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From: Serge Semin <Sergey.Semin@baikalelectronics.ru>
To: Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jslaby@suse.com>,
	Long Cheng <long.cheng@mediatek.com>
Cc: Maxime Ripard <mripard@kernel.org>, Stefan Roese <sr@denx.de>,
	linux-mips@vger.kernel.org, Paul Burton <paulburton@kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-kernel@vger.kernel.org,
	Russell King <linux@armlinux.org.uk>,
	Ralf Baechle <ralf@linux-mips.org>,
	Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>,
	Serge Semin <Sergey.Semin@baikalelectronics.ru>,
	Mika Westerberg <mika.westerberg@linux.intel.com>,
	Lukas Wunner <lukas@wunner.de>,
	linux-mediatek@lists.infradead.org,
	Serge Semin <fancer.lancer@gmail.com>,
	linux-serial@vger.kernel.org,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	Vignesh Raghavendra <vigneshr@ti.com>
Subject: [PATCH v3 1/4] serial: 8250: Fix max baud limit in generic 8250 port
Date: Thu, 7 May 2020 02:31:32 +0300
Message-ID: <20200506233136.11842-2-Sergey.Semin@baikalelectronics.ru> (raw)
In-Reply-To: <20200506233136.11842-1-Sergey.Semin@baikalelectronics.ru>

Standard 8250 UART ports are designed in a way so they can communicate
with baud rates up to 1/16 of a reference frequency. It's expected from
most of the currently supported UART controllers. That's why the former
version of serial8250_get_baud_rate() method called uart_get_baud_rate()
with min and max baud rates passed as (port->uartclk / 16 / UART_DIV_MAX)
and ((port->uartclk + tolerance) / 16) respectively. Doing otherwise, like
it was suggested in commit ("serial: 8250_mtk: support big baud rate."),
caused acceptance of bauds, which was higher than the normal UART
controllers actually supported. As a result if some user-space program
requested to set a baud greater than (uartclk / 16) it would have been
permitted without truncation, but then serial8250_get_divisor(baud)
(which calls uart_get_divisor() to get the reference clock divisor) would
have returned a zero divisor. Setting zero divisor will cause an
unpredictable effect varying from chip to chip. In case of DW APB UART the
communications just stop.

Lets fix this problem by getting back the limitation of (uartclk +
tolerance) / 16 maximum baud supported by the generic 8250 port. Mediatek
8250 UART ports driver developer shouldn't have touched it in the first
place  notably seeing he already provided a custom version of set_termios()
callback in that glue-driver which took into account the extended baud
rate values and accordingly updated the standard and vendor-specific
divisor latch registers anyway.

Fixes: 81bb549fdf14 ("serial: 8250_mtk: support big baud rate.")
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Long Cheng <long.cheng@mediatek.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-mips@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
---
 drivers/tty/serial/8250/8250_port.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index f77bf820b7a3..4d83c85a7389 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2615,6 +2615,8 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
 					     struct ktermios *termios,
 					     struct ktermios *old)
 {
+	unsigned int tolerance = port->uartclk / 100;
+
 	/*
 	 * Ask the core to calculate the divisor for us.
 	 * Allow 1% tolerance at the upper limit so uart clks marginally
@@ -2623,7 +2625,7 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
 	 */
 	return uart_get_baud_rate(port, termios, old,
 				  port->uartclk / 16 / UART_DIV_MAX,
-				  port->uartclk);
+				  (port->uartclk + tolerance) / 16);
 }
 
 void
-- 
2.25.1


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Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20200306130231.05BBC8030795@mail.baikalelectronics.ru>
2020-03-23  2:46 ` [PATCH v2] serial: 8250_dw: Fix common clocks usage race condition Sergey.Semin
2020-03-23  9:20   ` Andy Shevchenko
2020-03-23 11:11     ` Sergey Semin
2020-03-23 11:52       ` Andy Shevchenko
2020-03-23 17:07         ` Sergey Semin
2020-03-23 10:01   ` Maxime Ripard
2020-03-23 13:50     ` Sergey Semin
2020-03-24  9:41       ` Maxime Ripard
2020-03-24 10:12       ` Andy Shevchenko
2020-03-25 17:11         ` Sergey Semin
2020-03-26  1:44           ` Stephen Boyd
2020-03-27  9:12             ` Sergey Semin
2020-05-06 23:31   ` [PATCH v3 0/4] serial: 8250_dw: Fix ref clock usage Serge Semin
2020-05-06 23:31     ` Serge Semin [this message]
2020-05-15 13:48       ` [PATCH v3 1/4] serial: 8250: Fix max baud limit in generic 8250 port Andy Shevchenko
2020-05-06 23:31     ` [PATCH v3 2/4] serial: 8250: Add 8250 port clock update method Serge Semin
2020-05-15 12:45       ` Greg Kroah-Hartman
2020-05-15 14:32         ` Serge Semin
2020-05-06 23:31     ` [PATCH v3 3/4] serial: 8250_dw: Simplify the ref clock rate setting procedure Serge Semin
2020-05-15 14:05       ` Andy Shevchenko
2020-05-15 14:50         ` Serge Semin
2020-05-15 15:05           ` Andy Shevchenko
2020-05-06 23:31     ` [PATCH v3 4/4] serial: 8250_dw: Fix common clocks usage race condition Serge Semin
2020-05-15 14:10       ` Andy Shevchenko
2020-05-15 15:19         ` Serge Semin
2020-05-07 18:29     ` [PATCH v3 0/4] serial: 8250_dw: Fix ref clock usage Andy Shevchenko

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