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@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);
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
