linux-kernel - Re: [PATCH v2] serial: port: Don't block system suspend even if bytes are left to xmit

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Date: Fri, 31 May 2024 08:01:55 +0300
From: Tony Lindgren <tony.lindgren@...ux.intel.com>
To: Douglas Anderson <dianders@...omium.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Johan Hovold <johan+linaro@...nel.org>,
	Yicong Yang <yangyicong@...ilicon.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
	John Ogness <john.ogness@...utronix.de>,
	linux-arm-msm@...r.kernel.org, Tony Lindgren <tony@...mide.com>,
	Guanbing Huang <albanhuang@...cent.com>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH v2] serial: port: Don't block system suspend even if
 bytes are left to xmit

On Thu, May 30, 2024 at 08:48:46AM -0700, Douglas Anderson wrote:
> The DEFINE_RUNTIME_DEV_PM_OPS() used by the serial_port code means
> that the system suspend function will be pm_runtime_force_suspend().
> In pm_runtime_force_suspend() we can see that before calling the
> runtime suspend function we'll call pm_runtime_disable(). This should
> be a reliable way to detect that we're called from system suspend and
> that we shouldn't look for busyness.

OK makes sense, one comment below though.

> --- a/drivers/tty/serial/serial_port.c
> +++ b/drivers/tty/serial/serial_port.c
> @@ -64,6 +64,16 @@ static int serial_port_runtime_suspend(struct device *dev)
>  	if (port->flags & UPF_DEAD)
>  		return 0;
>  
> +	/*
> +	 * We only want to check the busyness of the port if runtime PM is
> +	 * enabled. Specifically runtime PM will be disabled by
> +	 * pm_runtime_force_suspend() during system suspend and we don't want
> +	 * to block system suspend even if there is data still left to
> +	 * transmit. We only want to block regular runtime PM transitions.
> +	 */
> +	if (!pm_runtime_enabled(dev))
> +		return 0;
> +

How about change the comment a bit to describe why this happens so it's
easy to remember the next time looking at the code. I'd suggest just
something like this:

Nothing to do on pm_runtime_force_suspend(), see DEFINE_RUNTIME_DEV_PM_OPS

Other than that:

Reviewed-by: Tony Lindgren <tony.lindgren@...ux.intel.com>