linux-kernel - [PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio

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Date:	Fri, 28 Sep 2012 16:19:08 +0400
From:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:	<gregkh@...uxfoundation.org>, <alan@...ux.intel.com>
CC:	<linux-serial@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<Vineet.Gupta1@...opsys.com>, <Alexey.Brodkin@...opsys.com>
Subject: [PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio

Modify divisor to select the nearest baud rate divider rather than the
lowest. It minimizes baud rate errors especially on low UART clock
frequencies.

For example, if uartclk is 33000000 and baud is 115200 the ratio is
about 17.9 The current code selects 17 (5% error) but should select 18
(0.5% error).

On the same lines as following:
http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.9-rc3/2.6.9-rc3-mm2/broken-out/serial-pick-nearest-baud-rate-divider.patch

Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
---
 drivers/tty/serial/8250_early.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250_early.c b/drivers/tty/serial/8250_early.c
index eaafb98..cfc46b7 100644
--- a/drivers/tty/serial/8250_early.c
+++ b/drivers/tty/serial/8250_early.c
@@ -140,7 +140,7 @@ static void __init init_port(struct early_serial8250_device *device)
 	serial_out(port, UART_FCR, 0);		/* no fifo */
 	serial_out(port, UART_MCR, 0x3);	/* DTR + RTS */
 
-	divisor = port->uartclk / (16 * device->baud);
+	divisor = (port->uartclk + (8 * device->baud)) / (16 *	device->baud);
 	c = serial_in(port, UART_LCR);
 	serial_out(port, UART_LCR, c | UART_LCR_DLAB);
 	serial_out(port, UART_DLL, divisor & 0xff);
-- 
1.7.9.5

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