Re: [PATCH v8 06/11] dma-buf: provide phys_vec to scatter-gather mapping routine

Nicolin Chen
2025-11-19T00:06:32Z

On Tue, Nov 11, 2025 at 11:57:48AM +0200, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@nvidia.com>
>
> Add dma_buf_map() and dma_buf_unmap() helpers to convert an array of
> MMIO physical address ranges into scatter-gather tables with proper
> DMA mapping.
>
> These common functions are a starting point and support any PCI
> drivers creating mappings from their BAR's MMIO addresses. VFIO is one
> case, as shortly will be RDMA. We can review existing DRM drivers to
> refactor them separately. We hope this will evolve into routines to
> help common DRM that include mixed CPU and MMIO mappings.
>
> Compared to the dma_map_resource() abuse this implementation handles
> the complicated PCI P2P scenarios properly, especially when an IOMMU
> is enabled:
>
> - Direct bus address mapping without IOVA allocation for
> PCI_P2PDMA_MAP_BUS_ADDR, using pci_p2pdma_bus_addr_map(). This
> happens if the IOMMU is enabled but the PCIe switch ACS flags allow
> transactions to avoid the host bridge.
>
> Further, this handles the slightly obscure, case of MMIO with a
> phys_addr_t that is different from the physical BAR programming
> (bus offset). The phys_addr_t is converted to a dma_addr_t and
> accommodates this effect. This enables certain real systems to
> work, especially on ARM platforms.
>
> - Mapping through host bridge with IOVA allocation and DMA_ATTR_MMIO
> attribute for MMIO memory regions (PCI_P2PDMA_MAP_THRU_HOST_BRIDGE).
> This happens when the IOMMU is enabled and the ACS flags are forcing
> all traffic to the IOMMU - ie for virtualization systems.
>
> - Cases where P2P is not supported through the host bridge/CPU. The
> P2P subsystem is the proper place to detect this and block it.
>
> Helper functions fill_sg_entry() and calc_sg_nents() handle the
> scatter-gather table construction, splitting large regions into
> UINT_MAX-sized chunks to fit within sg->length field limits.
>
> Since the physical address based DMA API forbids use of the CPU list
> of the scatterlist this will produce a mangled scatterlist that has
> a fully zero-length and NULL'd CPU list. The list is 0 length,
> all the struct page pointers are NULL and zero sized. This is stronger
> and more robust than the existing mangle_sg_table() technique. It is
> a future project to migrate DMABUF as a subsystem away from using
> scatterlist for this data structure.
>
> Tested-by: Alex Mastro <amastro@fb.com>
> Tested-by: Nicolin Chen <nicolinc@nvidia.com>
> Signed-off-by: Leon Romanovsky <leonro@nvidia.com>

Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>

With a nit:

> +err_unmap_dma:
> + if (!i || !dma->state) {
> + ; /* Do nothing */
> + } else if (dma_use_iova(dma->state)) {
> + dma_iova_destroy(attach->dev, dma->state, mapped_len, dir,
> + DMA_ATTR_MMIO);
> + } else {
> + for_each_sgtable_dma_sg(&dma->sgt, sgl, i)
> + dma_unmap_phys(attach->dev, sg_dma_address(sgl),
> + sg_dma_len(sgl), dir, DMA_ATTR_MMIO);

Would it be safer to skip dma_unmap_phys() the range [i, nents)?