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

[Yicong Yang]

On 2024/2/6 17:44, Greg KH wrote:
> 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.
>>
>> Cc: Tony Lindgren <tony@xxxxxxxxxxx>
>> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> 
> 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?
> 

Sorry for missing it. The fix tag should be:

Fixes: 84a9582fd203 ("serial: core: Start managing serial controllers to enable runtime PM")

Thanks.