From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 42152C433EF for ; Fri, 3 Dec 2021 13:07:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244571AbhLCNKz (ORCPT ); Fri, 3 Dec 2021 08:10:55 -0500 Received: from foss.arm.com ([217.140.110.172]:49102 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235590AbhLCNKw (ORCPT ); Fri, 3 Dec 2021 08:10:52 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id F307B1396; Fri, 3 Dec 2021 05:07:27 -0800 (PST) Received: from [10.57.34.58] (unknown [10.57.34.58]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D8E7A3F5A1; Fri, 3 Dec 2021 05:07:25 -0800 (PST) Message-ID: <46d9c8dd-d2dc-0d2a-aeb4-08dff9dcb733@arm.com> Date: Fri, 3 Dec 2021 13:07:21 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:91.0) Gecko/20100101 Thunderbird/91.3.2 From: Robin Murphy Subject: Re: [PATCH 0/3] Allow restricted-dma-pool to customize IO_TLB_SEGSIZE To: Tomasz Figa Cc: Hsin-Yi Wang , Christoph Hellwig , Marek Szyprowski , iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, Rob Herring , Maxime Ripard , - , devicetree@vger.kernel.org, Matthias Brugger , linux-arm-kernel@lists.infradead.org, linux-mediatek@lists.infradead.org, senozhatsky@chromium.org References: <20211123112104.3530135-1-hsinyi@chromium.org> <75ea1026-63e5-165a-9e07-27b5ac4c7579@arm.com> Content-Language: en-GB In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021-11-25 07:35, Tomasz Figa wrote: > Hi Robin, > > On Tue, Nov 23, 2021 at 8:59 PM Robin Murphy wrote: >> >> On 2021-11-23 11:21, Hsin-Yi Wang wrote: >>> Default IO_TLB_SEGSIZE (128) slabs may be not enough for some use cases. >>> This series adds support to customize io_tlb_segsize for each >>> restricted-dma-pool. >>> >>> Example use case: >>> >>> mtk-isp drivers[1] are controlled by mtk-scp[2] and allocate memory through >>> mtk-scp. In order to use the noncontiguous DMA API[3], we need to use >>> the swiotlb pool. mtk-scp needs to allocate memory with 2560 slabs. >>> mtk-isp drivers also needs to allocate memory with 200+ slabs. Both are >>> larger than the default IO_TLB_SEGSIZE (128) slabs. >> >> Are drivers really doing streaming DMA mappings that large? If so, that >> seems like it might be worth trying to address in its own right for the >> sake of efficiency - allocating ~5MB of memory twice and copying it back >> and forth doesn't sound like the ideal thing to do. >> >> If it's really about coherent DMA buffer allocation, I thought the plan >> was that devices which expect to use a significant amount and/or size of >> coherent buffers would continue to use a shared-dma-pool for that? It's >> still what the binding implies. My understanding was that >> swiotlb_alloc() is mostly just a fallback for the sake of drivers which >> mostly do streaming DMA but may allocate a handful of pages worth of >> coherent buffers here and there. Certainly looking at the mtk_scp >> driver, that seems like it shouldn't be going anywhere near SWIOTLB at all. > > First, thanks a lot for taking a look at this patch series. > > The drivers would do streaming DMA within a reserved region that is > the only memory accessible to them for security reasons. This seems to > exactly match the definition of the restricted pool as merged > recently. Huh? Of the drivers indicated, the SCP driver is doing nothing but coherent allocations, and I'm not entirely sure what those ISP driver patches are supposed to be doing but I suspect it's probably just buffer allocation too. I don't see any actual streaming DMA anywhere :/ > The new dma_alloc_noncontiguous() API would allow allocating suitable > memory directly from the pool, which would eliminate the need to copy. Can you clarify what's being copied, and where? I'm not all that familiar with the media APIs, but I thought it was all based around preallocated DMA buffers (the whole dedicated "videobuf" thing)? The few instances of actual streaming DMA I can see in drivers/media/ look to be mostly PCI drivers mapping private descriptors, whereas the MTK ISP appears to be entirely register-based. > However, for a restricted pool, this would exercise the SWIOTLB > allocator, which currently suffers from the limitation as described by > Hsin-Yi. Since the allocator in general is quite general purpose and > already used for coherent allocations as per the current restricted > pool implementation, I think it indeed makes sense to lift the > limitation, rather than trying to come up with yet another thing. No, just fix the dma_alloc_noncontiguous() fallback case to split the allocation into dma_max_mapping_size() chunks. *That* makes sense. Thanks, Robin. > > Best regards, > Tomasz > >> >> Robin. >> >>> [1] (not in upstream) https://patchwork.kernel.org/project/linux-media/cover/20190611035344.29814-1-jungo.lin@mediatek.com/ >>> [2] https://elixir.bootlin.com/linux/latest/source/drivers/remoteproc/mtk_scp.c >>> [3] https://patchwork.kernel.org/project/linux-media/cover/20210909112430.61243-1-senozhatsky@chromium.org/ >>> >>> Hsin-Yi Wang (3): >>> dma: swiotlb: Allow restricted-dma-pool to customize IO_TLB_SEGSIZE >>> dt-bindings: Add io-tlb-segsize property for restricted-dma-pool >>> arm64: dts: mt8183: use restricted swiotlb for scp mem >>> >>> .../reserved-memory/shared-dma-pool.yaml | 8 +++++ >>> .../arm64/boot/dts/mediatek/mt8183-kukui.dtsi | 4 +-- >>> include/linux/swiotlb.h | 1 + >>> kernel/dma/swiotlb.c | 34 ++++++++++++++----- >>> 4 files changed, 37 insertions(+), 10 deletions(-) >>>