linux-kernel - [PATCH 3/3] at91_serial: Fix roundoff error in at91_console_get

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Message-Id: <11545303082800-git-send-email-hskinnemoen@atmel.com>
Date:	Wed, 02 Aug 2006 16:51:48 +0200
From:	Haavard Skinnemoen <hskinnemoen@...el.com>
To:	Andrew Victor <andrew@...people.com>
Cc:	Russell King <rmk+serial@....linux.org.uk>,
	linux-kernel@...r.kernel.org,
	Haavard Skinnemoen <hskinnemoen@...el.com>
Subject: [PATCH 3/3] at91_serial: Fix roundoff error in at91_console_get_options

The at91_console_get_options() function initializes the baud,
parity and bits settings from the actual hardware setup, in
case it has been initialized by a e.g. boot loader.

The baud rate, however, is not necessarily exactly equal to one of
the standard baud rates (115200, etc.) This means that the baud rate
calculated by this function may be slightly higher or slightly lower
than one of the standard baud rates.

If the baud rate is slightly lower than the target, this causes
problems when uart_set_option() tries to match the detected baud rate
against the standard baud rate, as it will always select a baud rate
that is lower or equal to the target rate. For example if the
detected baud rate is slightly lower than 115200, usart_set_options()
will select 57600.

This patch fixes the problem by subtracting 1 from the value in BRGR
when calculating the baud rate. The detected baud rate will thus
always be higher than the nearest standard baud rate, and
uart_set_options() will end up doing the right thing.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@...el.com>
---
 drivers/serial/at91_serial.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/drivers/serial/at91_serial.c b/drivers/serial/at91_serial.c
index c759e7c..528d717 100644
--- a/drivers/serial/at91_serial.c
+++ b/drivers/serial/at91_serial.c
@@ -806,8 +806,14 @@ static void __init at91_console_get_opti
 	else if (mr == AT91_US_PAR_ODD)
 		*parity = 'o';

+	/*
+	 * The serial core only rounds down when matching this to a
+	 * supported baud rate. Make sure we don't end up slightly
+	 * lower than one of those, as it would make us fall through
+	 * to a much lower baud rate than we really want.
+	 */
 	quot = UART_GET_BRGR(port);
-	*baud = port->uartclk / (16 * (quot));
+	*baud = port->uartclk / (16 * (quot - 1));
 }

 static int __init at91_console_setup(struct console *co, char *options)
-- 
1.4.0

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