From mboxrd@z Thu Jan 1 00:00:00 1970 From: nwatters-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org Subject: Re: [PATCH v2] iommu/arm-smmu-v3: limit use of 2-level stream tables Date: Thu, 14 Jul 2016 13:33:50 -0400 Message-ID: <103772215d30a7c862f66200bbcc16ea@codeaurora.org> References: <1468260059-10759-1-git-send-email-nwatters@codeaurora.org> <1468347560-29924-1-git-send-email-nwatters@codeaurora.org> <20160714133107.GK29455@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20160714133107.GK29455-5wv7dgnIgG8@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Will Deacon Cc: iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: iommu@lists.linux-foundation.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.