Add UART Half-DMA transfer mode (#2)

os/hal/include/hal_uart.h

@@ -298,6 +298,26 @@ typedef enum {
   _uart_wakeup_rx_complete_isr(uartp);                                      \
 }
 
+/**
+ * @brief   Common ISR code for RX half-transfer data.
+ * @details This code handles the portable part of the ISR code:
+ *          - Callback invocation.
+ *          - Waiting thread wakeup, if any.
+ *          - Driver state transitions.
+ *          .
+ * @note    This macro is meant to be used in the low level drivers
+ *          implementation only.
+ *
+ * @param[in] uartp     pointer to the @p UARTDriver object
+ * @param[in] full      flag set to 1 for the second half, and 0 for the first half
+ *
+ * @notapi
+ */
+#define _uart_rx_half_isr_code(uartp, full) {                               \
+  if ((uartp)->config->rxhalf_cb != NULL)                                   \
+    (uartp)->config->rxhalf_cb(uartp, full);                                \
+}
+
 /**
  * @brief   Common ISR code for RX error.
  * @details This code handles the portable part of the ISR code:

os/hal/ports/STM32/LLD/USARTv1/hal_uart_lld.c

@@ -262,6 +262,11 @@ static void usart_start(UARTDriver *uartp) {
   /* Mustn't ever set TCIE here - if done, it causes an immediate
      interrupt.*/
   cr1 = USART_CR1_UE | USART_CR1_PEIE | USART_CR1_TE | USART_CR1_RE;
+
+  /* Add Idle interrupt if needed */
+  if (uartp->config->timeout_cb != NULL)
+    cr1 |= USART_CR1_IDLEIE;
+
   u->CR1 = uartp->config->cr1 | cr1;
 
   /* Starting the receiver idle loop.*/
@@ -290,6 +295,15 @@ static void uart_lld_serve_rx_end_irq(UARTDriver *uartp, uint32_t flags) {
        received character and then the driver stays in the same state.*/
     _uart_rx_idle_code(uartp);
   }
+  /* DMA half-transter interrupt handling - for the 1st/2nd half transfers. */
+  else if (uartp->config->rxhalf_cb != NULL) {
+    if ((flags & STM32_DMA_ISR_HTIF) != 0) {
+      _uart_rx_half_isr_code(uartp, 0);
+    }
+    else if ((flags & STM32_DMA_ISR_TCIF) != 0) {
+      _uart_rx_half_isr_code(uartp, 1);
+    }
+  }
   else {
     /* Receiver in active state, a callback is generated, if enabled, after
        a completed transfer.*/
@@ -975,8 +989,14 @@ void uart_lld_start_receive(UARTDriver *uartp, size_t n, void *rxbuf) {
   /* RX DMA channel preparation.*/
   dmaStreamSetMemory0(uartp->dmarx, rxbuf);
   dmaStreamSetTransactionSize(uartp->dmarx, n);
-  dmaStreamSetMode(uartp->dmarx, uartp->dmarxmode  | STM32_DMA_CR_DIR_P2M |
+
-                                 STM32_DMA_CR_MINC | STM32_DMA_CR_TCIE);
+  uint32_t mode = STM32_DMA_CR_DIR_P2M | STM32_DMA_CR_MINC | STM32_DMA_CR_TCIE;
+
+  /* DMA half-transfer interrupt & circular mode, if needed */
+  if (uartp->config->rxhalf_cb != NULL)
+    mode |= STM32_DMA_CR_HTIE | STM32_DMA_CR_CIRC;
+
+  dmaStreamSetMode(uartp->dmarx, uartp->dmarxmode | mode);
 
   /* Starting transfer.*/
   dmaStreamEnable(uartp->dmarx);

os/hal/ports/STM32/LLD/USARTv1/hal_uart_lld.h

@@ -577,6 +577,14 @@ typedef void (*uartccb_t)(UARTDriver *uartp, uint16_t c);
  */
 typedef void (*uartecb_t)(UARTDriver *uartp, uartflags_t e);
 
+/**
+ * @brief   Receive Half-transfer UART notification callback type.
+ *
+ * @param[in] uartp     pointer to the @p UARTDriver object
+ * @param[in] full      flag set to 1 for the second half, and 0 for the first half
+ */
+typedef void (*uarthcb_t)(UARTDriver *uartp, uartflags_t full);
+
 /**
  * @brief   Driver configuration structure.
  * @note    It could be empty on some architectures.
@@ -625,6 +633,11 @@ typedef struct {
    * @brief Initialization value for the CR3 register.
    */
   uint16_t                  cr3;
+  /* Additional (optional) handlers. Placed here for the struct compatibility.*/
+  /**
+   * @brief Half-transfer receive buffer callback.
+   */
+  uarthcb_t                 rxhalf_cb;
 } UARTConfig;
 
 /**