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

[Tony Lindgren <tony@atomide.com>]

* Yicong Yang <yangyicong@huawei.com> [240204 03:24]:
> From: Yicong Yang <yangyicong@hisilicon.com>
> 
> 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()

Can you please confirm if this still happens also with commit 6f699743aebf
("serial: core: Fix runtime PM handling for pending tx")? It adds a check
for -EINPROGRESS.

> 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 request an runtime resume for
> handling this.

OK. Let's look at this further after you have checked what happens with
commit 6f699743aebf.

If needed, to me it seems that flushing tx and returning -EBUSY from
serial_port_runtime_suspend() if busy might do the trick though.

Regards,

Tony