On Wed, Aug 19, 2020 at 03:07:04PM +0100, Robin Murphy wrote:
FWIW, I asked back in time what the plan is for non-coherent allocations and it seemed like DMA_ATTR_NON_CONSISTENT and dma_sync_*() was supposed to be the right thing to go with. [2] The same thread also explains why dma_alloc_pages() isn't suitable for the users of dma_alloc_attrs() and DMA_ATTR_NON_CONSISTENT.
AFAICS even back then Christoph was implying getting rid of NON_CONSISTENT and *replacing* it with something streaming-API-based - i.e. this series - not encouraging mixing the existing APIs. It doesn't seem impossible to implement a remapping version of this new dma_alloc_pages() for IOMMU-backed ops if it's really warranted (although at that point it seems like "non-coherent" vb2-dc starts to have significant conceptual overlap with vb2-sg).
You can alway vmap the returned pages from dma_alloc_pages, but it will make cache invalidation hell - you'll need to use invalidate_kernel_vmap_range and flush_kernel_vmap_range to properly handle virtually indexed caches.
Or with remapping you mean using the iommu do de-scatter/gather?
You can implement that trivially implement it yourself for the iommu case:
{ merge_boundary = dma_get_merge_boundary(dev); if (!merge_boundary || merge_boundary > chunk_size - 1) { /* can't coalesce */ return -EINVAL; }
nents = DIV_ROUND_UP(total_size, chunk_size); sg = sgl_alloc(); for_each_sgl() { sg->page = __alloc_pages(get_order(chunk_size)) sg->len = chunk_size; } dma_map_sg(sg, DMA_ATTR_SKIP_CPU_SYNC); // you are guaranteed to get a single dma_addr out }
Of course this still uses the scatterlist structure with its annoying mix of input and output parametes, so I'd rather not expose it as an official API at the DMA layer.