bus/spi: add RefCell, CriticalSection and Mutex shared bus implementations.

Author: Dirbaio

embedded-hal-bus/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Added
 - i2c: add bus sharing implementations.
+- spi: add bus sharing implementations.
 
 ## [v0.1.0-alpha.1] - 2022-09-28
 

embedded-hal-bus/src/spi.rs

@@ -0,0 +1,133 @@
+use core::cell::RefCell;
+use critical_section::Mutex;
+use embedded_hal::digital::OutputPin;
+use embedded_hal::spi::{
+    ErrorType, Operation, SpiBus, SpiBusRead, SpiBusWrite, SpiDevice, SpiDeviceRead, SpiDeviceWrite,
+};
+
+use super::DeviceError;
+
+/// `critical-section`-based shared bus [`SpiDevice`] implementation.
+///
+/// This allows for sharing an [`SpiBus`](embedded_hal::spi::blocking::SpiBus), obtaining multiple [`SpiDevice`] instances,
+/// each with its own `CS` pin.
+///
+/// Sharing is implemented with a `critical-section` [`Mutex`](critical_section::Mutex). A critical section is taken for
+/// the entire duration of a transaction. This allows sharing a single bus across multiple threads (interrupt priority levels).
+/// The downside is critical sections typically require globally disabling interrupts, so `CriticalSectionDevice` will likely
+/// negatively impact real-time properties, such as interrupt latency. If you can, prefer using
+/// [`RefCellDevice`](super::RefCellDevice) instead, which does not require taking critical sections.
+pub struct CriticalSectionDevice<'a, BUS, CS> {
+    bus: &'a Mutex<RefCell<BUS>>,
+    cs: CS,
+}
+
+impl<'a, BUS, CS> CriticalSectionDevice<'a, BUS, CS> {
+    /// Create a new ExclusiveDevice
+    pub fn new(bus: &'a Mutex<RefCell<BUS>>, cs: CS) -> Self {
+        Self { bus, cs }
+    }
+}
+
+impl<'a, BUS, CS> ErrorType for CriticalSectionDevice<'a, BUS, CS>
+where
+    BUS: ErrorType,
+    CS: OutputPin,
+{
+    type Error = DeviceError<BUS::Error, CS::Error>;
+}
+
+impl<'a, Word: Copy + 'static, BUS, CS> SpiDeviceRead<Word> for CriticalSectionDevice<'a, BUS, CS>
+where
+    BUS: SpiBusRead<Word>,
+    CS: OutputPin,
+{
+    fn read_transaction(&mut self, operations: &mut [&mut [Word]]) -> Result<(), Self::Error> {
+        critical_section::with(|cs| {
+            let bus = &mut *self.bus.borrow_ref_mut(cs);
+
+            self.cs.set_low().map_err(DeviceError::Cs)?;
+
+            let mut op_res = Ok(());
+            for buf in operations {
+                if let Err(e) = bus.read(buf) {
+                    op_res = Err(e);
+                    break;
+                }
+            }
+
+            // On failure, it's important to still flush and deassert CS.
+            let flush_res = bus.flush();
+            let cs_res = self.cs.set_high();
+
+            op_res.map_err(DeviceError::Spi)?;
+            flush_res.map_err(DeviceError::Spi)?;
+            cs_res.map_err(DeviceError::Cs)?;
+
+            Ok(())
+        })
+    }
+}
+
+impl<'a, Word: Copy + 'static, BUS, CS> SpiDeviceWrite<Word> for CriticalSectionDevice<'a, BUS, CS>
+where
+    BUS: SpiBusWrite<Word>,
+    CS: OutputPin,
+{
+    fn write_transaction(&mut self, operations: &[&[Word]]) -> Result<(), Self::Error> {
+        critical_section::with(|cs| {
+            let bus = &mut *self.bus.borrow_ref_mut(cs);
+
+            self.cs.set_low().map_err(DeviceError::Cs)?;
+
+            let mut op_res = Ok(());
+            for buf in operations {
+                if let Err(e) = bus.write(buf) {
+                    op_res = Err(e);
+                    break;
+                }
+            }
+
+            // On failure, it's important to still flush and deassert CS.
+            let flush_res = bus.flush();
+            let cs_res = self.cs.set_high();
+
+            op_res.map_err(DeviceError::Spi)?;
+            flush_res.map_err(DeviceError::Spi)?;
+            cs_res.map_err(DeviceError::Cs)?;
+
+            Ok(())
+        })
+    }
+}
+
+impl<'a, Word: Copy + 'static, BUS, CS> SpiDevice<Word> for CriticalSectionDevice<'a, BUS, CS>
+where
+    BUS: SpiBus<Word>,
+    CS: OutputPin,
+{
+    fn transaction(&mut self, operations: &mut [Operation<'_, Word>]) -> Result<(), Self::Error> {
+        critical_section::with(|cs| {
+            let bus = &mut *self.bus.borrow_ref_mut(cs);
+
+            self.cs.set_low().map_err(DeviceError::Cs)?;
+
+            let op_res = operations.iter_mut().try_for_each(|op| match op {
+                Operation::Read(buf) => bus.read(buf),
+                Operation::Write(buf) => bus.write(buf),
+                Operation::Transfer(read, write) => bus.transfer(read, write),
+                Operation::TransferInPlace(buf) => bus.transfer_in_place(buf),
+            });
+
+            // On failure, it's important to still flush and deassert CS.
+            let flush_res = bus.flush();
+            let cs_res = self.cs.set_high();
+
+            op_res.map_err(DeviceError::Spi)?;
+            flush_res.map_err(DeviceError::Spi)?;
+            cs_res.map_err(DeviceError::Cs)?;
+
+            Ok(())
+        })
+    }
+}