From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	stable <stable@kernel.org>,
	Raul E Rangel <rrangel@google.com>
Subject: [PATCH 6.12 345/460] serial: 8250: Fix TX deadlock when using DMA
Date: Mon, 23 Mar 2026 14:45:41 +0100
Message-ID: <20260323134535.020663691@linuxfoundation.org>
In-Reply-To: <20260323134526.647552166@linuxfoundation.org>
References: <20260323134526.647552166@linuxfoundation.org>
User-Agent: quilt/0.69
Precedence: bulk
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit

6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Raul E Rangel <rrangel@chromium.org>

commit a424a34b8faddf97b5af41689087e7a230f79ba7 upstream.

`dmaengine_terminate_async` does not guarantee that the
`__dma_tx_complete` callback will run. The callback is currently the
only place where `dma->tx_running` gets cleared. If the transaction is
canceled and the callback never runs, then `dma->tx_running` will never
get cleared and we will never schedule new TX DMA transactions again.

This change makes it so we clear `dma->tx_running` after we terminate
the DMA transaction. This is "safe" because `serial8250_tx_dma_flush`
is holding the UART port lock. The first thing the callback does is also
grab the UART port lock, so access to `dma->tx_running` is serialized.

Fixes: 9e512eaaf8f4 ("serial: 8250: Fix fifo underflow on flush")
Cc: stable <stable@kernel.org>
Signed-off-by: Raul E Rangel <rrangel@google.com>
Link: https://patch.msgid.link/20260209135815.1.I16366ecb0f62f3c96fe3dd5763fcf6f3c2b4d8cd@changeid
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/serial/8250/8250_dma.c |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

--- a/drivers/tty/serial/8250/8250_dma.c
+++ b/drivers/tty/serial/8250/8250_dma.c
@@ -162,7 +162,22 @@ void serial8250_tx_dma_flush(struct uart
 	 */
 	dma->tx_size = 0;
 
+	/*
+	 * We can't use `dmaengine_terminate_sync` because `uart_flush_buffer` is
+	 * holding the uart port spinlock.
+	 */
 	dmaengine_terminate_async(dma->txchan);
+
+	/*
+	 * The callback might or might not run. If it doesn't run, we need to ensure
+	 * that `tx_running` is cleared so that we can schedule new transactions.
+	 * If it does run, then the zombie callback will clear `tx_running` again
+	 * and perform a no-op since `tx_size` was cleared above.
+	 *
+	 * In either case, we ASSUME the DMA transaction will terminate before we
+	 * issue a new `serial8250_tx_dma`.
+	 */
+	dma->tx_running = 0;
 }
 
 int serial8250_rx_dma(struct uart_8250_port *p)