From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757732Ab2I1MMi (ORCPT ); Fri, 28 Sep 2012 08:12:38 -0400 Received: from us02smtp2.synopsys.com ([198.182.60.77]:43646 "EHLO alvesta.synopsys.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757519Ab2I1MMf (ORCPT ); Fri, 28 Sep 2012 08:12:35 -0400 X-Greylist: delayed 547 seconds by postgrey-1.27 at vger.kernel.org; Fri, 28 Sep 2012 08:12:35 EDT From: Alexey Brodkin To: , , , , , Subject: [PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio Date: Fri, 28 Sep 2012 16:03:04 +0400 Message-ID: <1348833784-31385-1-git-send-email-abrodkin@synopsys.com> X-Mailer: git-send-email 1.7.9.5 MIME-Version: 1.0 Content-Type: text/plain X-Originating-IP: [10.225.15.77] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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