From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751317AbcGNReA (ORCPT ); Thu, 14 Jul 2016 13:34:00 -0400 Received: from smtp.codeaurora.org ([198.145.29.96]:55290 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751074AbcGNRdw (ORCPT ); Thu, 14 Jul 2016 13:33:52 -0400 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Thu, 14 Jul 2016 13:33:50 -0400 From: nwatters@codeaurora.org To: Will Deacon Cc: Robin Murphy , Joerg Roedel , linux-arm-kernel@lists.infradead.org, iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] iommu/arm-smmu-v3: limit use of 2-level stream tables In-Reply-To: <20160714133107.GK29455@arm.com> References: <1468260059-10759-1-git-send-email-nwatters@codeaurora.org> <1468347560-29924-1-git-send-email-nwatters@codeaurora.org> <20160714133107.GK29455@arm.com> Message-ID: <103772215d30a7c862f66200bbcc16ea@codeaurora.org> User-Agent: Roundcube Webmail/1.1.4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2016-07-14 09:31, Will Deacon wrote: > On Tue, Jul 12, 2016 at 02:19:20PM -0400, Nate Watterson wrote: >> In the current arm-smmu-v3 driver, all smmus that support 2-level >> stream tables are being forced to use them. This is suboptimal for >> smmus that support fewer stream id bits than would fill in a single >> second level table. This patch limits the use of 2-level tables to >> smmus that both support the feature and whose first level table can >> possibly contain more than a single entry. > > Just to be clear, what exactly are you seeing as being suboptimal here? > Is it the memory wastage from overallocating the L2 table, or something > more? Disregarding the config cache, fetching an STE when 2-level tables are being used will require the hw to perform more memory accesses than it would have to with a linear table since the L1 descriptor must also be fetched. Presumably this is why the spec states, "ARM recommends that a more efficient linear table is used instead of programming SPLIT > LOG2SIZE". My understanding is that the only benefit to using 2-level tables is that it can save space when stream ids are sparsely distributed. Are there any other compelling reasons to use them? > > if it's just the memory allocation, I'd sooner restrict the span field > in the L1 desc. Although I am not especially concerned about the memory allocation, even if the span was reduced, we would still be wasting a page for the L1 table unless L1 and L2 tables were allocated in a single dmam_alloc_coherent call. > > Will Nate -- Qualcomm Datacenter Technologies, Inc. on behalf of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.