[PATCH 1/2] serial: port: Don't block system suspend even if bytes are left to xmit

[Douglas Anderson]

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 problem 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 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. 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: 43066e32227e ("serial: port: Don't suspend if the port is still busy")
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---

 drivers/tty/serial/serial_port.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/tty/serial/serial_port.c b/drivers/tty/serial/serial_port.c
index 91a338d3cb34..b781227cc996 100644
--- a/drivers/tty/serial/serial_port.c
+++ b/drivers/tty/serial/serial_port.c
@@ -64,6 +64,16 @@ static int serial_port_runtime_suspend(struct device *dev)
 	if (port->flags & UPF_DEAD)
 		return 0;
 
+	/*
+	 * We only want to check the busyness of the port if PM Runtime is
+	 * enabled. Specifically PM Runtime will be disabled by
+	 * pm_runtime_force_suspend() during system suspend and we don't want
+	 * to block system suspend even if there is data still left to
+	 * transmit. We only want to block regulator PM Runtime transitions.
+	 */
+	if (!pm_runtime_enabled(dev))
+		return 0;
+
 	uart_port_lock_irqsave(port, &flags);
 	if (!port_dev->tx_enabled) {
 		uart_port_unlock_irqrestore(port, flags);
-- 
2.45.1.288.g0e0cd299f1-goog