From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Chris Friesen" Subject: Re: [BUG] serial base baud misdetected in 2.6.27 Date: Tue, 04 Nov 2008 14:29:33 -0600 Message-ID: <4910B0AD.3020802@nortel.com> References: <4910936D.4030605@nortel.com> <20081104192253.151d6ef3@lxorguk.ukuu.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from zcars04e.nortel.com ([47.129.242.56]:37691 "EHLO zcars04e.nortel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754402AbYKDU3j (ORCPT ); Tue, 4 Nov 2008 15:29:39 -0500 In-Reply-To: <20081104192253.151d6ef3@lxorguk.ukuu.org.uk> Sender: linux-serial-owner@vger.kernel.org List-Id: linux-serial@vger.kernel.org To: Alan Cox Cc: linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org Alan Cox wrote: > On Tue, 04 Nov 2008 12:24:45 -0600 > "Chris Friesen" wrote: > >> I'm running an ATCA7101 board. This board has two 16550 ports that get >> detected as ttyS4 and ttyS5. >> >> With a much earlier kernel (2.6.14) the serial port base speed is >> correctly detected as 230400, which gives the desired 115200 when I >> specify that on the kernel commandline. >> >> With 2.6.16 and 2.6.27 (not sure about the intervening kernels) the >> serial port base speed is detected as 921600, which means that I need to >> specify 460800 on the commandline to get a real speed of 115200 (as >> verified by an external terminal server). >> >> Any ideas what could be causing this misdetection? I'm having lab >> issues right now so I can't track it down, but I'll have a go at it >> eventually. > > If this is a PCI card it may be matching against a PCI entry for a > similar card with different crystals and need the right svid/sdid adding. This is an ATCA board, so it's not a separate PCI card, but it does look like it's hanging off the PCI bus. The vendor/device matches to an Exar Corp. XR17C152, which is given a base_baud of 921600 in drivers/serial/8250_pci.c. According to the chip specs, 921600 is correct with the default crystal and control register values. However, other crystals can be used and there's also a control register bit to enable a prescalar which cuts the baud rate by a factor of four. I suspect that one of these has been done on our boards. The confusing thing is that the 2.6.14 code seems to also use a value of 921600, but somehow it works. I wonder if our kernel vendor has tweaked something in the code to adjust this value later on. Chris