Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751313AbWILF51 (ORCPT ); Tue, 12 Sep 2006 01:57:27 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751314AbWILF51 (ORCPT ); Tue, 12 Sep 2006 01:57:27 -0400 Received: from gate.crashing.org ([63.228.1.57]:12473 "EHLO gate.crashing.org") by vger.kernel.org with ESMTP id S1751313AbWILF50 (ORCPT ); Tue, 12 Sep 2006 01:57:26 -0400 Subject: Re: [RFC] MMIO accessors & barriers documentation From: Benjamin Herrenschmidt To: "Eric W. Biederman" Cc: Alan Cox , Linux Kernel list , Jesse Barnes , "David S. Miller" , Jeff Garzik , Paul Mackerras , Linus Torvalds , Andrew Morton , Segher Boessenkool In-Reply-To: References: <1157947414.31071.386.camel@localhost.localdomain> <1157965071.23085.84.camel@localhost.localdomain> <1157966269.3879.23.camel@localhost.localdomain> <1157969261.23085.108.camel@localhost.localdomain> Content-Type: text/plain Date: Tue, 12 Sep 2006 15:56:45 +1000 Message-Id: <1158040605.15465.70.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Evolution 2.6.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1185 Lines: 28 > Frame buffers are rarely cachable as such, on x86 they are usually > write-combining. Which means that the writes can be merged and > possibly reordered while they are being written but they can't be > cached. Most arches I believe have something that roughly corresponds > to write combining. > > Ensuring we can still use this optimization to mmio space is > moderately important. I've not gone too much in details about write combining (we need to do something about it but I don't want to mix problems) but I did define that the ordered accessors aren't guaranteed to provide write combining on storage mapped with WC enabled while the relaxed or non ordered ones are. That should be enough at this point. Later, we should look into providing an ioremap_wc() and possibly page table flags for write combining userland mappings. Time to get rid of MTRRs for graphics :) And infiniband-style stuff seems to want that too. Ben. - 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/