Re: [PATCH] serial: core: Fix checks for tx runtime PM state

[Maximilian Luz <luzmaximilian@gmail.com>]

On 10/5/23 09:56, Tony Lindgren wrote:
> Maximilian reported that surface_serial_hub serdev tx does not work during
> system suspend. During system suspend, runtime PM gets disabled in
> __device_suspend_late(), and tx is unable to wake-up the serial core port
> device that we use to check if tx is safe to start. Johan summarized the
> regression noting that serdev tx no longer always works as earlier when the
> serdev device is runtime PM active.
> 
> The serdev device and the serial core controller devices are siblings of
> the serial port hardware device. The runtime PM usage count from serdev
> device does not propagate to the serial core device siblings, it only
> propagates to the parent.
> 
> In addition to the tx issue for suspend, testing for the serial core port
> device can cause an unnecessary delay in enabling tx while waiting for the
> serial core port device to wake-up. The serial core port device wake-up is
> only needed to flush pending tx when the serial port hardware device was
> in runtime PM suspended state.
> 
> To fix the regression, we need to check the runtime PM state of the parent
> serial port hardware device for tx instead of the serial core port device.
> 
> As the serial port device drivers may or may not implement runtime PM, we
> need to also add a check for pm_runtime_enabled().
> 
> Reported-by: Maximilian Luz <luzmaximilian@gmail.com>
> Fixes: 84a9582fd203 ("serial: core: Start managing serial controllers to enable runtime PM")
> Signed-off-by: Tony Lindgren <tony@atomide.com>

Thanks!

Tested-by: Maximilian Luz <luzmaximilian@gmail.com>

> ---
>   drivers/tty/serial/serial_core.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -156,7 +156,7 @@ static void __uart_start(struct uart_state *state)
>   	 * enabled, serial_port_runtime_resume() calls start_tx() again
>   	 * after enabling the device.
>   	 */
> -	if (pm_runtime_active(&port_dev->dev))
> +	if (!pm_runtime_enabled(port->dev) || pm_runtime_active(port->dev))
>   		port->ops->start_tx(port);
>   	pm_runtime_mark_last_busy(&port_dev->dev);
>   	pm_runtime_put_autosuspend(&port_dev->dev);