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

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Feb 06, 2024 at 03:33:22PM +0800, Yicong Yang wrote:
> 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()
> 
> 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.
> 
> Cc: Tony Lindgren <tony@atomide.com>
> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>

Is this a regression that was caused by the port code?  If so, what
commit id does this fix?  Should it be backported to older kernels?

thanks,

greg k-h