Prevent starting a new I2C transaction before previous stop finishes.

When a stop condition is generated by the I2C DMA callback, the code
does not wait for the finish of the stop condition. If while the stop
condition is being generated and a new I2C transaction attempts to
start, the new transaction will be inadvertently terminated by the
previous stop condition.

Since ISRs should finish as quickly as possible, the ISRs should not
spin-wait until the stop condition finishes. Instead, the spin-wait
should be placed before starting a new transaction.

The patch also removes incorrect comments that falsely indicate that
the code will spin-wait until the stop condition finishes.

Author: zyma98

src/i2c.rs

@@ -285,6 +285,9 @@ impl<I2C: Instance> I2c<I2C> {
     /// Sends START and Address for writing
     #[inline(always)]
     fn prepare_write(&self, addr: u8) -> Result<(), Error> {
+        // Wait until a previous STOP condition finishes
+        while self.i2c.cr1.read().stop().bit_is_set() {}
+
         // Send a START condition
         self.i2c.cr1.modify(|_, w| w.start().set_bit());
 
@@ -327,6 +330,9 @@ impl<I2C: Instance> I2c<I2C> {
 
     /// Sends START and Address for reading
     fn prepare_read(&self, addr: u8) -> Result<(), Error> {
+        // Wait until a previous STOP condition finishes
+        while self.i2c.cr1.read().stop().bit_is_set() {}
+
         // Send a START condition and set ACK bit
         self.i2c
             .cr1
@@ -443,9 +449,6 @@ impl<I2C: Instance> I2c<I2C> {
             // Receive last byte
             *last = self.recv_byte()?;
 
-            // Wait for the STOP to be sent.
-            while self.i2c.cr1.read().stop().bit_is_set() {}
-
             // Fallthrough is success
             Ok(())
         } else {
@@ -465,9 +468,6 @@ impl<I2C: Instance> I2c<I2C> {
         // Send a STOP condition
         self.i2c.cr1.modify(|_, w| w.stop().set_bit());
 
-        // Wait for STOP condition to transmit.
-        while self.i2c.cr1.read().stop().bit_is_set() {}
-
         // Fallthrough is success
         Ok(())
     }
@@ -482,9 +482,6 @@ impl<I2C: Instance> I2c<I2C> {
         // Send a STOP condition
         self.i2c.cr1.modify(|_, w| w.stop().set_bit());
 
-        // Wait for STOP condition to transmit.
-        while self.i2c.cr1.read().stop().bit_is_set() {}
-
         // Fallthrough is success
         Ok(())
     }

src/i2c/dma.rs

@@ -628,7 +628,6 @@ where
                 // Wait for BTF
                 while self.hal_i2c.i2c.sr1.read().btf().bit_is_clear() {}
 
-                // Generate stop and wait for it
                 self.send_stop();
             }
         }
@@ -668,7 +667,7 @@ where
 
                 // Clear ACK
                 self.hal_i2c.i2c.cr1.modify(|_, w| w.ack().clear_bit());
-                // Generate stop and wait for it
+
                 self.send_stop();
             }
         }
@@ -730,7 +729,6 @@ where
 
                     self.rx.rx_transfer.as_mut().unwrap().start(|_| {});
                 } else {
-                    // Generate stop and wait for it
                     self.send_stop();
                 }
             }
@@ -756,7 +754,7 @@ where
 
                 // Clear ACK
                 self.hal_i2c.i2c.cr1.modify(|_, w| w.ack().clear_bit());
-                // Generate stop and wait for it
+
                 self.send_stop();
             }
         }