[PATCH] serial: qcom-geni: do not advance stale DMA completions

[Guangshuo Li]

The qcom GENI serial DMA TX completion path advances the transmit fifo by
the number of bytes recorded in port->tx_remaining.

If uart_flush_buffer() runs after the hardware has completed a DMA
transfer but before the DMA completion interrupt has been handled, the
serial core resets the transmit fifo while port->tx_remaining still
describes the old DMA transfer.

A previous fix avoided advancing an empty fifo by checking that the fifo
length is at least tx_remaining. That still does not distinguish the old
DMA payload from new bytes written after the flush. If userspace writes
new data before the stale DMA completion interrupt is handled, the fifo
can again contain at least tx_remaining bytes and the stale completion
can advance and discard those new bytes.

Mark an in-flight DMA transfer stale when the transmit fifo is flushed.
The later completion still unprepares the original DMA mapping using the
saved length, but it no longer advances the transmit fifo.

Fixes: 2aaa43c70778 ("tty: serial: qcom-geni-serial: add support for serial engine DMA")
Signed-off-by: Guangshuo Li <lgs201920130244@xxxxxxxxx>
---
 drivers/tty/serial/qcom_geni_serial.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
index 7ead87b4eb65..ab3dbee3e526 100644
--- a/drivers/tty/serial/qcom_geni_serial.c
+++ b/drivers/tty/serial/qcom_geni_serial.c
@@ -143,6 +143,7 @@ struct qcom_geni_serial_port {
 
 	unsigned int tx_remaining;
 	unsigned int tx_queued;
+	bool tx_dma_stale;
 	int wakeup_irq;
 	bool rx_tx_swap;
 	bool cts_rts_swap;
@@ -697,6 +698,7 @@ static void qcom_geni_serial_start_tx_dma(struct uart_port *uport)
 	}
 
 	port->tx_remaining = xmit_size;
+	port->tx_dma_stale = false;
 }
 
 static void qcom_geni_serial_start_tx_fifo(struct uart_port *uport)
@@ -1029,6 +1031,7 @@ static void qcom_geni_serial_handle_tx_dma(struct uart_port *uport)
 	struct qcom_geni_serial_port *port = to_dev_port(uport);
 	struct tty_port *tport = &uport->state->port;
 	unsigned int fifo_len = kfifo_len(&tport->xmit_fifo);
+	bool tx_dma_stale = port->tx_dma_stale;
 
 	/*
 	 * Only advance the kfifo if it still contains the bytes that were
@@ -1039,12 +1042,13 @@ static void qcom_geni_serial_handle_tx_dma(struct uart_port *uport)
 	 * kfifo->in, making kfifo_len() wrap to UART_XMIT_SIZE - tx_remaining
 	 * and triggering a spurious large DMA transfer of stale data.
 	 */
-	if (fifo_len >= port->tx_remaining)
+	if (!tx_dma_stale && fifo_len >= 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;
+	port->tx_dma_stale = false;
 
 	if (!kfifo_is_empty(&tport->xmit_fifo))
 		qcom_geni_serial_start_tx_dma(uport);
@@ -1182,6 +1186,10 @@ static void qcom_geni_serial_shutdown(struct uart_port *uport)
 
 static void qcom_geni_serial_flush_buffer(struct uart_port *uport)
 {
+	struct qcom_geni_serial_port *port = to_dev_port(uport);
+
+	if (port->tx_dma_addr)
+		port->tx_dma_stale = true;
 	qcom_geni_serial_cancel_tx_cmd(uport);
 }
 
-- 
2.43.0