[PATCH v2] serial: port: Don't suspend if the port is still busy

[Yicong Yang <yangyicong@huawei.com>]

From: Yicong Yang <yangyicong@hisilicon.com>

We accidently met the issue that the bash prompt is not shown after the
previous command done and until the next input if there's only one CPU
(In our issue other CPUs are isolated by isolcpus=). Further analysis
shows it's because the port entering runtime suspend even if there's
still pending chars in the buffer and the pending chars will only be
processed in next device resuming. We are using amba-pl011 and the
problematic flow is like below:

Bash                                         kworker
tty_write()
  file_tty_write()
    n_tty_write()
      uart_write()
        __uart_start()
          pm_runtime_get() // wakeup waker
            queue_work()
                                             pm_runtime_work()
                                               rpm_resume()
                                                status = RPM_RESUMING
                                                serial_port_runtime_resume()
                                                  port->ops->start_tx()
                                                    pl011_tx_chars()
                                                      uart_write_wakeup()
        […]
        __uart_start()
          pm_runtime_get() < 0 // because runtime status = RPM_RESUMING
                               // later data are not commit to the port driver
                                                status = RPM_ACTIVE
                                                rpm_idle() -> rpm_suspend()

This patch tries to fix this by checking the port busy before entering
runtime suspending. A runtime_suspend callback is added for the port
driver. When entering runtime suspend the callback is invoked, if there's
still pending chars in the buffer then flush the buffer.

Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
---
Change since v1:
- Use port lock wrapper per John
- Flush the pending chars and return -EBUSY per Tony.
Thanks.
Link: https://lore.kernel.org/all/20240204031957.58176-1-yangyicong@huawei.com/

 drivers/tty/serial/serial_port.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/serial_port.c b/drivers/tty/serial/serial_port.c
index 88975a4df306..0617d5158235 100644
--- a/drivers/tty/serial/serial_port.c
+++ b/drivers/tty/serial/serial_port.c
@@ -46,8 +46,32 @@ static int serial_port_runtime_resume(struct device *dev)
 	return 0;
 }
 
+static int serial_port_runtime_suspend(struct device *dev)
+{
+	struct serial_port_device *port_dev = to_serial_base_port_device(dev);
+	struct uart_port *port;
+	unsigned long flags;
+	int ret = 0;
+
+	port = port_dev->port;
+
+	if (port->flags & UPF_DEAD)
+		return ret;
+
+	uart_port_lock_irqsave(port, &flags);
+	if (__serial_port_busy(port)) {
+		port->ops->start_tx(port);
+		pm_runtime_mark_last_busy(dev);
+		ret = -EBUSY;
+	}
+	uart_port_unlock_irqrestore(port, flags);
+
+	return ret;
+}
+
 static DEFINE_RUNTIME_DEV_PM_OPS(serial_port_pm,
-				 NULL, serial_port_runtime_resume, NULL);
+				 serial_port_runtime_suspend,
+				 serial_port_runtime_resume, NULL);
 
 static int serial_port_probe(struct device *dev)
 {
-- 
2.24.0