linux-kernel - Re: [PATCH v12 1/1] serial: core: Start managing serial controllers to enable runtime PM

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Date:   Wed, 4 Oct 2023 13:01:23 +0300
From:   Tony Lindgren <tony@...mide.com>
To:     Johan Hovold <johan@...nel.org>
Cc:     Maximilian Luz <luzmaximilian@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Dhruva Gole <d-gole@...com>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        John Ogness <john.ogness@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-omap@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH v12 1/1] serial: core: Start managing serial controllers
 to enable runtime PM

* Johan Hovold <johan@...nel.org> [231004 09:14]:
> On Wed, Oct 04, 2023 at 12:03:20PM +0300, Tony Lindgren wrote:
> > The serial port device and serdev device are siblings of the physical
> > serial port controller device as seen in the hierarcy printed out by
> > Maximilian.
> 
> Yeah, and that's precisely the broken part. Keeping the serdev
> controller active is supposed to keep the serial controller active. Your
> serial core rework appears to have broken just that.

Hmm OK good point, tx can currently have an extra delay if a serdev
device is active, and the serial port controller device is not active.

So we can check for active port->dev instead of &port_dev->dev though
to know when when start_tx() is safe to do as below.

Thanks.

Tony

8< -----------------
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 6207f0051f23d..defecc5b04422 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -156,7 +156,7 @@ static void __uart_start(struct uart_state *state)
 	 * enabled, serial_port_runtime_resume() calls start_tx() again
 	 * after enabling the device.
 	 */
-	if (pm_runtime_active(&port_dev->dev))
+	if (!pm_runtime_enabled(port->dev) || pm_runtime_active(port->dev))
 		port->ops->start_tx(port);
 	pm_runtime_mark_last_busy(&port_dev->dev);
 	pm_runtime_put_autosuspend(&port_dev->dev);