[PATCH 2/5] serial: tegra: don't advance the TX kfifo for a flushed transfer

[Samarth Narang]

tegra_uart_flush_buffer() terminates the TX DMA transfer after the
serial core has reset the transmit kfifo, but leaves tx_in_progress set.
A completion callback that was already running may have calculated its
byte count from tx_bytes_requested before acquiring the port lock,
allowing it to call uart_xmit_advance() on the reset kfifo after the
flush. This can advance the kfifo tail past its head, causing
kfifo_len() to wrap and the driver to transmit stale buffer contents.
The same race was fixed in qcom-geni by commit 452d6fa37ae9 ("serial:
qcom_geni: fix kfifo underflow when flush precedes DMA completion IRQ").

Clear tx_in_progress in the flush hook and have the completion callback
skip the kfifo update if the transfer has already been retired by either
the flush or tegra_uart_stop_tx(), which also advances the kfifo itself.

Fixes: e9ea096dd225 ("serial: tegra: add serial driver")
Cc: stable at vger.kernel.org
Assisted-by: LLM
Signed-off-by: Samarth Narang <samarth.colleges at gmail.com>
---
 drivers/tty/serial/serial-tegra.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/tty/serial/serial-tegra.c b/drivers/tty/serial/serial-tegra.c
index b0fc4ae3686f..8b712a9bc293 100644
--- a/drivers/tty/serial/serial-tegra.c
+++ b/drivers/tty/serial/serial-tegra.c
@@ -537,6 +537,11 @@ static void tegra_uart_tx_dma_complete(void *args)
 	count = tup->tx_bytes_requested - state.residue;
 	async_tx_ack(tup->tx_dma_desc);
 	uart_port_lock_irqsave(&tup->uport, &flags);
+	/* Already retired by stop_tx or flush_buffer */
+	if (tup->tx_in_progress != TEGRA_UART_TX_DMA) {
+		uart_port_unlock_irqrestore(&tup->uport, flags);
+		return;
+	}
 	uart_xmit_advance(&tup->uport, count);
 	tup->tx_in_progress = 0;
 	if (kfifo_len(&tport->xmit_fifo) < WAKEUP_CHARS)
@@ -1276,6 +1281,9 @@ static void tegra_uart_flush_buffer(struct uart_port *u)
 	tup->tx_bytes = 0;
 	if (tup->tx_dma_chan)
 		dmaengine_terminate_all(tup->tx_dma_chan);
+	/* The kfifo has been reset: the completion must not advance it */
+	if (tup->tx_in_progress == TEGRA_UART_TX_DMA)
+		tup->tx_in_progress = 0;
 }
 
 static void tegra_uart_shutdown(struct uart_port *u)
-- 
2.34.1