Hi, On the Apple M1 there's this slightly annoying situation where the DART IOMMU has a hard-wired page size of 16KB. Additionally, the DARTs for some hardware (USB A ports, WiFi, Ethernet, Thunderbolt PCIe) cannot be switched to bypass mode and it's also not easily possible to program a software bypass mode.
This is a problem for kernels configured with 4K pages. Unfortunately, most distributions ship with those by default. There's not much that can be done for IOMMU_DOMAIN_UNMANAGED domains since most API clients likely expect to be able to map single CPU pages. For IOMMU_DOMAIN_DMA domains however, dma-iommu.c is the only code that uses the raw IOMMU API to manage these domains and can possibly be adapted to still work correctly. Essentially, I changed some relevant alignments to happen with respect to both PAGE_SIZE and iovad->granule. The sglist code also can't use the optimization for a single IOVA allocation anymore since most phys_addrs will not be aligned to the IOMMU page size. I'd like to get some early feedback on this approach to see if it's feasible to continue working on this or if a different approach will work better or if this setup just won't be supported. I'm not very confident I've covered all necessary cases but I'll take a closer look at every function in dma-iommu.c if there's a chance that this will be accepted eventually. The current changes are enough to boot from a USB device and use the Ethernet adapter on my M1 Mini with 4kb pages though. One issue I see is that this will end up wasting memory. There's e.g. dma_pool_*, which will dma_alloc_coherent PAGE_SIZE bytes and stuff the individual allocations into those buffers. These will get padded to SZ_16K but dma_pool will be completely unaware that it got 4x as much memory as requested and will leave it unused :-( The other issue I'm aware of is v4l2 which expects that a page-aligned sglist can be represented contiguously in IOVA space [1]. Best, Sven [1] https://lore.kernel.org/linux-iommu/0d20bd6b-d0a1-019c-6398-b12f83f4f...@arm.com/ Sven Peter (3): iommu: Move IOMMU pagesize check to attach_device iommu/dma-iommu: Support iovad->granule > PAGE_SIZE iommu: Introduce __IOMMU_DOMAIN_LARGE_PAGES drivers/iommu/dma-iommu.c | 87 ++++++++++++++++++++++++++++++++++----- drivers/iommu/iommu.c | 36 ++++++++++++++-- drivers/iommu/iova.c | 7 ++-- include/linux/iommu.h | 14 ++++--- 4 files changed, 123 insertions(+), 21 deletions(-) -- 2.25.1 _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu