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

[Tomasz Figa]

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(-)
> >