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23 Mar 2020 02:20:50 -0700 Received: from andy by smile with local (Exim 4.93) (envelope-from ) id 1jGJGN-00CCZd-2B; Mon, 23 Mar 2020 11:20:51 +0200 Date: Mon, 23 Mar 2020 11:20:51 +0200 From: Andy Shevchenko To: Sergey.Semin@baikalelectronics.ru Subject: Re: [PATCH v2] serial: 8250_dw: Fix common clocks usage race condition Message-ID: <20200323092051.GB1922688@smile.fi.intel.com> References: <20200306130231.05BBC8030795@mail.baikalelectronics.ru> <20200323024611.16039-1-Sergey.Semin@baikalelectronics.ru> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200323024611.16039-1-Sergey.Semin@baikalelectronics.ru> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200323_022101_728280_8EAA95A2 X-CRM114-Status: GOOD ( 23.74 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Andrew Lunn , Kefeng Wang , Heikki Krogerus , Heiko Stuebner , Catalin Marinas , Michael Turquette , Serge Semin , Will Deacon , linux-clk@vger.kernel.org, Florian Fainelli , Maxim Kaurkin , Ramil Zaripov , Gregory Clement , Russell King , Wei Xu , Chen-Yu Tsai , Ekaterina Skachko , Jiri Slaby , Sebastian Hesselbarth , Jason Cooper , Ray Jui , Maxime Ripard , Alexey Malahov , linux-serial@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Thomas Bogendoerfer , Vadim Vlasov , Paul Burton , Scott Branden , Stephen Boyd , Greg Kroah-Hartman , linux-kernel@vger.kernel.org, Ralf Baechle , Jisheng Zhang , Pavel Parkhomenko Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, Mar 23, 2020 at 05:46:09AM +0300, Sergey.Semin@baikalelectronics.ru wrote: > From: Serge Semin The question to CLK framework maintainers, is it correct approach in general for this case? > There are races possible in the dw8250_set_termios() callback method > and while the device is in PM suspend state. A race condition may > happen if the baudrate clock source device is shared with some other > device (in our machine 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 enable an exclusive > reference clock rate access in case if "baudclk" device is specified. > > So if some other device also acquires the rate exclusivity during the > time of a DW UART 8250 port being opened, then DW UART 8250 driver > won't be able to alter the baud-clock. It shall just use the available > clock rate. Similarly another device also won't manage to change the > rate at that time. If nothing else have the exclusive rate access > acquired except DW UART 8250 driver, then the driver will be able to > alter the rate as much as it needs to in accordance with the currently > implemented logic. Thank you for an update, my comments below. ... > +static int dw8250_startup(struct uart_port *p) > +{ > + struct dw8250_data *d = to_dw8250_data(p->private_data); > + > + /* > + * 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 nothing will change the rate behind our back and second > + * the tty/serial subsystem knows the actual reference clock rate of > + * the port. > + */ > + if (clk_rate_exclusive_get(d->clk)) { > + dev_warn(p->dev, "Couldn't lock the clock rate\n"); So, if this fails, in ->shutdown you will disbalance reference count, or did I miss something? > + } else if (d->clk) { > + p->uartclk = clk_get_rate(d->clk); > + if (!p->uartclk) { > + clk_rate_exclusive_put(d->clk); > + dev_err(p->dev, "Clock rate not defined\n"); > + return -EINVAL; > + } This operations I didn't get. If we have d->clk and suddenly get 0 as a rate (and note, that we still update uartclk member!), we try to put (why?) the exclusiveness of rate. > + } > + > + 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); > + > + clk_rate_exclusive_put(d->clk); > +} -- With Best Regards, Andy Shevchenko _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel