From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1876FC433FE for ; Mon, 11 Apr 2022 12:01:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344731AbiDKMDq (ORCPT ); Mon, 11 Apr 2022 08:03:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42346 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1345661AbiDKMDp (ORCPT ); Mon, 11 Apr 2022 08:03:45 -0400 Received: from muru.com (muru.com [72.249.23.125]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 0EDDF369FB; Mon, 11 Apr 2022 05:01:31 -0700 (PDT) Received: from localhost (localhost [127.0.0.1]) by muru.com (Postfix) with ESMTPS id E6373809F; Mon, 11 Apr 2022 11:58:59 +0000 (UTC) Date: Mon, 11 Apr 2022 15:01:29 +0300 From: Tony Lindgren To: Johan Hovold Cc: Greg Kroah-Hartman , Andy Shevchenko , Jiri Slaby , Sebastian Andrzej Siewior , Vignesh Raghavendra , linux-serial@vger.kernel.org, linux-omap@vger.kernel.org, linux-kernel@vger.kernel.org, "Matwey V . Kornilov" , Steffen Trumtrar , Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= Subject: Re: [PATCH 1/2] serial: 8250: Fix runtime PM for start_tx() for RS485 Message-ID: References: <20220411094805.45696-1-tony@atomide.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-omap@vger.kernel.org * Johan Hovold [220411 11:53]: > On Mon, Apr 11, 2022 at 01:32:58PM +0300, Tony Lindgren wrote: > > * Johan Hovold [220411 10:23]: > > > On Mon, Apr 11, 2022 at 01:10:34PM +0300, Tony Lindgren wrote: > > > > * Johan Hovold [220411 09:54]: > > > > > On Mon, Apr 11, 2022 at 12:48:04PM +0300, Tony Lindgren wrote: > > > > > > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c > > > > > > --- a/drivers/tty/serial/8250/8250_port.c > > > > > > +++ b/drivers/tty/serial/8250/8250_port.c > > > > > > @@ -1681,8 +1681,10 @@ static void serial8250_start_tx(struct uart_port *port) > > > > > > return; > > > > > > > > > > > > if (em485 && > > > > > > - em485->active_timer == &em485->start_tx_timer) > > > > > > + em485->active_timer == &em485->start_tx_timer) { > > > > > > + serial8250_rpm_put_tx(up); > > > > > > return; > > > > > > + } > > > > The problem is that that serial8250_rpm_put_tx() you're adding may > > > suspend the device unconditionally (i.e. regardless of any previous > > > calls to serial8250_rpm_get_tx()). > > > > > > If rs485 tx is just being deferred you mustn't suspend the device before > > > it has had a chance to start transmitting. > > > > Hmm I'm pretty sure rs485 has the runtime PM usage count is currently > > unbalanced. To me it seems em485->start_tx_timer calls start_tx() > > again from serial8250_em485_handle_start_tx(). > > It appears to call __start_tx(), but note that the only call to > serial8250_rpm_get_tx() is in serial8250_start_tx() which this patch > would have cancelled out. OK > Also note that the serial8250_rpm_get/set_tx() calls aren't supposed to > be balanced. get() can be called multiple times and will only increment > the PM usage counter once, while put() will decrement the counter > whenever get() has been called once (and hence potentially suspend the > device immediately). > > Messy indeed. Yeah that is not nice. I'll send a patch to prepare things for runtime PM that will hopefully make things a bit easier as discussed earlier. Regards, Tony