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

From: Robin Murphy
Date: Tue Nov 23 2021 - 06:58:58 EST

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

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.


[1] (not in upstream)

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(-)