drivers: serial: silabs: Update ISR to check for DMA device presence

Since the symbol CONFIG_UART_ASYNC_API is shared between EUSART and USART
drivers, it creates a scenario where EUSART can have
CONFIG_UART_SILABS_USART_ASYNC=y but with no DMA device declared for it
in the DTS (because we only want async API for the USART driver).
We handled this case by disabling async transfer when the DMA
device is null, but when we activate CONFIG_PM, we have a hard fault due
to null pointer dereference in the ISR.
This bug was only discovered today when trying to enable CONFIG_PM over the
uart_async_api test, which led to a hard fault in the ISR. This occurs
because we only enable interrupts when the PM is enabled in the "poll_out"
function to put the PM lock back.

Signed-off-by: Martin Hoff <[email protected]>

Author: Martinhoff-maker

drivers/serial/uart_silabs_eusart.c

@@ -778,7 +778,9 @@ static void eusart_isr(const struct device *dev)
 	}
 #endif
 #ifdef CONFIG_UART_SILABS_EUSART_ASYNC
-
+	if (!data->dma_tx.dma_dev) {
+		return;
+	}
 	if (flags & EUSART_IF_RXTO) {
 		if (data->dma_rx.timeout == 0) {
 			eusart_dma_rx_flush(data);

drivers/serial/uart_silabs_usart.c

@@ -765,6 +765,9 @@ static void uart_silabs_isr(const struct device *dev)
 	}
 #endif
 #ifdef CONFIG_UART_SILABS_USART_ASYNC
+	if (!data->dma_tx.dma_dev) {
+		return;
+	}
 	if (flags & USART_IF_TCMP1) {
 
 		data->dma_rx.timeout_cnt++;