Added transactional SPI interface

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

Author: ryankurte

src/blocking/spi.rs

@@ -104,3 +104,26 @@ pub mod write_iter {
         }
     }
 }
+
+/// Operation for transactional SPI trait
+///
+/// This allows composition of SPI operations into a single bus transaction
+#[cfg(feature = "unproven")]
+#[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
+    WriteRead(&'a mut [W]),
+}
+
+/// Transactional trait allows multiple actions to be executed
+/// as part of a single SPI transaction
+#[cfg(feature = "unproven")]
+pub trait Transactional<W: 'static> {
+    /// Associated error type
+    type Error;
+
+    /// Execute the provided transactions
+    fn exec<'a>(&mut self, operations: &mut [Operation<'a, W>]) -> Result<(), Self::Error>;
+}