linux-kernel - Re: [PATCH 4/6] serial: 8250: Implement prep

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Date:   Thu, 23 Sep 2021 18:05:36 +0300
From:   Tony Lindgren <tony@...mide.com>
To:     Johan Hovold <johan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-serial@...r.kernel.org, linux-omap@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] serial: 8250: Implement prep_tx for power management

* Johan Hovold <johan@...nel.org> [210923 12:50]:
> On Tue, Sep 21, 2021 at 01:33:44PM +0300, Tony Lindgren wrote:
> > +static int serial8250_prep_tx(struct uart_port *port)
> > +{
> > +	struct uart_8250_port *up = up_to_u8250p(port);
> > +	struct device *dev = up->port.dev;
> > +	int err;
> > +
> > +	if (!(up->capabilities & UART_CAP_RPM))
> > +		return 0;
> > +
> > +	if (!pm_runtime_suspended(dev)) {
> > +		pm_runtime_mark_last_busy(dev);
> > +		return 0;
> > +	}
> > +
> > +	err = pm_request_resume(dev);
> > +	if (err < 0) {
> > +		dev_warn(dev, "prep_tx wakeup failed: %d\n", err);
> > +		return err;
> > +	}
> 
> How is this supposed to work without a runtime PM usage-counter
> increment? What's to prevent the port from suspending again while it's
> transmitting?

Hmm yeah we should at pm_runtime_get() and pm_runtime_put() to write()
unless serial8250_rpm_get() and serial8250_rpm_put() are doing it.

Or pair prep with finish and deal with the usage count there.

Regards,

Tony