Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933433Ab0KOSUx (ORCPT ); Mon, 15 Nov 2010 13:20:53 -0500 Received: from wolverine01.qualcomm.com ([199.106.114.254]:39503 "EHLO wolverine01.qualcomm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933033Ab0KOSUw (ORCPT ); Mon, 15 Nov 2010 13:20:52 -0500 X-IronPort-AV: E=McAfee;i="5400,1158,6168"; a="62930436" Subject: Re: [PATCH 09/14] msm: iommu: Kconfig option for cacheable page tables From: Daniel Walker To: Stepan Moskovchenko Cc: davidb@codeaurora.org, bryanh@codeaurora.org, linux-arm-msm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org In-Reply-To: <8bd62d321211c93af883cd2ad404e802.squirrel@www.codeaurora.org> References: <1289619000-13167-1-git-send-email-stepanm@codeaurora.org> <1289619000-13167-10-git-send-email-stepanm@codeaurora.org> <1289765858.24270.240.camel@m0nster> <8bd62d321211c93af883cd2ad404e802.squirrel@www.codeaurora.org> Content-Type: text/plain; charset="UTF-8" Date: Mon, 15 Nov 2010 10:20:36 -0800 Message-ID: <1289845236.4050.25.camel@c-dwalke-linux.qualcomm.com> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2541 Lines: 58 On Sun, 2010-11-14 at 18:56 -0800, Stepan Moskovchenko wrote: > > On Fri, 2010-11-12 at 19:29 -0800, Stepan Moskovchenko wrote: > >> > >> +config IOMMU_PGTABLES_L2 > >> + depends on ARCH_MSM8X60 > >> + depends on MMU > >> + depends on CPU_DCACHE_DISABLE=n > >> + depends on SMP > >> + bool "Cacheable IOMMU page tables" > >> + default y > >> + help > >> + Allows the IOMMU page tables to be brought into the L2 cache. This > >> + improves the TLB miss latency at the expense of potential pollution > >> + of the L2 cache. This option has been shown to improve multimedia > >> + performance in some cases. > >> + > >> + If unsure, say Y here. > > > > Why would someone want this off? > > > > The other thing is that you usually want this included with the code > > that uses the option. > > The code that uses it had gone in during a previous patch series, but I > didn't want to meddle in the Kconfig just yet, especially since the option > only improves performance and does not add new functionality at a high > level. This patch should be the last of what is needed for this feature. > > You would want to turn this off if you wanted more deterministic behavior > from the multimedia subsystem, such as when trying to run benchmarks for > the worst-case behavior in terms of memory latency and TLB misses. You > might also want to turn it off if you are debugging memory problems that > you suspect might be related to the cache maintenance code, in which case > turning this off would give an idea as to whether that is the problem. > Similarly, this needs to be off (and will be, due to the dependencies) if > certain required things (like the Dcache, MMU, etc) have been disabled. > Finally, if you are doing things with the MMSS that you know will not > result in many TLB misses, (or if you know that you can tolerate high miss > latency) you may as well turn this off to avoid the (small) bit of cache > pollution. It sounds like you don't really want it off unless your a developer (or you turn one of the dependencies) .. I think this might be better as a hidden option just cause of the developer centric nature of it. Daniel -- Sent by a consultant of the Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum. -- 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/