
 use core::*;
+use std::os::unix::prelude::*;
 
 pub trait I2CSMBus {
     /// This sends a single bit to the device, at the place of the Rd/Wr bit