Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755038Ab1FSXtk (ORCPT ); Sun, 19 Jun 2011 19:49:40 -0400 Received: from terminus.zytor.com ([198.137.202.10]:42271 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754883Ab1FSXti (ORCPT ); Sun, 19 Jun 2011 19:49:38 -0400 Message-ID: <4DFE89E0.5020509@zytor.com> Date: Sun, 19 Jun 2011 16:44:32 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110428 Fedora/3.1.10-1.fc15 Thunderbird/3.1.10 MIME-Version: 1.0 To: Ryan Mallon CC: Ingo Molnar , Petr Tesarik , Andrew Morton , Fenghua Yu , Ingo Molnar , Paul Mundt , Russell King , Thomas Gleixner , Tony Luck , x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org, linux-kernel@vger.kernel.org, Arjan van de Ven , Dave Jones , Linus Torvalds Subject: Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary physical addresses References: <201106171038.25988.ptesarik@suse.cz> <20110617093032.GA19235@elte.hu> <4DFE7FF9.9070406@gmail.com> In-Reply-To: <4DFE7FF9.9070406@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2034 Lines: 50 On 06/19/2011 04:02 PM, Ryan Mallon wrote: > On 17/06/11 19:30, Ingo Molnar wrote: >> * Petr Tesarik wrote: >> >>> This patch series enhances /dev/mem, so that read and write is >>> possible at any address. The patchset includes actual >>> implementation for x86. >> This series lacks a description of why this is desired. >> >> My strong opinion is that it's not desired at all: /dev/mem never >> worked beyond 4G addresses so by today it has become largely obsolete >> and is on the way out really. >> >> I'm aware of these current /dev/mem uses: >> >> - Xorg maps below 4G non-RAM addresses and the video BIOS >> >> - It used to have some debugging role but these days kexec and kgdb >> has largely taken over that role - partly due to the 4G limit. >> >> - there's some really horrible out-of-tree drivers that do mmap()s >> via /dev/mem, those should be fixed if they want to move beyond >> 4G: their char device should be mmap()able. > > There are drivers where this makes sense. For example an FPGA device > with a proprietary register layout on the memory bus can be done this > way. The FPGA can simply be mapped in user-space via /dev/mem and > handled there. If the device requires no access other than memory bus > reads and writes then writing a custom char device driver just to get an > mmap function seems a bit overkill. > There are some test drivers which really want /dev/mem to work. FPGA devices like that really should be exported as resources from a platform driver or device tree driver, at which point those resources can be memory-mapped. That being said, using /dev/mem for fixed resources is semicommon. -hpa -- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/