linux-kernel - [PATCH] tty/serial: atmel: fix fractional baud rate computation

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Message-ID: <20160921104414.16241-1-nicolas.ferre@atmel.com>
Date:   Wed, 21 Sep 2016 12:44:14 +0200
From:   Nicolas Ferre <nicolas.ferre@...el.com>
To:     <aystarik@...il.com>,
        Ludovic Desroches <ludovic.desroches@...el.com>,
        Boris BREZILLON <boris.brezillon@...e-electrons.com>,
        <richard.genoud@...il.com>, <linux-serial@...r.kernel.org>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        Cyrille Pitchen <cyrille.pitchen@...el.com>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nicolas Ferre <nicolas.ferre@...el.com>
Subject: [PATCH] tty/serial: atmel: fix fractional baud rate computation

From: Alexey Starikovskiy <aystarik@...il.com>

The problem with previous code was it rounded values in wrong
place and produced wrong baud rate in some cases.

Signed-off-by: Alexey Starikovskiy <aystarik@...il.com>
[nicolas.ferre@...el.com: port to newer kernel and add commit log]
Signed-off-by: Nicolas Ferre <nicolas.ferre@...el.com>
---
 drivers/tty/serial/atmel_serial.c | 10 ++++++----
 include/linux/atmel_serial.h      |  1 +
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index 5f550d9feed9..fd8aa1f4ba78 100644
--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c
@@ -2170,13 +2170,15 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
 	 * accurately. This feature is enabled only when using normal mode.
 	 * baudrate = selected clock / (8 * (2 - OVER) * (CD + FP / 8))
 	 * Currently, OVER is always set to 0 so we get
-	 * baudrate = selected clock (16 * (CD + FP / 8))
+	 * baudrate = selected clock / (16 * (CD + FP / 8))
+	 * then
+	 * 8 CD + FP = selected clock / (2 * baudrate)
 	 */
 	if (atmel_port->has_frac_baudrate &&
 	    (mode & ATMEL_US_USMODE) == ATMEL_US_USMODE_NORMAL) {
-		div = DIV_ROUND_CLOSEST(port->uartclk, baud);
-		cd = div / 16;
-		fp = DIV_ROUND_CLOSEST(div % 16, 2);
+		div = DIV_ROUND_CLOSEST(port->uartclk, baud * 2);
+		cd = div >> 3;
+		fp = div & ATMEL_US_FP_MASK;
 	} else {
 		cd = uart_get_divisor(port, baud);
 	}
diff --git a/include/linux/atmel_serial.h b/include/linux/atmel_serial.h
index f8e452aa48d7..bd2560502f3c 100644
--- a/include/linux/atmel_serial.h
+++ b/include/linux/atmel_serial.h
@@ -119,6 +119,7 @@
 #define ATMEL_US_BRGR		0x20	/* Baud Rate Generator Register */
 #define	ATMEL_US_CD		GENMASK(15, 0)	/* Clock Divider */
 #define ATMEL_US_FP_OFFSET	16	/* Fractional Part */
+#define ATMEL_US_FP_MASK	0x7
 
 #define ATMEL_US_RTOR		0x24	/* Receiver Time-out Register for USART */
 #define ATMEL_UA_RTOR		0x28	/* Receiver Time-out Register for UART */
-- 
2.9.0