[PATCH 6.6.y] serial: qcom_geni: Fix RX DMA stall when SE_DMA_RX_LEN_IN is zero

[Sasha Levin <sashal@kernel.org>]

From: Viken Dadhaniya <viken.dadhaniya@oss.qualcomm.com>

[ Upstream commit b93062b6d8a1b2d9bad235cac25558a909819026 ]

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().

Fixes: 2aaa43c70778 ("tty: serial: qcom-geni-serial: add support for serial engine DMA")
Cc: stable@vger.kernel.org
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Signed-off-by: Viken Dadhaniya <viken.dadhaniya@oss.qualcomm.com>
Link: https://patch.msgid.link/20260528-serial-rx-0-byte-fix-v2-1-b4195cfe342f@oss.qualcomm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/tty/serial/qcom_geni_serial.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
index 5083eaddd635a9..ab5b4c6c29de08 100644
--- a/drivers/tty/serial/qcom_geni_serial.c
+++ b/drivers/tty/serial/qcom_geni_serial.c
@@ -838,12 +838,9 @@ 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, drop);
 
 	ret = geni_se_rx_dma_prep(&port->se, port->rx_buf,
-- 
2.53.0