linux-kernel - Re: [PATCH v4] serial: port: Don't suspend if the port is still busy

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Date: Fri, 23 Feb 2024 17:44:31 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Yicong Yang <yangyicong@...wei.com>
Cc: gregkh@...uxfoundation.org, jirislaby@...nel.org, tony@...mide.com,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	john.ogness@...utronix.de, tglx@...utronix.de,
	yangyicong@...ilicon.com, linuxarm@...wei.com,
	prime.zeng@...ilicon.com, jonathan.cameron@...wei.com,
	fanghao11@...wei.com
Subject: Re: [PATCH v4] serial: port: Don't suspend if the port is still busy

On Fri, Feb 23, 2024 at 04:39:03PM +0800, Yicong Yang wrote:
> From: Yicong Yang <yangyicong@...ilicon.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@...mide.com>

No need to Cc to people whose tags you already have. The Git tools will add
them to the Cc list.

> Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> Cc: Jiri Slaby <jirislaby@...nel.org>

In general, Cc is better to be either supplied with --cc or be located after
the cutter '---' line, so they won't pollute the commit message.

..

Code wise LGTM, thanks.
Minor remarks below, with them addressed,
Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

> +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;

You can assign here

	struct uart_port *port = port_dev->port;

and save 2 LoCs.

> +	unsigned long flags;
> +	bool busy;
> +
> +	port = port_dev->port;

> +

If you want to have assignment separated, this blank line may be dropped.

> +	if (port->flags & UPF_DEAD)
> +		return 0;
> +
> +	uart_port_lock_irqsave(port, &flags);
> +	busy = __serial_port_busy(port);
> +	if (busy)
> +		port->ops->start_tx(port);
> +	uart_port_unlock_irqrestore(port, flags);
> +
> +	if (busy)
> +		pm_runtime_mark_last_busy(dev);
> +
> +	return busy ? -EBUSY : 0;
> +}

-- 
With Best Regards,
Andy Shevchenko