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Add CoherentHandle, an opaque DMA allocation type for buffers that are only ever accessed by hardware. Unlike Coherent<T>, it does not provide CPU access to the allocated memory. CoherentHandle implicitly sets DMA_ATTR_NO_KERNEL_MAPPING and stores the value returned by dma_alloc_attrs() as an opaque handle (NonNull<c_void>) rather than a typed pointer, since with this flag the C API returns an opaque cookie (e.g. struct page *), not a CPU pointer to the allocated memory. Only the DMA bus address is exposed to drivers; the opaque handle is used solely to free the allocation on drop. This commit is for reference only; there is currently no in-tree user. Signed-off-by: Danilo Krummrich <dakr@kernel.org> Reviewed-by: Alexandre Courbot <acourbot@nvidia.com> Link: https://patch.msgid.link/20260321172749.592387-2-dakr@kernel.org [acourbot: fix conflict in dma.rs.] Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>