From mboxrd@z Thu Jan 1 00:00:00 1970 From: Robin Murphy Subject: Re: [PATCH 0/3] Allow restricted-dma-pool to customize IO_TLB_SEGSIZE Date: Fri, 3 Dec 2021 13:07:21 +0000 Message-ID: <46d9c8dd-d2dc-0d2a-aeb4-08dff9dcb733@arm.com> References: <20211123112104.3530135-1-hsinyi@chromium.org> <75ea1026-63e5-165a-9e07-27b5ac4c7579@arm.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: Content-Language: en-GB In-Reply-To: List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Tomasz Figa Cc: Hsin-Yi Wang , Christoph Hellwig , Marek Szyprowski , iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Rob Herring , Maxime Ripard , - , devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Matthias Brugger , linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, linux-mediatek-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, senozhatsky-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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-NuS5LvNUpcJWk0Htik3J/w@public.gmane.org/ >>> [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-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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(-) >>>