linux-kernel - Re: [PATCH] serial: 8250: Fix PM usage

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Date:   Wed, 29 Jun 2022 12:21:21 +0300
From:   Tony Lindgren <tony@...mide.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] serial: 8250: Fix PM usage_count for console handover

* Andy Shevchenko <andriy.shevchenko@...ux.intel.com> [220628 21:09]:
> On Tue, Jun 28, 2022 at 07:58:34PM +0300, Ilpo Järvinen wrote:
> > When console is enabled, univ8250_console_setup() calls
> > serial8250_console_setup() before .dev is set to uart_port. Therefore,
> > it will not call pm_runtime_get_sync(). Later, when the actual driver
> > is going to take over univ8250_console_exit() is called. As .dev is
> > already set, serial8250_console_exit() makes pm_runtime_put_sync() call
> > with usage count being zero triggering PM usage count warning
> > (extra debug for univ8250_console_setup(), univ8250_console_exit(), and
> > serial8250_register_ports()):

Hmm so serial8250_console_setup() calls pm_runtime_get_sync() if dev
exists..

> > diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> > index 57e86133af4f..2e83e7367441 100644
> > --- a/drivers/tty/serial/8250/8250_core.c
> > +++ b/drivers/tty/serial/8250/8250_core.c
> > @@ -23,6 +23,7 @@
> >  #include <linux/sysrq.h>
> >  #include <linux/delay.h>
> >  #include <linux/platform_device.h>
> > +#include <linux/pm_runtime.h>
> >  #include <linux/tty.h>
> >  #include <linux/ratelimit.h>
> >  #include <linux/tty_flip.h>
> > @@ -558,6 +559,9 @@ serial8250_register_ports(struct uart_driver *drv, struct device *dev)
> >  
> >  		up->port.dev = dev;
> >  
> > +		if (uart_console_enabled(&up->port))
> > +			pm_runtime_get_sync(up->port.dev);
> > +
> >  		serial8250_apply_quirks(up);
> >  		uart_add_one_port(drv, &up->port);
> >  	}

..and now we also call it here. Are there now cases where pm_runtime_get_sync()
gets called twice potentially or does uart_console_enabled() ensure that is
not the case already?

Regards,

Tony