FROMLIST: serial: qcom_geni: Fix RX DMA stall when SE_DMA_RX_LEN_IN is zero

In qcom_geni_serial_handle_rx_dma(), geni_se_rx_dma_unprep() clears
port->rx_dma_addr before SE_DMA_RX_LEN_IN is read. If the register is zero,
for example when the RX stale counter fires on an idle line, the handler
returns without calling geni_se_rx_dma_prep().

The next RX DMA interrupt then hits the !port->rx_dma_addr guard and
returns immediately, so the RX DMA buffer is never rearmed and later input
is lost.

Keep the handler on the rearm path when rx_in is zero. Warn about the
unexpected zero-length DMA completion, skip received-data handling, and
always call geni_se_rx_dma_prep().

Link: https://lore.kernel.org/all/20260528-serial-rx-0-byte-fix-v1-1-dc4e876c7368@oss.qualcomm.com/
Fixes: 2aaa43c ("tty: serial: qcom-geni-serial: add support for serial engine DMA")
Signed-off-by: Viken Dadhaniya <viken.dadhaniya@oss.qualcomm.com>

Author: vdadhani

drivers/tty/serial/qcom_geni_serial.c

@@ -905,12 +905,9 @@ static void qcom_geni_serial_handle_rx_dma(struct uart_port *uport, bool drop)
 	port->rx_dma_addr = 0;
 
 	rx_in = readl(uport->membase + SE_DMA_RX_LEN_IN);
-	if (!rx_in) {
-		dev_warn(uport->dev, "serial engine reports 0 RX bytes in!\n");
-		return;
-	}
-
-	if (!drop)
+	if (!rx_in)
+		dev_warn_ratelimited(uport->dev, "serial engine reports 0 RX bytes in!\n");
+	else if (!drop)
 		handle_rx_uart(uport, rx_in);
 
 	ret = geni_se_rx_dma_prep(&port->se, port->rx_buf,