Re: [PATCH 0/3] Allow restricted-dma-pool to customize IO_TLB_SEGSIZE

[Hsin-Yi Wang <hsinyi@chromium.org>]

On Tue, Nov 23, 2021 at 7:58 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.
>
mtk_scp on its own can use the shared-dma-pool, which it currently uses.
The reason we switched to restricted-dma-pool is that we want to use
the noncontiguous DMA API for mtk-isp. The noncontiguous DMA API is
designed for devices with iommu, and if a device doesn't have an
iommu, it will fallback using swiotlb. But currently noncontiguous DMA
API doesn't work with the shared-dma-pool.

vb2_dc_alloc() -> dma_alloc_noncontiguous() -> alloc_single_sgt() ->
__dma_alloc_pages() -> dma_direct_alloc_pages() ->
__dma_direct_alloc_pages() -> swiotlb_alloc().


> 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(-)
> >