Added transactional SPI interface

See: rust-embedded#178 for work to get here

Author: ryankurte

src/blocking/spi.rs

@@ -102,3 +102,56 @@ pub mod write_iter {
         }
     }
 }
+
+/// Operation for transactional SPI trait
+///
+/// This allows composition of SPI operations into a single bus transaction
+#[derive(Debug, PartialEq)]
+pub enum Operation<'a, W: 'static> {
+    /// Write data from the provided buffer, discarding read data
+    Write(&'a [W]),
+    /// Write data out while reading data into the provided buffer
+    Transfer(&'a mut [W]),
+}
+
+/// Transactional trait allows multiple actions to be executed
+/// as part of a single SPI transaction
+pub trait Transactional<W: 'static> {
+    /// Associated error type
+    type Error;
+
+    /// Execute the provided transactions
+    fn try_exec<'a>(&mut self, operations: &mut [Operation<'a, W>]) -> Result<(), Self::Error>;
+}
+
+/// Blocking transactional impl over spi::Write and spi::Transfer
+pub mod transactional {
+    use super::{Operation, Write, Transfer};
+
+    /// Default implementation of `blocking::spi::Transactional<W>` for implementers of
+    /// `spi::Write<W>` and `spi::Transfer<W>`
+    pub trait Default<W, E> {}
+
+    impl<W: 'static, E, S> super::Transactional<W> for S
+    where
+        S: Write<W, Error=E> + Transfer<W, Error=E>,
+        W: Copy + Clone,
+    {
+        type Error = E;
+
+        fn try_exec<'a>(
+            &mut self,
+            operations: &mut [super::Operation<'a, W>],
+            
+        ) -> Result<(), E> {
+            for op in operations {
+                match op {
+                    Operation::Write(w) => self.try_write(w)?,
+                    Operation::Transfer(t) => self.try_transfer(t).map(|_| ())?,
+                }
+            }
+
+            Ok(())
+        }
+    }
+}