Re: [PATCH v1 09/16] dma-mapping: handle MMIO flow in dma_map|unmap_page
From: Jason Gunthorpe <jgg@nvidia.com>
Date: 2025-08-07 13:08:19
Also in:
linux-block, linux-doc, linux-iommu, linux-mm, linux-nvme, linux-trace-kernel, lkml, rust-for-linux, virtualization, xen-devel
On Mon, Aug 04, 2025 at 03:42:43PM +0300, Leon Romanovsky wrote:
From: Leon Romanovsky <leonro@nvidia.com> Extend base DMA page API to handle MMIO flow.
I would mention here this follows the long ago agreement that we don't need to enable P2P in the legacy dma_ops area. Simply failing when getting an ATTR_MMIO is OK.
quoted hunk ↗ jump to hunk
--- a/kernel/dma/mapping.c +++ b/kernel/dma/mapping.c@@ -158,6 +158,7 @@ dma_addr_t dma_map_page_attrs(struct device *dev, struct page *page, { const struct dma_map_ops *ops = get_dma_ops(dev); phys_addr_t phys = page_to_phys(page) + offset; + bool is_mmio = attrs & DMA_ATTR_MMIO; dma_addr_t addr; BUG_ON(!valid_dma_direction(dir));@@ -166,12 +167,23 @@ dma_addr_t dma_map_page_attrs(struct device *dev, struct page *page, return DMA_MAPPING_ERROR; if (dma_map_direct(dev, ops) || - arch_dma_map_phys_direct(dev, phys + size)) + (!is_mmio && arch_dma_map_phys_direct(dev, phys + size))) addr = dma_direct_map_phys(dev, phys, size, dir, attrs);
I don't know this area, maybe explain a bit in the commit message how you see ATTR_MMIO interacts with arch_dma_map_phys_direct ?
else if (use_dma_iommu(dev))
addr = iommu_dma_map_phys(dev, phys, size, dir, attrs);
- else
+ else if (is_mmio) {
+ if (!ops->map_resource)
+ return DMA_MAPPING_ERROR;
+
+ addr = ops->map_resource(dev, phys, size, dir, attrs);
+ } else {
+ /*
+ * All platforms which implement .map_page() don't support
+ * non-struct page backed addresses.
+ */
addr = ops->map_page(dev, page, offset, size, dir, attrs);Comment could be clearer maybe just: The dma_ops API contract for ops->map_page() requires kmappable memory, while ops->map_resource() does not. But this approach looks good to me, it prevents non-kmappable phys from going down to the legacy dma_ops map_page where it cannot work. From here you could do what Marek and Christoph asked to flush the struct page out of the ops->map_page() and replace it with kmap_local_phys(). Jason