[PATCH 6.6.y] serial: qcom_geni: fix kfifo underflow when flush precedes DMA completion IRQ

[Sasha Levin <sashal@kernel.org>]

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

[ Upstream commit 452d6fa37ae9b021f4f6d397dbae077f7296f6f4 ]

When uart_flush_buffer() runs before the DMA completion IRQ is delivered,
the following race can occur (all steps serialized by uart_port_lock):

  1. DMA starts: tx_remaining = N, kfifo contains N bytes
  2. DMA completes in hardware; IRQ is pending but not yet delivered
  3. uart_flush_buffer() acquires the port lock and calls kfifo_reset(),
     making kfifo_len() = 0 while tx_remaining remains N
  4. uart_flush_buffer() releases the port lock
  5. DMA IRQ fires; handle_tx_dma() acquires the port lock and calls
     uart_xmit_advance(uport, tx_remaining) on an empty kfifo

uart_xmit_advance() increments kfifo->out by tx_remaining. Since
kfifo_reset() already set both in and out to 0, out wraps past in,
causing kfifo_len() to return UART_XMIT_SIZE - tx_remaining. The next
start_tx_dma() call then submits a DMA transfer of stale buffer data.

Fix this by snapshotting kfifo_len() at the start of handle_tx_dma()
and skipping uart_xmit_advance() when fifo_len < tx_remaining, which
indicates the kfifo was reset by a preceding flush.

Fixes: 2aaa43c70778 ("tty: serial: qcom-geni-serial: add support for serial engine DMA")
Cc: stable <stable@kernel.org>
Signed-off-by: Viken Dadhaniya <viken.dadhaniya@oss.qualcomm.com>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260506-serial-dma-stale-tx-buf-v1-1-e3ccb360d719@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 | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
index f820a09cb5c39b..b97faf0c804bfd 100644
--- a/drivers/tty/serial/qcom_geni_serial.c
+++ b/drivers/tty/serial/qcom_geni_serial.c
@@ -962,8 +962,21 @@ static void qcom_geni_serial_handle_tx_dma(struct uart_port *uport)
 {
 	struct qcom_geni_serial_port *port = to_dev_port(uport);
 	struct circ_buf *xmit = &uport->state->xmit;
+	unsigned int chars_pending = uart_circ_chars_pending(xmit);
+
+	/*
+	 * Only advance the buffer if it still contains the bytes that were
+	 * transferred. uart_flush_buffer() may have run before this IRQ
+	 * fired: it clears the circular buffer under the port lock, making
+	 * chars_pending = 0 while tx_remaining remains non-zero. Calling
+	 * uart_xmit_advance() in that case would advance xmit->tail past
+	 * xmit->head, making uart_circ_chars_pending() wrap to
+	 * UART_XMIT_SIZE - tx_remaining and triggering a spurious large DMA
+	 * transfer of stale data.
+	 */
+	if (chars_pending >= port->tx_remaining)
+		uart_xmit_advance(uport, port->tx_remaining);
 
-	uart_xmit_advance(uport, port->tx_remaining);
 	geni_se_tx_dma_unprep(&port->se, port->tx_dma_addr, port->tx_remaining);
 	port->tx_dma_addr = 0;
 	port->tx_remaining = 0;
-- 
2.53.0