serial: port: Don't block system suspend even if bytes are left to xmit

Recently, suspend testing on sc7180-trogdor based devices has started
to sometimes fail with messages like this:

  port a88000.serial:0.0: PM: calling pm_runtime_force_suspend+0x0/0xf8 @ 28934, parent: a88000.serial:0
  port a88000.serial:0.0: PM: dpm_run_callback(): pm_runtime_force_suspend+0x0/0xf8 returns -16
  port a88000.serial:0.0: PM: pm_runtime_force_suspend+0x0/0xf8 returned -16 after 33 usecs
  port a88000.serial:0.0: PM: failed to suspend: error -16

I could reproduce these problems by logging in via an agetty on the
debug serial port (which was _not_ used for kernel console) and
running:
  cat /var/log/messages
...and then (via an SSH session) forcing a few suspend/resume cycles.

Tracing through the code and doing some printf()-based debugging shows
that the -16 (-EBUSY) comes from the recently added
serial_port_runtime_suspend().

The idea of the serial_port_runtime_suspend() function is to prevent
the port from being _runtime_ suspended if it still has bytes left to
transmit. Having bytes left to transmit isn't a reason to block
_system_ suspend, though. If a serdev device in the kernel needs to
block system suspend it should block its own suspend and it can use
serdev_device_wait_until_sent() to ensure bytes are sent.

The DEFINE_RUNTIME_DEV_PM_OPS() used by the serial_port code means
that the system suspend function will be pm_runtime_force_suspend().
In pm_runtime_force_suspend() we can see that before calling the
runtime suspend function we'll call pm_runtime_disable(). This should
be a reliable way to detect that we're called from system suspend and
that we shouldn't look for busyness.

Fixes: 43066e32 ("serial: port: Don't suspend if the port is still busy")
Cc: stable@vger.kernel.org
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20240531080914.v3.1.I2395e66cf70c6e67d774c56943825c289b9c13e4@changeidSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_port.c

@@ -64,6 +64,13 @@ static int serial_port_runtime_suspend(struct device *dev)
 	if (port->flags & UPF_DEAD)
 		return 0;
 
+	/*
+	 * Nothing to do on pm_runtime_force_suspend(), see
+	 * DEFINE_RUNTIME_DEV_PM_OPS.
+	 */
+	if (!pm_runtime_enabled(dev))
+		return 0;
+
 	uart_port_lock_irqsave(port, &flags);
 	if (!port_dev->tx_enabled) {
 		uart_port_unlock_irqrestore(port, flags);