Re: [PATCH v2] serial: port: Don't suspend if the port is still busy

[Andy Shevchenko]

On Tue, Feb 06, 2024 at 03:33:22PM +0800, Yicong Yang wrote:
> From: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> 
> We accidently met the issue that the bash prompt is not shown after the
> previous command done and until the next input if there's only one CPU
> (In our issue other CPUs are isolated by isolcpus=). Further analysis
> shows it's because the port entering runtime suspend even if there's
> still pending chars in the buffer and the pending chars will only be
> processed in next device resuming. We are using amba-pl011 and the
> problematic flow is like below:
> 
> Bash                                         kworker
> tty_write()
>   file_tty_write()
>     n_tty_write()
>       uart_write()
>         __uart_start()
>           pm_runtime_get() // wakeup waker
>             queue_work()
>                                              pm_runtime_work()
>                                                rpm_resume()
>                                                 status = RPM_RESUMING
>                                                 serial_port_runtime_resume()
>                                                   port->ops->start_tx()
>                                                     pl011_tx_chars()
>                                                       uart_write_wakeup()
>         […]
>         __uart_start()
>           pm_runtime_get() < 0 // because runtime status = RPM_RESUMING
>                                // later data are not commit to the port driver
>                                                 status = RPM_ACTIVE
>                                                 rpm_idle() -> rpm_suspend()
> 
> This patch tries to fix this by checking the port busy before entering
> runtime suspending. A runtime_suspend callback is added for the port
> driver. When entering runtime suspend the callback is invoked, if there's
> still pending chars in the buffer then flush the buffer.

..

> +static int serial_port_runtime_suspend(struct device *dev)
> +{
> +	struct serial_port_device *port_dev = to_serial_base_port_device(dev);
> +	struct uart_port *port;
> +	unsigned long flags;
> +	int ret = 0;
> +
> +	port = port_dev->port;
> +
> +	if (port->flags & UPF_DEAD)
> +		return ret;
> +
> +	uart_port_lock_irqsave(port, &flags);
> +	if (__serial_port_busy(port)) {
> +		port->ops->start_tx(port);

> +		pm_runtime_mark_last_busy(dev);

Do you think we need to call this under a lock?

> +		ret = -EBUSY;
> +	}
> +	uart_port_unlock_irqrestore(port, flags);
> +
> +	return ret;
> +}

With the above I would rather write it as

static int __serial_port_busy(struct uart_port *port)
{
	if (uart_tx_stopped(port))
		return 0;

	if (uart_circ_chars_pending(&port->state->xmit)
		return -EBUSY;

	return 0;
}

static int serial_port_runtime_suspend(struct device *dev)
{
	int ret;
	...
	uart_port_lock_irqsave(port, &flags);
	ret = __serial_port_busy(port);
	if (ret)
		port->ops->start_tx(port);
	uart_port_unlock_irqrestore(port, flags);

	if (ret)
		pm_runtime_mark_last_busy(dev);

	return ret;
}

It also seems aligned with the resume implementation above.

..

For the consistency's sake the resume can be refactored as

static int serial_port_runtime_resume(struct device *dev)
{
	...
	int ret;
	...
	ret = __serial_port_busy(port);
	if (ret)
	...
}

but this can be done later.

-- 
With Best Regards,
Andy Shevchenko