linux-kernel - [PATCH 3.2 60/95] serial: mxs-auart: Fix baudrate calculation

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Message-ID: <lsq.1500213398.197314054@decadent.org.uk>
Date:   Sun, 16 Jul 2017 14:56:38 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Uwe Kleine-König" 
        <u.kleine-koenig@...gutronix.de>
Subject: [PATCH 3.2 60/95] serial: mxs-auart: Fix baudrate calculation

3.2.91-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>

commit a6040bc610554c66088fda3608ae5d6307c548e4 upstream.

The reference manual for the i.MX28 recommends to calculate the divisor
as

	divisor = (UARTCLK * 32) / baud rate, rounded to the nearest integer

, so let's do this. For a typical setup of UARTCLK = 24 MHz and baud
rate = 115200 this changes the divisor from 6666 to 6667 and so the
actual baud rate improves from 115211.521 Bd (error ≅ 0.01 %) to
115194.240 Bd (error ≅ 0.005 %).

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
[bwh: Backported to 3.2: adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/tty/serial/mxs-auart.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/tty/serial/mxs-auart.c
+++ b/drivers/tty/serial/mxs-auart.c
@@ -366,7 +366,7 @@ static void mxs_auart_settermios(struct
 	baud_min = DIV_ROUND_UP(u->uartclk * 32, AUART_LINECTRL_BAUD_DIV_MAX);
 	baud_max = u->uartclk * 32 / AUART_LINECTRL_BAUD_DIV_MIN;
 	baud = uart_get_baud_rate(u, termios, old, baud_min, baud_max);
-	div = u->uartclk * 32 / baud;
+	div = DIV_ROUND_CLOSEST(u->uartclk * 32, baud);
 	ctrl |= AUART_LINECTRL_BAUD_DIVFRAC(div & 0x3F);
 	ctrl |= AUART_LINECTRL_BAUD_DIVINT(div >> 6);