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

[Jason Gunthorpe <jgg@ziepe.ca>]

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>
> ---
>  drivers/dma-buf/dma-buf.c | 235 ++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/dma-buf.h   |  18 ++++
>  2 files changed, 253 insertions(+)

I've looked at this enough times now, the logic for DMA mapping and
the construction of the scatterlist is good:

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason