On Tue, Nov 18, 2025 at 04:06:11PM -0800, Nicolin Chen wrote: > On Tue, Nov 11, 2025 at 11:57:48AM +0200, Leon Romanovsky wrote: > > From: Leon Romanovsky <[email protected]> > > > > 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 <[email protected]> > > Tested-by: Nicolin Chen <[email protected]> > > Signed-off-by: Leon Romanovsky <[email protected]> > > Reviewed-by: Nicolin Chen <[email protected]> > > 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)?
[i, nents) is not supposed to be in SG list which we are iterating. Thanks
