Auto merge of #73 - japaric:itm, r=japaric

make `Stim::write_*` methods take `&mut self` instead of `&self`

this prevents people from overlapping non-atomic write operations on the same stimulus port when
working with generators (cooperative tasks).

For example, with this change the following code won't compile

``` rust
let stim = &mut ITM.stim[0];
let a = || {
    loop {
        // ..
        for byte in b"Hello, world!".iter() {
            while !stim.is_fifo_ready() { yield }
            stim.write_u8(*byte);
        }
        // ..
    }
};

let b = || {
    loop {
        // ..
        for byte in b"The quick brown fox jumps over the lazy dog".iter() {
            while !stim.is_fifo_ready() { yield }
            stim.write_u8(*byte);
        }
        // ..
    }
};
```

A possible fix for the above code is to use different stimulus ports in each task (generator).

1 files changed, 9 insertions, 3 deletions

diff --git a/src/peripheral/mod.rs b/src/peripheral/mod.rs
index d462bdb..bd658a4 100644
--- a/src/peripheral/mod.rs
+++ b/src/peripheral/mod.rs
@@ -9,7 +9,7 @@
 #![allow(private_no_mangle_statics)]
 
 use core::marker::PhantomData;
-use core::ops::Deref;
+use core::ops::{Deref, DerefMut};
 
 use interrupt;
 
@@ -262,8 +262,8 @@ pub struct ITM {
 
 impl ITM {
     /// Returns a pointer to the register block
-    pub fn ptr() -> *const itm::RegisterBlock {
-        0xE000_0000 as *const _
+    pub fn ptr() -> *mut itm::RegisterBlock {
+        0xE000_0000 as *mut _
     }
 }
 
@@ -275,6 +275,12 @@ impl Deref for ITM {
     }
 }
 
+impl DerefMut for ITM {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        unsafe { &mut *Self::ptr() }
+    }
+}
+
 /// Memory Protection Unit
 pub struct MPU {
     _marker: PhantomData<*const ()>,