HAL_ChibiOS: prevent hw flow control from hogging a DMA channel

When hw flow control is enabled check the CTS pin before we grab the
DMA channel to prevent a long timeout trying to send to a blocked port
from holding a DMA channel against another device

this fixes issue #16587

libraries/AP_HAL_ChibiOS/UARTDriver.cpp

@@ -829,6 +829,19 @@ void UARTDriver::write_pending_bytes_DMA(uint32_t n)
     if (tx_len == 0) {
         return;
     }
+
+    if (_flow_control != FLOW_CONTROL_DISABLE &&
+        sdef.cts_line != 0 &&
+        palReadLine(sdef.cts_line)) {
+        // we are using hw flow control and the CTS line is high. We
+        // will hold off trying to transmit until the CTS line goes
+        // low to indicate the receiver has space. We do this before
+        // we take the DMA lock to prevent a high CTS line holding a
+        // DMA channel that may be needed by another device
+        tx_len = 0;
+        return;
+    }
+
     if (!dma_handle->lock_nonblock()) {
         tx_len = 0;
         return;

libraries/AP_HAL_ChibiOS/UARTDriver.h

@@ -81,6 +81,7 @@ public:
         ioline_t tx_line;
         ioline_t rx_line;
         ioline_t rts_line;
+        ioline_t cts_line;
         int8_t rxinv_gpio;
         uint8_t rxinv_polarity;
         int8_t txinv_gpio;

libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py

@@ -1214,6 +1214,7 @@ def write_UART_config(f):
         tx_line = make_line(dev + '_TX')
         rx_line = make_line(dev + '_RX')
         rts_line = make_line(dev + '_RTS')
+        cts_line = make_line(dev + '_CTS')
         if rts_line != "0":
             have_rts_cts = True
             f.write('#define HAL_HAVE_RTSCTS_SERIAL%u\n' % uart_serial_num[dev])
@@ -1234,10 +1235,10 @@ def write_UART_config(f):
                 "#define HAL_%s_CONFIG { (BaseSequentialStream*) &SD%u, %u, false, "
                 % (dev, n, n))
             if mcu_series.startswith("STM32F1"):
-                f.write("%s, %s, %s, " % (tx_line, rx_line, rts_line))
+                f.write("%s, %s, %s, %s, " % (tx_line, rx_line, rts_line, cts_line))
             else:
-                f.write("STM32_%s_RX_DMA_CONFIG, STM32_%s_TX_DMA_CONFIG, %s, %s, %s, " %
-                        (dev, dev, tx_line, rx_line, rts_line))
+                f.write("STM32_%s_RX_DMA_CONFIG, STM32_%s_TX_DMA_CONFIG, %s, %s, %s, %s, " %
+                        (dev, dev, tx_line, rx_line, rts_line, cts_line))
 
             # add inversion pins, if any
             f.write("%d, " % get_gpio_bylabel(dev + "_RXINV"))