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 X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8D5B0C433E1 for ; Tue, 26 May 2020 16:57:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 758C020776 for ; Tue, 26 May 2020 16:57:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729650AbgEZQ5D (ORCPT ); Tue, 26 May 2020 12:57:03 -0400 Received: from mga12.intel.com ([192.55.52.136]:58729 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726930AbgEZQ5C (ORCPT ); Tue, 26 May 2020 12:57:02 -0400 IronPort-SDR: oIHRVtthYxI7JBehFK1KuCLOzFT0WArfgLts5cZ+1vAxwaffI70j2ojBuRXsAZbUpiGZYz/GRP R6opfkIno4GA== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 26 May 2020 09:57:02 -0700 IronPort-SDR: Er2zNXmgAzjRa4OAXcIq2b/xKyApj7yAEdoa4+gewOIBv7hNZ1+ASgdTNPQGRDP2fUT7g6IaUD cmfxEw6lq4OA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.73,437,1583222400"; d="scan'208";a="301780640" Received: from smile.fi.intel.com (HELO smile) ([10.237.68.40]) by fmsmga002.fm.intel.com with ESMTP; 26 May 2020 09:56:59 -0700 Received: from andy by smile with local (Exim 4.93) (envelope-from ) id 1jdcsv-0092P9-QH; Tue, 26 May 2020 19:57:01 +0300 Date: Tue, 26 May 2020 19:57:01 +0300 From: Andy Shevchenko To: Serge Semin Cc: Thomas Bogendoerfer , Greg Kroah-Hartman , Jiri Slaby , Serge Semin , Alexey Malahov , Arnd Bergmann , Maxime Ripard , Will Deacon , Russell King , linux-mips@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 3/3] serial: 8250_dw: Fix common clocks usage race condition Message-ID: <20200526165701.GX1634618@smile.fi.intel.com> References: <20200526160316.26136-1-Sergey.Semin@baikalelectronics.ru> <20200526160316.26136-4-Sergey.Semin@baikalelectronics.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200526160316.26136-4-Sergey.Semin@baikalelectronics.ru> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 26, 2020 at 07:03:16PM +0300, Serge Semin wrote: > The race condition may happen if the UART reference clock is shared with > some other device (on Baikal-T1 SoC it's another DW UART port). In this > case if that device changes the clock rate while serial console is using > it the DW 8250 UART port might not only end up with an invalid uartclk > value saved, but may also experience a distorted output data since > baud-clock could have been changed. In order to fix this lets at least > try to adjust the 8250 port setting like UART clock rate in case if the > reference clock rate change is discovered. The driver will call the new > method to update 8250 UART port clock rate settings. It's done by means of > the clock event notifier registered at the port startup and unregistered > in the shutdown callback method. > > Note 1. In order to avoid deadlocks we had to execute the UART port update > method in a dedicated deferred work. This is due to (in my opinion > redundant) the clock update implemented in the dw8250_set_termios() > method. > Note 2. Before the ref clock is manually changed by the custom > set_termios() function we swap the port uartclk value with new rate > adjusted to be suitable for the requested baud. It is necessary in > order to effectively disable a functionality of the ref clock events > handler for the current UART port, since uartclk update will be done > a bit further in the generic serial8250_do_set_termios() function. ... > +static void dw8250_clk_work_cb(struct work_struct *work) > +{ > + struct dw8250_data *d = work_to_dw8250_data(work); > + struct uart_8250_port *up; > + unsigned long rate; > + > + rate = clk_get_rate(d->clk); > + if (rate) { if (rate <= 0) return; ? > + up = serial8250_get_port(d->data.line); > + > + serial8250_update_uartclk(&up->port, rate); > + } > +} ... > +static int dw8250_startup(struct uart_port *p) > +{ > + struct dw8250_data *d = to_dw8250_data(p->private_data); > + int ret; > + > + /* > + * Some platforms may provide a reference clock shared between several > + * devices. In this case before using the serial port first we have to > + * make sure that any clock state change is known to the UART port at > + * least post factum. > + */ > + if (d->clk) { Do you need this? > + ret = clk_notifier_register(d->clk, &d->clk_notifier); Okay, seems clk_notifier_register() and its counterpart should be fixed for optional clocks. > + if (ret) > + dev_warn(p->dev, "Failed to set the clock notifier\n"); So, what does this warning mean on the platforms which does not need notifier at all (i.o.w. all but baikal)? > + /* > + * Get current reference clock rate to make sure the UART port > + * is equipped with an up-to-date value before it's started up. > + */ Why? We call ->set_termios() for it, no? > + p->uartclk = clk_get_rate(d->clk); > + if (!p->uartclk) { > + dev_err(p->dev, "Clock rate not defined\n"); > + return -EINVAL; > + } > + } > + > + return serial8250_do_startup(p); > +} > + > +static void dw8250_shutdown(struct uart_port *p) > +{ > + struct dw8250_data *d = to_dw8250_data(p->private_data); > + > + serial8250_do_shutdown(p); > + > + if (d->clk) { Ditto. > + clk_notifier_unregister(d->clk, &d->clk_notifier); > + > + flush_work(&d->clk_work); > + } > +} -- With Best Regards, Andy Shevchenko