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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: 43066e32227e ("serial: port: Don't suspend if the port is still busy") Cc: stable@vger.kernel.org Reviewed-by: Tony Lindgren Signed-off-by: Douglas Anderson --- In v1 [1] this was part of a 2-patch series. I'm now just sending this patch on its own since the Qualcomm GENI serial driver has ended up having a whole pile of problems that are taking a while to unravel. It makes sense to disconnect the two efforts. The core problem fixed by this patch and the geni problems never had any dependencies anyway. [1] https://lore.kernel.org/r/20240523162207.1.I2395e66cf70c6e67d774c56943825c289b9c13e4@changeid/ Changes in v3: - Adjust comment as per Tony Lindgren. - Add Cc: stable. Changes in v2: - Fix "regulator" => "regular" in comment. - Fix "PM Runtime" => "runtime PM" in comment. - Commit messages says how serdev devices should ensure bytes xfered. drivers/tty/serial/serial_port.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/drivers/tty/serial/serial_port.c b/drivers/tty/serial/serial_port.c index 91a338d3cb34..d35f1d24156c 100644 --- a/drivers/tty/serial/serial_port.c +++ b/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);