Re: [PATCH v6 00/16] dma-mapping: migrate to physical address-based API

Marek Szyprowski
2025-09-11T22:25:45Z

On 09.09.2025 15:27, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@nvidia.com>
>
> Changelog:
> v6:
> * Based on "dma-debug: don't enforce dma mapping check on noncoherent
> allocations" patch.
> * Removed some unused variables from kmsan conversion.
> * Fixed missed ! in dma check.
> v5: https://lore.kernel.org/all/cover.1756822782.git.leon@kernel.org
> * Added Jason's and Keith's Reviewed-by tags
> * Fixed DMA_ATTR_MMIO check in dma_direct_map_phys
> * Jason's cleanup suggestions
> v4: https://lore.kernel.org/all/cover.1755624249.git.leon@kernel.org/
> * Fixed kbuild error with mismatch in kmsan function declaration due to
> rebase error.
> v3: https://lore.kernel.org/all/cover.1755193625.git.leon@kernel.org
> * Fixed typo in "cacheable" word
> * Simplified kmsan patch a lot to be simple argument refactoring
> v2: https://lore.kernel.org/all/cover.1755153054.git.leon@kernel.org
> * Used commit messages and cover letter from Jason
> * Moved setting IOMMU_MMIO flag to dma_info_to_prot function
> * Micro-optimized the code
> * Rebased code on v6.17-rc1
> v1: https://lore.kernel.org/all/cover.1754292567.git.leon@kernel.org
> * Added new DMA_ATTR_MMIO attribute to indicate
> PCI_P2PDMA_MAP_THRU_HOST_BRIDGE path.
> * Rewrote dma_map_* functions to use thus new attribute
> v0: https://lore.kernel.org/all/cover.1750854543.git.leon@kernel.org/
> ------------------------------------------------------------------------
>
> This series refactors the DMA mapping to use physical addresses
> as the primary interface instead of page+offset parameters. This
> change aligns the DMA API with the underlying hardware reality where
> DMA operations work with physical addresses, not page structures.
>
> The series maintains export symbol backward compatibility by keeping
> the old page-based API as wrapper functions around the new physical
> address-based implementations.
>
> This series refactors the DMA mapping API to provide a phys_addr_t
> based, and struct-page free, external API that can handle all the
> mapping cases we want in modern systems:
>
> - struct page based cacheable DRAM
> - struct page MEMORY_DEVICE_PCI_P2PDMA PCI peer to peer non-cacheable
> MMIO
> - struct page-less PCI peer to peer non-cacheable MMIO
> - struct page-less "resource" MMIO
>
> Overall this gets much closer to Matthew's long term wish for
> struct-pageless IO to cacheable DRAM. The remaining primary work would
> be in the mm side to allow kmap_local_pfn()/phys_to_virt() to work on
> phys_addr_t without a struct page.
>
> The general design is to remove struct page usage entirely from the
> DMA API inner layers. For flows that need to have a KVA for the
> physical address they can use kmap_local_pfn() or phys_to_virt(). This
> isolates the struct page requirements to MM code only. Long term all
> removals of struct page usage are supporting Matthew's memdesc
> project which seeks to substantially transform how struct page works.
>
> Instead make the DMA API internals work on phys_addr_t. Internally
> there are still dedicated 'page' and 'resource' flows, except they are
> now distinguished by a new DMA_ATTR_MMIO instead of by callchain. Both
> flows use the same phys_addr_t.
>
> When DMA_ATTR_MMIO is specified things work similar to the existing
> 'resource' flow. kmap_local_pfn(), phys_to_virt(), phys_to_page(),
> pfn_valid(), etc are never called on the phys_addr_t. This requires
> rejecting any configuration that would need swiotlb. CPU cache
> flushing is not required, and avoided, as ATTR_MMIO also indicates the
> address have no cacheable mappings. This effectively removes any
> DMA API side requirement to have struct page when DMA_ATTR_MMIO is
> used.
>
> In the !DMA_ATTR_MMIO mode things work similarly to the 'page' flow,
> except on the common path of no cache flush, no swiotlb it never
> touches a struct page. When cache flushing or swiotlb copying
> kmap_local_pfn()/phys_to_virt() are used to get a KVA for CPU
> usage. This was already the case on the unmap side, now the map side
> is symmetric.
>
> Callers are adjusted to set DMA_ATTR_MMIO. Existing 'resource' users
> must set it. The existing struct page based MEMORY_DEVICE_PCI_P2PDMA
> path must also set it. This corrects some existing bugs where iommu
> mappings for P2P MMIO were improperly marked IOMMU_CACHE.
>
> Since ATTR_MMIO is made to work with all the existing DMA map entry
> points, particularly dma_iova_link(), this finally allows a way to use
> the new DMA API to map PCI P2P MMIO without creating struct page. The
> VFIO DMABUF series demonstrates how this works. This is intended to
> replace the incorrect driver use of dma_map_resource() on PCI BAR
> addresses.
>
> This series does the core code and modern flows. A followup series
> will give the same treatment to the legacy dma_ops implementation.

Applied patches 1-13 into dma-mapping-for-next branch. Let's check if it
works fine in linux-next.

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland