Re: [PATCH] tty/serial: atmel: fix fractional baud rate computation
From: Uwe Kleine-König <hidden>
Date: 2016-09-22 07:08:11
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linux-arm-kernel, lkml
On Wed, Sep 21, 2016 at 12:44:14PM +0200, Nicolas Ferre wrote:
quoted hunk ↗ jump to hunk
From: Alexey Starikovskiy <redacted> 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 <redacted> [nicolas.ferre@atmel.com: port to newer kernel and add commit log] Signed-off-by: Nicolas Ferre <redacted> --- 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;
given baud = 115200 and uartclk = 5414300 this results in: div = DIV_ROUND_CLOSEST(5414300, 115200 * 2) = 23 cd = 2 fp = 7 which yields a rate of 5414300 / 46 = 117702.17. With cd = 3 and fp = 0 however the resulting rate is 5414300 / 48 = 112797.92. Which one is better?
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} 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
Is there another user of this header? If not, this can be folded into the driver. Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ |