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

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Tue, Feb 06, 2024 at 03:09:32PM +0200, Andy Shevchenko wrote:
> 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.

...

> > +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);

And obvious question here: why in case of 0 we can't mark this as busy as well?
I.o.w. why do we need to mark it only when error is set?

> 	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