linux-kernel - Re: [PATCH 1/2] serial: port: Don't block system suspend even if bytes are left to xmit

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Date: Mon, 27 May 2024 20:54:26 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Douglas Anderson <dianders@...omium.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	John Ogness <john.ogness@...utronix.de>,
	Tony Lindgren <tony@...mide.com>, linux-arm-msm@...r.kernel.org,
	Johan Hovold <johan+linaro@...nel.org>,
	Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
	Yicong Yang <yangyicong@...ilicon.com>,
	Guanbing Huang <albanhuang@...cent.com>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH 1/2] serial: port: Don't block system suspend even if
 bytes are left to xmit

On Thu, May 23, 2024 at 04:22:12PM -0700, Douglas Anderson wrote:
> Recently, suspend testing on sc7180-trogdor based devices has started
> to sometimes fail with messages like this:
> 
>   port a88000.serial:0.0: PM: calling pm_runtime_force_suspend+0x0/0xf8 @ 28934, parent: a88000.serial:0
>   port a88000.serial:0.0: PM: dpm_run_callback(): pm_runtime_force_suspend+0x0/0xf8 returns -16
>   port a88000.serial:0.0: PM: pm_runtime_force_suspend+0x0/0xf8 returned -16 after 33 usecs
>   port a88000.serial:0.0: PM: failed to suspend: error -16
> 
> I could reproduce these problem by logging in via an agetty on the
> debug serial port (which was _not_ used for kernel console) and
> running:
>   cat /var/log/messages
> ...and then (via an SSH session) forcing a few suspend/resume cycles.
> 
> Tracing through the code and doing some printf debugging shows that

printf()

..or...

printf()-based

> the -16 (-EBUSY) comes from the recently added
> serial_port_runtime_suspend().
> 
> The idea of the serial_port_runtime_suspend() function is to prevent
> the port from being _runtime_ suspended if it still has bytes left to
> transmit. Having bytes left to transmit isn't a reason to block
> _system_ suspend, though. 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.

..

> +	/*
> +	 * We only want to check the busyness of the port if PM Runtime is
> +	 * enabled. Specifically PM Runtime 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 regulator PM Runtime transitions.

regular

> +	 */
> +	if (!pm_runtime_enabled(dev))
> +		return 0;

-- 
With Best Regards,
Andy Shevchenko