serial: qcom-geni: fix hard lockup on buffer flush

The Qualcomm GENI serial driver does not handle buffer flushing and used
to continue printing discarded characters when the circular buffer was
cleared. Since commit 1788cf6a ("tty: serial: switch from circ_buf
to kfifo") this instead results in a hard lockup due to
qcom_geni_serial_send_chunk_fifo() spinning indefinitely in the
interrupt handler.

This is easily triggered by interrupting a command such as dmesg in a
serial console but can also happen when stopping a serial getty on
reboot.

Implement the flush_buffer() callback and use it to cancel any active TX
command when the write buffer has been emptied.
Reported-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/lkml/20240610222515.3023730-1-dianders@chromium.org/
Fixes: 1788cf6a ("tty: serial: switch from circ_buf to kfifo")
Fixes: a1fee899 ("tty: serial: qcom_geni_serial: Fix softlock")
Cc: stable@vger.kernel.org	# 5.0
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20240704101805.30612-3-johan+linaro@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/qcom_geni_serial.c

@@ -906,13 +906,17 @@ static void qcom_geni_serial_handle_tx_fifo(struct uart_port *uport,
 	else
 		pending = kfifo_len(&tport->xmit_fifo);
 
-	/* All data has been transmitted and acknowledged as received */
-	if (!pending && !status && done) {
+	/* All data has been transmitted or command has been cancelled */
+	if (!pending && done) {
 		qcom_geni_serial_stop_tx_fifo(uport);
 		goto out_write_wakeup;
 	}
 
-	avail = port->tx_fifo_depth - (status & TX_FIFO_WC);
+	if (active)
+		avail = port->tx_fifo_depth - (status & TX_FIFO_WC);
+	else
+		avail = port->tx_fifo_depth;
+
 	avail *= BYTES_PER_FIFO_WORD;
 
 	chunk = min(avail, pending);
@@ -1091,6 +1095,11 @@ static void qcom_geni_serial_shutdown(struct uart_port *uport)
 	qcom_geni_serial_cancel_tx_cmd(uport);
 }
 
+static void qcom_geni_serial_flush_buffer(struct uart_port *uport)
+{
+	qcom_geni_serial_cancel_tx_cmd(uport);
+}
+
 static int qcom_geni_serial_port_setup(struct uart_port *uport)
 {
 	struct qcom_geni_serial_port *port = to_dev_port(uport);
@@ -1547,6 +1556,7 @@ static const struct uart_ops qcom_geni_console_pops = {
 	.request_port = qcom_geni_serial_request_port,
 	.config_port = qcom_geni_serial_config_port,
 	.shutdown = qcom_geni_serial_shutdown,
+	.flush_buffer = qcom_geni_serial_flush_buffer,
 	.type = qcom_geni_serial_get_type,
 	.set_mctrl = qcom_geni_serial_set_mctrl,
 	.get_mctrl = qcom_geni_serial_get_mctrl,