Fix STM32 I2C v2 async transfer not doing a repeated start (#197)

* Fix STM32 I2C v2 async transfer not doing a repeated start

* Clarify docs

Author: Jamie Smith

drivers/include/drivers/I2C.h

@@ -377,7 +377,8 @@ class I2C : private NonCopyable<I2C> {
      *
      * The %I2C peripheral will begin a transmit and/or receive operation in the background.  If only a transmit
      * or receive buffer is specified, only a transmit or receive will be done.  If both buffers are specified,
-     * first the transmission is done to the given slave address, then the specified number of bytes are received.
+     * first the transmission is done to the given slave address, then the MCU performs a repeated start
+     * and the specified number of bytes are received.
      *
      * If you wish to find out when the transfer is done, pass a callback function to the callback argument
      * and set the event argument to the events you wish to receive.
@@ -400,7 +401,7 @@ class I2C : private NonCopyable<I2C> {
      *        of the flags I2C_EVENT_ERROR, I2C_EVENT_ERROR_NO_SLAVE, I2C_EVENT_TRANSFER_COMPLETE, or I2C_EVENT_TRANSFER_EARLY_NACK
      * @param callback  The event callback function
      * @param repeated Set up for a repeated start.  If true, the Mbed processor does not relinquish the bus after
-     *       this write operation.  You may then call write(), read(), or start() again to start another operation.
+     *       this operation.  You may then call write(), read(), start(), or transfer() again to start another operation.
      *
      * @returns Zero if the transfer has started, or -1 if I2C peripheral is busy
      */
@@ -415,7 +416,8 @@ class I2C : private NonCopyable<I2C> {
      *
      * The %I2C peripheral will begin a transmit and/or receive operation in the background.  If only a transmit
      * or receive buffer is specified, only a transmit or receive will be done.  If both buffers are specified,
-     * first the transmission is done to the given slave address, then the specified number of bytes are received.
+     * first the transmission is done to the given slave address, then the MCU performs a repeated start
+     * and the specified number of bytes are received.
      *
      * Internally, the chip vendor may implement this function using either DMA or interrupts.
      *
@@ -428,7 +430,7 @@ class I2C : private NonCopyable<I2C> {
      * @param rx_length The length of RX buffer in bytes  If 0, no reception is done.
      * @param timeout timeout value.  Use #rtos::Kernel::wait_for_u32_forever to wait forever (the default).
      * @param repeated Set up for a repeated start.  If true, the Mbed processor does not relinquish the bus after
-     *       this operation.  You may then call write(), read(), or start() again to start another operation.
+     *       this operation.  You may then call write(), read(), start(), or transfer() again to start another operation.
      *
      * @returns Result code describing whether the transfer succeeded or not.
      */

targets/TARGET_STM/i2c_api.c

@@ -1389,7 +1389,7 @@ void HAL_I2C_MasterTxCpltCallback(I2C_HandleTypeDef *hi2c)
         if(obj->rx_buff.length > 0)
         {
             // This is a write-then-read transaction, switch to reading.
-            uint32_t xferOptions = obj_s->stop ? I2C_FIRST_AND_LAST_FRAME : I2C_FIRST_FRAME;
+            uint32_t xferOptions = get_hal_xfer_options(obj_s, obj_s->stop);
             HAL_I2C_Master_Seq_Receive_IT(hi2c, obj_s->address, (uint8_t *) obj->rx_buff.buffer, obj->rx_buff.length,
                                           xferOptions);
             obj_s->state = STM_I2C_ASYNC_READ_IN_PROGRESS;
@@ -1784,7 +1784,11 @@ void i2c_transfer_asynch(i2c_t *obj, const void *tx, size_t tx_length, void *rx,
 
     prep_for_restart_if_needed(obj_s);
 
-    uint32_t xferOptions = get_hal_xfer_options(obj_s, stop);
+    // If we are doing both a tx and an rx, then we never want to send a stop on the
+    // first transfer.
+    bool sendStopOnFirstTransfer = stop && ((tx_length == 0) || (rx_length == 0));
+
+    uint32_t xferOptions = get_hal_xfer_options(obj_s, sendStopOnFirstTransfer);
 
     if(!i2c_is_ready_for_transaction_start(obj_s->state))
     {