uart: add simple uart driver

Printf-debugging FTW!

Signed-off-by: Sean Cross <sean@xobs.io>

src/uart.rs

@@ -0,0 +1,66 @@
+use core::fmt::{Error, Write};
+
+pub struct Uart {}
+
+impl Uart {
+    pub fn new() -> Uart {
+        Uart {}
+    }
+
+    pub fn putc(&self, c: u8) {
+        let ptr = 0xE000_1800 as *mut u32;
+        unsafe {
+            while ptr.add(1).read_volatile() == 0 {}
+            ptr.add(0).write_volatile(c as u32);
+        }
+    }
+
+    fn get(&self, base_addr: usize) -> Option<u8> {
+        let ptr = 0xE000_1800 as *mut u32;
+        unsafe {
+            if ptr.add(2).read_volatile() == 0 {
+                Some(ptr.add(0).read_volatile() as u8)
+            } else {
+                None
+            }
+        }
+    }
+}
+
+// This is a slightly different syntax. Write is this "trait", meaning it is much like
+// an interface where we're just guaranteeing a certain function signature. In the Write
+// trait, one is absolutely required to be implemented, which is write_str. There are other
+// functions, but they all rely on write_str(), so their default implementation is OK for now.
+impl Write for Uart {
+    // The trait Write expects us to write the function write_str
+    // which looks like:
+    fn write_str(&mut self, s: &str) -> Result<(), Error> {
+        for c in s.bytes() {
+            self.putc(c);
+        }
+        // Return that we succeeded.
+        Ok(())
+    }
+}
+
+#[macro_export]
+macro_rules! print
+{
+	($($args:tt)+) => ({
+			use core::fmt::Write;
+			let _ = write!(crate::uart::Uart::new(), $($args)+);
+	});
+}
+#[macro_export]
+macro_rules! println
+{
+	() => ({
+		print!("\r\n")
+	});
+	($fmt:expr) => ({
+		print!(concat!($fmt, "\r\n"))
+	});
+	($fmt:expr, $($args:tt)+) => ({
+		print!(concat!($fmt, "\r\n"), $($args)+)
+	});
+}