Hi Robin, On Tue, Nov 23, 2021 at 8:59 PM Robin Murphy <robin.murphy@xxxxxxx> 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. The new dma_alloc_noncontiguous() API would allow allocating suitable memory directly from the pool, which would eliminate the need to copy. 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. Best regards, Tomasz > > Robin. > > > [1] (not in upstream) https://patchwork.kernel.org/project/linux-media/cover/20190611035344.29814-1-jungo.lin@xxxxxxxxxxxx/ > > [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@xxxxxxxxxxxx/ > > > > 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(-) > >
