drivers: serial: nrfx_uarte: Add workaround for bytes dropping

Add workaround for development kits on which interface chip (segger chip)
is dropping bytes when they are sent in bursts. Issue has been seen on
DK which have multiple virtual com ports on the USB.

A workaround is adding periodic busy waits to introduce gaps in the
transmission.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>

drivers/serial/Kconfig.nrfx

@@ -27,6 +27,32 @@ config UART_ASYNC_TX_CACHE_SIZE
 	  in RAM, because EasyDMA in UARTE peripherals can only transfer data
 	  from RAM.
 
+config UART_NRF_DK_SERIAL_WORKAROUND
+	bool
+	help
+	  On some development kits there are characters being dropped by the
+	  controller chip (Segger interface) if there are burst of bytes being
+	  printed. It can lead to test failures since test out is corrupted.
+	  A workaround is to enforce gaps (by adding busy waits) in the transmission.
+	  It is recommended to enable the workaround only in the test since
+	  busy waits can impact application flow.
+
+if UART_NRF_DK_SERIAL_WORKAROUND
+
+config UART_NRF_DK_SERIAL_WORKAROUND_COUNT
+	int
+	default 32
+	help
+	  Number of bytes transferred after which a busy wait is added.
+
+config UART_NRF_DK_SERIAL_WORKAROUND_WAIT_MS
+	int
+	default 10
+	help
+	  Busy wait time (in milliseconds).
+endif
+
+
 # ----------------- port 0 -----------------
 config UART_0_NRF_UART
 	def_bool HAS_HW_NRF_UART0

drivers/serial/uart_nrfx_uarte.c

@@ -1539,6 +1539,32 @@ static void uarte_nrfx_poll_out(const struct device *dev, unsigned char c)
 	bool isr_mode = k_is_in_isr() || k_is_pre_kernel();
 	unsigned int key;
 
+#if CONFIG_UART_NRF_DK_SERIAL_WORKAROUND
+	/* On some boards (usually those which have multiple virtual coms) it can
+	 * be seen that bytes are dropped on the console serial (serial that goes
+	 * through Segger interface chip) when working in virtual environment.
+	 * It's the Segger chip that drops those bytes. A workaround is to enforce
+	 * periodic gaps which allows to handle the traffic correctly.
+	 */
+	if (dev == DEVICE_DT_GET(DT_CHOSEN(zephyr_console))) {
+		static int cnt;
+		static uint32_t t;
+		uint32_t now = k_uptime_get_32();
+
+		if ((now - t) >= CONFIG_UART_NRF_DK_SERIAL_WORKAROUND_WAIT_MS) {
+			cnt = 0;
+		} else {
+			cnt++;
+			if (cnt >= CONFIG_UART_NRF_DK_SERIAL_WORKAROUND_COUNT) {
+				k_busy_wait(1000 * CONFIG_UART_NRF_DK_SERIAL_WORKAROUND_WAIT_MS);
+				cnt = 0;
+			}
+		}
+
+		t = now;
+	}
+#endif
+
 	if (isr_mode) {
 		while (1) {
 			key = irq_lock();