serial: 8250: Fall back to non-DMA Rx if IIR_RDI occurs

DW UART sometimes triggers IIR_RDI during DMA Rx when IIR_RX_TIMEOUT
should have been triggered instead. Since IIR_RDI has higher priority
than IIR_RX_TIMEOUT, this causes the Rx to hang into interrupt loop.
The problem seems to occur at least with some combinations of
small-sized transfers (I've reproduced the problem on Elkhart Lake PSE
UARTs).

If there's already an on-going Rx DMA and IIR_RDI triggers, fall
graciously back to non-DMA Rx. That is, behave as if IIR_RX_TIMEOUT had
occurred.

8250_omap already considers IIR_RDI similar to this change so its
nothing unheard of.

Fixes: 75df022 ("serial: 8250_dma: Fix RX handling")
Cc: <[email protected]>
Co-developed-by: Srikanth Thokala <[email protected]>
Signed-off-by: Srikanth Thokala <[email protected]>
Co-developed-by: Aman Kumar <[email protected]>
Signed-off-by: Aman Kumar <[email protected]>
Signed-off-by: Ilpo Järvinen <[email protected]>
Reviewed-by: Andy Shevchenko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: ij-intel

drivers/tty/serial/8250/8250_port.c

@@ -1897,6 +1897,10 @@ EXPORT_SYMBOL_GPL(serial8250_modem_status);
 static bool handle_rx_dma(struct uart_8250_port *up, unsigned int iir)
 {
 	switch (iir & 0x3f) {
+	case UART_IIR_RDI:
+		if (!up->dma->rx_running)
+			break;
+		fallthrough;
 	case UART_IIR_RX_TIMEOUT:
 		serial8250_rx_dma_flush(up);
 		fallthrough;