serial: port: Don't suspend if the port is still busy

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.

Fixes: 84a9582f

 ("serial: core: Start managing serial controllers to enable runtime PM")
Cc: stable <stable@kernel.org>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Link: https://lore.kernel.org/r/20240226152351.40924-1-yangyicong@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>