optimized Spi::write

Author: burrbull

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Changed
 
+ - Optimized version of blocking SPI write
  - Support `u16` read/write for SPI
  - Use `bool` for BIDI mode type
  - `PwmHz::get_period`: fix computation of return value, prevent division by zero

src/spi.rs

@@ -659,6 +659,40 @@ impl<SPI: Instance, PINS, const BIDI: bool, W: FrameSize, OPERATION>
             nb::Error::WouldBlock
         })
     }
+
+    // Implement write as per the "Transmit only procedure"
+    // RM SPI::3.5. This is more than twice as fast as the
+    // default Write<> implementation (which reads and drops each
+    // received value)
+    fn spi_write<WI>(&mut self, words: WI) -> Result<(), Error>
+    where
+        WI: IntoIterator<Item = W>,
+    {
+        if BIDI {
+            self.spi.cr1.modify(|_, w| w.bidioe().set_bit());
+        }
+        // Write each word when the tx buffer is empty
+        for word in words {
+            loop {
+                let sr = self.spi.sr.read();
+                if sr.txe().bit_is_set() {
+                    self.write_data_reg(word);
+                    if sr.modf().bit_is_set() {
+                        return Err(Error::ModeFault);
+                    }
+                    break;
+                }
+            }
+        }
+        // Wait for final TXE
+        while !self.is_tx_empty() {}
+        // Wait for final !BSY
+        while self.is_busy() {}
+        // Clear OVR set due to dropped received values
+        let _ = self.read_data_reg();
+        let _ = self.spi.sr.read();
+        Ok(())
+    }
 }
 
 // Spi DMA

src/spi/hal_02.rs

@@ -97,7 +97,7 @@ mod blocking {
         type Error = Error;
 
         fn write(&mut self, words: &[u8]) -> Result<(), Self::Error> {
-            self.write_iter(words.iter().copied())
+            self.spi_write(words.iter().copied())
         }
     }
 
@@ -111,14 +111,7 @@ mod blocking {
         where
             WI: IntoIterator<Item = u8>,
         {
-            for word in words.into_iter() {
-                nb::block!(self.send(word))?;
-                if !BIDI {
-                    nb::block!(self.read())?;
-                }
-            }
-
-            Ok(())
+            self.spi_write(words)
         }
     }
 

src/spi/hal_1.rs

@@ -111,14 +111,7 @@ mod blocking {
         SPI: Instance,
     {
         fn write(&mut self, words: &[W]) -> Result<(), Self::Error> {
-            for word in words {
-                nb::block!(<Self as FullDuplex<W>>::write(self, *word))?;
-                if !BIDI {
-                    nb::block!(<Self as FullDuplex<W>>::read(self))?;
-                }
-            }
-
-            Ok(())
+            self.spi_write(words.iter().copied())
         }
     }