From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755967AbXLKBrT (ORCPT ); Mon, 10 Dec 2007 20:47:19 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752461AbXLKBrL (ORCPT ); Mon, 10 Dec 2007 20:47:11 -0500 Received: from terminus.zytor.com ([198.137.202.10]:55346 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752453AbXLKBrK (ORCPT ); Mon, 10 Dec 2007 20:47:10 -0500 Message-ID: <475DEBD8.2080608@zytor.com> Date: Mon, 10 Dec 2007 17:46:00 -0800 From: "H. Peter Anvin" User-Agent: Thunderbird 2.0.0.9 (X11/20071115) MIME-Version: 1.0 To: David Newall CC: Krzysztof Halasa , Rene Herman , Pavel Machek , Andi Kleen , Alan Cox , "David P. Reed" , linux-kernel@vger.kernel.org, Thomas Gleixner , Ingo Molnar Subject: Re: RFC: outb 0x80 in inb_p, outb_p harmful on some modern AMD64 with MCP51 laptops References: <475879CD.9080006@reed.com> <20071207160439.71b7f46a@the-village.bc.nu> <20071209125458.GB4381@ucw.cz> <20071209165908.GA15910@one.firstfloor.org> <20071209212513.GC24284@elf.ucw.cz> <475CBDD7.5050602@keyaccess.nl> <475DE37F.20706@davidnewall.com> <475DE6F4.80702@zytor.com> <475DEB23.1000304@davidnewall.com> In-Reply-To: <475DEB23.1000304@davidnewall.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org David Newall wrote: > H. Peter Anvin wrote: >> David Newall wrote: >>> Where did the 8us delay come from? The documentation and source is >>> careful not to say how long the delay is. Would changing it to, say >>> 1us, be technically wrong? Is code that requires 8us correct? >> >> I think a single ISA bus transaction is 1 µs, so two of them back to >> back should be 2 µs, not 8 µs... > > Exactly. You think it's 2us, but the documentation doesn't say. The _p > functions are generic inasmuch as they provide an unspecified delay. > Drivers which work across platforms, and which use _p, therefore have > different delays on different platforms. Should the length of the delay > be unimportant? I wouldn't have thought so. If it is important, does > that mean that such drivers are buggy on some platforms? > What it specifically does is it generates a delay which is proportional to the ISA/LPC clock. > I really *hate* the idea that access to non-present hardware is used to > generate a delay. That sucks so badly. It's worthy of a school-aged > hacker, not of a world-leading operating system. It's so not > best-practice that it's worst-practice. > Perhaps you do, but it's the de facto standard on the platform. Every BIOS uses the same technique, because it works. *Now*, the real question is how many drivers actually need these delays. My guess is most don't at all. -hpa