Re: [PATCH 4/6] serial: 8250: Implement prep_tx for power management

[Johan Hovold <johan@kernel.org>]

On Tue, Sep 21, 2021 at 01:33:44PM +0300, Tony Lindgren wrote:
> We can use the prep_tx() call to wake up an idle serial port. This allows
> ust to remove the depedency to pm_runtime_irq_safe() for 8250_omap driver
> in the following patches.
> 
> Signed-off-by: Tony Lindgren <tony@atomide.com>
> ---
>  drivers/tty/serial/8250/8250_port.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -1650,6 +1650,29 @@ static enum hrtimer_restart serial8250_em485_handle_start_tx(struct hrtimer *t)
>  	return HRTIMER_NORESTART;
>  }
>  
> +static int serial8250_prep_tx(struct uart_port *port)
> +{
> +	struct uart_8250_port *up = up_to_u8250p(port);
> +	struct device *dev = up->port.dev;
> +	int err;
> +
> +	if (!(up->capabilities & UART_CAP_RPM))
> +		return 0;
> +
> +	if (!pm_runtime_suspended(dev)) {
> +		pm_runtime_mark_last_busy(dev);
> +		return 0;
> +	}
> +
> +	err = pm_request_resume(dev);
> +	if (err < 0) {
> +		dev_warn(dev, "prep_tx wakeup failed: %d\n", err);
> +		return err;
> +	}

How is this supposed to work without a runtime PM usage-counter
increment? What's to prevent the port from suspending again while it's
transmitting?

> +
> +	return -EINPROGRESS;
> +}

Johan