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    SubjectRe: [PATCH 4/9] soc: apple: Add SART driver
    On Mon, Mar 21, 2022, at 18:17, Alyssa Rosenzweig wrote:
    >> +/*
    >> + * Adds the region [paddr, paddr+size] to the DMA allow list.
    >> + *
    >> + * @sart: SART reference
    >> + * @paddr: Start address of the region to be used for DMA
    >> + * @size: Size of the region to be used for DMA.
    >> + */
    >> +int apple_sart_add_allowed_region(struct apple_sart *sart, phys_addr_t paddr,
    >> + size_t size);
    >> +
    >> +/*
    >> + * Removes the region [paddr, paddr+size] from the DMA allow list.
    >> + *
    >> + * Note that exact same paddr and size used for apple_sart_add_allowed_region
    >> + * have to be passed.
    >> + *
    >> + * @sart: SART reference
    >> + * @paddr: Start address of the region no longer used for DMA
    >> + * @size: Size of the region no longer used for DMA.
    >> + */
    >> +int apple_sart_remove_allowed_region(struct apple_sart *sart, phys_addr_t paddr,
    >> + size_t size);
    >
    > Might it be simpler for add_allowed_region to return a handle that
    > remove_allowed_region takes instead of paddr+size? Then
    > remove_allowed_region doesn't have to walk anything, and the requirement
    > to use the same paddr/size is implicit.

    I liked that idea and just prototyped it and them remembered why I have
    the API like it is:

    In a perfect world this would be an IOMMU and just implement the IOMMU device ops.
    In a slightly less perfect world this would implement dma_map_ops and transparently
    allow using the normal DMA API inside the NVMe driver.
    In Apple's world the NVMe driver needs two separate IOMMU-like filters and you can't
    have two separate dma_map_ops without ugly hacks.

    This will usually be used just next to dma_map_coherent and dma_free_coherent and
    for those you need to keep track of phys_addr, dma_addr and size anyway,
    e.g. (with the error handling removed here)

    bfr->buffer = dma_alloc_coherent(anv->dev, size, &iova, GFP_KERNEL);
    ret = apple_sart_add_allowed_region(anv->sart, iova, size);

    and later then

    apple_sart_remove_allowed_region(anv->sart, bfr->iova, bfr->size);
    dma_free_coherent(anv->dev, bfr->size, bfr->buffer, bfr->iova);



    Best,

    Sven

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