serial: qcom-geni: fix soft lockup on sw flow control and suspend

The stop_tx() callback is used to implement software flow control and
must not discard data as the Qualcomm GENI driver is currently doing
when there is an active TX command.

Cancelling an active command can also leave data in the hardware FIFO,
which prevents the watermark interrupt from being enabled when TX is
later restarted. This results in a soft lockup and is easily triggered
by stopping TX using software flow control in a serial console but this
can also happen after suspend.

Fix this by only stopping any active command, and effectively clearing
the hardware fifo, when shutting down the port. When TX is later
restarted, a transfer command may need to be issued to discard any stale
data that could prevent the watermark interrupt from firing.

Fixes: c4f52879 ("tty: serial: msm_geni_serial: Add serial driver support for GENI based QUP")
Cc: stable@vger.kernel.org	# 4.17
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20240704101805.30612-2-johan+linaro@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/qcom_geni_serial.c

@@ -649,15 +649,25 @@ static void qcom_geni_serial_start_tx_dma(struct uart_port *uport)
 
 static void qcom_geni_serial_start_tx_fifo(struct uart_port *uport)
 {
+	unsigned char c;
 	u32 irq_en;
 
-	if (qcom_geni_serial_main_active(uport) ||
-	    !qcom_geni_serial_tx_empty(uport))
-		return;
+	/*
+	 * Start a new transfer in case the previous command was cancelled and
+	 * left data in the FIFO which may prevent the watermark interrupt
+	 * from triggering. Note that the stale data is discarded.
+	 */
+	if (!qcom_geni_serial_main_active(uport) &&
+	    !qcom_geni_serial_tx_empty(uport)) {
+		if (uart_fifo_out(uport, &c, 1) == 1) {
+			writel(M_CMD_DONE_EN, uport->membase + SE_GENI_M_IRQ_CLEAR);
+			qcom_geni_serial_setup_tx(uport, 1);
+			writel(c, uport->membase + SE_GENI_TX_FIFOn);
+		}
+	}
 
 	irq_en = readl(uport->membase +	SE_GENI_M_IRQ_EN);
 	irq_en |= M_TX_FIFO_WATERMARK_EN | M_CMD_DONE_EN;
-
 	writel(DEF_TX_WM, uport->membase + SE_GENI_TX_WATERMARK_REG);
 	writel(irq_en, uport->membase +	SE_GENI_M_IRQ_EN);
 }
@@ -665,13 +675,17 @@ static void qcom_geni_serial_start_tx_fifo(struct uart_port *uport)
 static void qcom_geni_serial_stop_tx_fifo(struct uart_port *uport)
 {
 	u32 irq_en;
-	struct qcom_geni_serial_port *port = to_dev_port(uport);
 
 	irq_en = readl(uport->membase + SE_GENI_M_IRQ_EN);
 	irq_en &= ~(M_CMD_DONE_EN | M_TX_FIFO_WATERMARK_EN);
 	writel(0, uport->membase + SE_GENI_TX_WATERMARK_REG);
 	writel(irq_en, uport->membase + SE_GENI_M_IRQ_EN);
-	/* Possible stop tx is called multiple times. */
+}
+
+static void qcom_geni_serial_cancel_tx_cmd(struct uart_port *uport)
+{
+	struct qcom_geni_serial_port *port = to_dev_port(uport);
+
 	if (!qcom_geni_serial_main_active(uport))
 		return;
 
@@ -684,6 +698,8 @@ static void qcom_geni_serial_stop_tx_fifo(struct uart_port *uport)
 		writel(M_CMD_ABORT_EN, uport->membase + SE_GENI_M_IRQ_CLEAR);
 	}
 	writel(M_CMD_CANCEL_EN, uport->membase + SE_GENI_M_IRQ_CLEAR);
+
+	port->tx_remaining = 0;
 }
 
 static void qcom_geni_serial_handle_rx_fifo(struct uart_port *uport, bool drop)
@@ -1069,11 +1085,10 @@ static void qcom_geni_serial_shutdown(struct uart_port *uport)
 {
 	disable_irq(uport->irq);
 
-	if (uart_console(uport))
-		return;
-
 	qcom_geni_serial_stop_tx(uport);
 	qcom_geni_serial_stop_rx(uport);
+
+	qcom_geni_serial_cancel_tx_cmd(uport);
 }
 
 static int qcom_geni_serial_port_setup(struct uart_port *uport)