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
