spi: add Read and separate-buffers Transfer

Author: Dirbaio

CHANGELOG.md

@@ -16,6 +16,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added `IoPin` trait for pins that can change between being inputs or outputs
   dynamically.
 - Added `Debug` to all spi mode types.
+- spi: added `Read`
+- spi: added `Transfer` with separate buffers (single-buffer `Transfer` is now `TransferInplace`)
 
 ### Changed
 - Swap PWM channel arguments to references

src/spi/blocking.rs

@@ -4,15 +4,41 @@
 //! traits. To save boilerplate when that's the case a `Default` marker trait may be provided.
 //! Implementing that marker trait will opt in your type into a blanket implementation.
 
-/// Blocking transfer
+/// Blocking transfer with separate buffers
 pub trait Transfer<W> {
     /// Error type
     type Error: core::fmt::Debug;
 
+    /// Writes and reads simultaneously. `write` is written to the slave on MOSI and
+    /// words received on MISO are stored in `read`.
+    ///
+    /// It is allowed for `read` and `write` to have different lengths, even zero length.
+    /// The transfer runs for `max(read.len(), write.len())` words. If `read` is shorter,
+    /// incoming words after `read` has been filled will be discarded. If `write` is shorter,
+    /// the value of words sent in MOSI after all `write` has been sent is implementation defined,
+    /// typically `0x00`, `0xFF`, or configurable.
+    fn transfer(&mut self, read: &mut [W], write: &[W]) -> Result<(), Self::Error>;
+}
+
+/// Blocking transfer with single buffer (in-place)
+pub trait TransferInplace<W> {
+    /// Error type
+    type Error: core::fmt::Debug;
+
     /// Writes and reads simultaneously. The contents of `words` are
     /// written to the slave, and the received words are stored into the same
     /// `words` buffer, overwriting it.
-    fn transfer(&mut self, words: &mut [W]) -> Result<(), Self::Error>;
+    fn transfer_inplace(&mut self, words: &mut [W]) -> Result<(), Self::Error>;
+}
+
+/// Blocking read
+pub trait Read<W> {
+    /// Error type
+    type Error;
+
+    /// Reads `words` to the slave. The word value sent on MOSI during
+    /// reading is implementation defined, typically `0x00`, `0xFF`, or configurable.
+    fn read(&mut self, words: &mut [W]) -> Result<(), Self::Error>;
 }
 
 /// Blocking write
@@ -40,10 +66,14 @@ pub trait WriteIter<W> {
 /// This allows composition of SPI operations into a single bus transaction
 #[derive(Debug, PartialEq)]
 pub enum Operation<'a, W: 'static> {
+    /// Read data into the provided buffer.
+    Read(&'a mut [W]),
     /// 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]),
+    Transfer(&'a mut [W], &'a [W]),
+    /// Write data out while reading data into the provided buffer
+    TransferInplace(&'a mut [W]),
 }
 
 /// Transactional trait allows multiple actions to be executed