Messages in this thread | | | Date | Fri, 3 Dec 2021 13:07:21 +0000 | From | Robin Murphy <> | Subject | Re: [PATCH 0/3] Allow restricted-dma-pool to customize IO_TLB_SEGSIZE |
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On 2021-11-25 07:35, Tomasz Figa wrote: > Hi Robin, > > On Tue, Nov 23, 2021 at 8:59 PM Robin Murphy <robin.murphy@arm.com> 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(-) >>>
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