rp/uart: extract fifo draining from blocking_read

this will also be needed for dma operations.

embassy-rp/src/uart/mod.rs

@@ -238,16 +238,22 @@ impl<'d, T: Instance, M: Mode> UartRx<'d, T, M> {
         }
     }
 
-    pub fn blocking_read(&mut self, buffer: &mut [u8]) -> Result<(), Error> {
+    pub fn blocking_read(&mut self, mut buffer: &mut [u8]) -> Result<(), Error> {
-        let r = T::regs();
+        while buffer.len() > 0 {
-        unsafe {
+            let received = self.drain_fifo(buffer)?;
-            for b in buffer {
+            buffer = &mut buffer[received..];
-                *b = loop {
+        }
-                    if r.uartfr().read().rxfe() {
+        Ok(())
-                        continue;
     }
 
-                    let dr = r.uartdr().read();
+    fn drain_fifo(&mut self, buffer: &mut [u8]) -> Result<usize, Error> {
+        let r = T::regs();
+        for (i, b) in buffer.iter_mut().enumerate() {
+            if unsafe { r.uartfr().read().rxfe() } {
+                return Ok(i);
+            }
+
+            let dr = unsafe { r.uartdr().read() };
 
             if dr.oe() {
                 return Err(Error::Overrun);
@@ -258,12 +264,10 @@ impl<'d, T: Instance, M: Mode> UartRx<'d, T, M> {
             } else if dr.fe() {
                 return Err(Error::Framing);
             } else {
-                        break dr.data();
+                *b = dr.data();
-                    }
-                };
             }
         }
-        Ok(())
+        Ok(buffer.len())
     }
 }