Re: [PATCH 0/3] Allow restricted-dma-pool to customize IO_TLB_SEGSIZE
From: Hsin-Yi Wang
Date: Tue Nov 23 2021 - 22:55:36 EST
On Tue, Nov 23, 2021 at 7:58 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 are controlled by mtk-scp and allocate memory through
> > mtk-scp. In order to use the noncontiguous DMA API, 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().
> >  (not in upstream) https://patchwork.kernel.org/project/linux-media/cover/20190611035344.29814-1-jungo.lin@xxxxxxxxxxxx/
> >  https://elixir.bootlin.com/linux/latest/source/drivers/remoteproc/mtk_scp.c
> >  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(-)