From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758016Ab2I1NDG (ORCPT ); Fri, 28 Sep 2012 09:03:06 -0400 Received: from mail-we0-f174.google.com ([74.125.82.174]:49429 "EHLO mail-we0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757947Ab2I1NDD (ORCPT ); Fri, 28 Sep 2012 09:03:03 -0400 Message-ID: <5065A003.4090604@suse.cz> Date: Fri, 28 Sep 2012 15:02:59 +0200 From: Jiri Slaby User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/17.0 Thunderbird/17.0a2 MIME-Version: 1.0 To: Alexey Brodkin CC: gregkh@linuxfoundation.org, alan@linux.intel.com, linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org, Vineet.Gupta1@synopsys.com Subject: Re: [PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio References: <1348834748-31565-1-git-send-email-abrodkin@synopsys.com> In-Reply-To: <1348834748-31565-1-git-send-email-abrodkin@synopsys.com> X-Enigmail-Version: 1.5a1pre Content-Type: text/plain; charset=ISO-8859-2 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/28/2012 02:19 PM, Alexey Brodkin wrote: > 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); So this should be in fact DIV_ROUND_CLOSEST(), right? But anyway I'm missing any explanation of *why* this is needed? This should be part of the commit log. Is there any bug you are fixing here? thanks, -- js suse labs