dma-buf: provide phys_vec to scatter-gather mapping routine

Add dma_buf_phys_vec_to_sgt() and dma_buf_free_sgt() 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.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Alex Mastro <amastro@fb.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Ankit Agrawal <ankita@nvidia.com>
Link: https://lore.kernel.org/r/20251120-dmabuf-vfio-v9-6-d7f71607f371@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/include/linux/dma-buf-mapping.h b/include/linux/dma-buf-mapping.h
new file mode 100644
index 000000000000..a3c0ce2d3a42
--- /dev/null
+++ b/include/linux/dma-buf-mapping.h
@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * DMA BUF Mapping Helpers
+ *
+ */
+#ifndef __DMA_BUF_MAPPING_H__
+#define __DMA_BUF_MAPPING_H__
+#include <linux/dma-buf.h>
+
+struct sg_table *dma_buf_phys_vec_to_sgt(struct dma_buf_attachment *attach,
+					 struct p2pdma_provider *provider,
+					 struct dma_buf_phys_vec *phys_vec,
+					 size_t nr_ranges, size_t size,
+					 enum dma_data_direction dir);
+void dma_buf_free_sgt(struct dma_buf_attachment *attach, struct sg_table *sgt,
+		      enum dma_data_direction dir);
+#endif