[PATCH RFC 02/26] dma-buf: Add the SGT DMA mapping type

Jason Gunthorpe
2026-02-18T00:12:18Z

The SGT (Scatter Gather Table) DMA mapping type represents the existing
sg_table/scatterlist based DMA mapping. It provides a sg_table based
map/unmap interface that exactly matches how things work today.

dma_buf_sgt_exp_compat_match will be used in the next patch to
transparently wrap an unaware exporter with a mapping type.

The SGT type handles the allow_peer2peer flag directly through matching
logic. The importer indicates if it is willing to accept peer2peer and the
exporter indicates if it requires peer2peer. A required peer2peer exporter
will not match to an importer that does not accept peer2peer.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
---
drivers/dma-buf/dma-buf-mapping.c | 95 ++++++++++++++++++++++++++++
include/linux/dma-buf-mapping.h | 101 ++++++++++++++++++++++++++++++
include/linux/dma-buf.h | 18 +++++-
3 files changed, 213 insertions(+), 1 deletion(-)

diff --git a/drivers/dma-buf/dma-buf-mapping.c b/drivers/dma-buf/dma-buf-mapping.c
index 459c204cabb803..02f5cf8b3def40 100644
--- a/drivers/dma-buf/dma-buf-mapping.c
+++ b/drivers/dma-buf/dma-buf-mapping.c
@@ -6,6 +6,7 @@
#include <linux/dma-buf-mapping.h>
#include <linux/dma-resv.h>
#include <linux/dma-buf.h>
+#include <linux/seq_file.h>

static struct scatterlist *fill_sg_entry(struct scatterlist *sgl, size_t length,
dma_addr_t addr)
@@ -292,3 +293,97 @@ int dma_buf_match_mapping(struct dma_buf_match_args *args,
return -EINVAL;
}
EXPORT_SYMBOL_NS_GPL(dma_buf_match_mapping, "DMA_BUF");
+
+static int dma_buf_sgt_match(struct dma_buf *dmabuf,
+ const struct dma_buf_mapping_match *exp,
+ const struct dma_buf_mapping_match *imp)
+{
+ switch (exp->sgt_data.exporter_requires_p2p) {
+ case DMA_SGT_NO_P2P:
+ return 0;
+ case DMA_SGT_EXPORTER_REQUIRES_P2P_DISTANCE:
+ if (WARN_ON(!exp->sgt_data.exporting_p2p_device) ||
+ imp->sgt_data.importer_accepts_p2p !=
+ DMA_SGT_IMPORTER_ACCEPTS_P2P)
+ return -EOPNOTSUPP;
+ if (pci_p2pdma_distance(exp->sgt_data.exporting_p2p_device,
+ imp->sgt_data.importing_dma_device,
+ true) < 0)
+ return -EOPNOTSUPP;
+ return 0;
+ }
+ return 0;
+}
+
+static inline void
+dma_buf_sgt_finish_match(struct dma_buf_match_args *args,
+ const struct dma_buf_mapping_match *exp,
+ const struct dma_buf_mapping_match *imp)
+{
+ struct dma_buf_attachment *attach = args->attach;
+
+ attach->map_type = (struct dma_buf_mapping_match) {
+ .type = &dma_buf_mapping_sgt_type,
+ .exp_ops = exp->exp_ops,
+ .sgt_data = {
+ .importing_dma_device = imp->sgt_data.importing_dma_device,
+ /* exporting_p2p_device is left opaque */
+ .importer_accepts_p2p = imp->sgt_data.importer_accepts_p2p,
+ .exporter_requires_p2p = exp->sgt_data.exporter_requires_p2p,
+ },
+ };
+
+ /*
+ * Setup the SGT type variables stored in attach because importers and
+ * exporters that do not natively use mappings expect them to be there.
+ * When converting to use mappings users should use the match versions
+ * of these instead.
+ */
+ attach->dev = imp->sgt_data.importing_dma_device;
+ attach->peer2peer = attach->map_type.sgt_data.importer_accepts_p2p ==
+ DMA_SGT_IMPORTER_ACCEPTS_P2P;
+}
+
+static void dma_buf_sgt_debugfs_dump(struct seq_file *s,
+ struct dma_buf_attachment *attach)
+{
+ seq_printf(s, " %s", dev_name(dma_buf_sgt_dma_device(attach)));
+}
+
+struct dma_buf_mapping_type dma_buf_mapping_sgt_type = {
+ .name = "DMA Mapped Scatter Gather Table",
+ .match = dma_buf_sgt_match,
+ .finish_match = dma_buf_sgt_finish_match,
+ .debugfs_dump = dma_buf_sgt_debugfs_dump,
+};
+EXPORT_SYMBOL_NS_GPL(dma_buf_mapping_sgt_type, "DMA_BUF");
+
+static struct sg_table *
+dma_buf_sgt_compat_map_dma_buf(struct dma_buf_attachment *attach,
+ enum dma_data_direction dir)
+{
+ return attach->dmabuf->ops->map_dma_buf(attach, dir);
+}
+
+static void dma_buf_sgt_compat_unmap_dma_buf(struct dma_buf_attachment *attach,
+ struct sg_table *sgt,
+ enum dma_data_direction dir)
+{
+ attach->dmabuf->ops->unmap_dma_buf(attach, sgt, dir);
+}
+
+/* Route the classic map/unmap ops through the exp ops for old importers */
+static const struct dma_buf_mapping_sgt_exp_ops dma_buf_sgt_compat_exp_ops = {
+ .map_dma_buf = dma_buf_sgt_compat_map_dma_buf,
+ .unmap_dma_buf = dma_buf_sgt_compat_unmap_dma_buf,
+};
+
+/*
+ * This mapping type is used for unaware exporters that do not support
+ * match_mapping(). It wraps the dma_buf ops for SGT mappings into a mapping
+ * type so aware importers can transparently work with unaware exporters. This
+ * does not require p2p because old exporters will check it through the
+ * attach->peer2peer mechanism.
+ */
+const struct dma_buf_mapping_match dma_buf_sgt_exp_compat_match =
+ DMA_BUF_EMAPPING_SGT(&dma_buf_sgt_compat_exp_ops);
diff --git a/include/linux/dma-buf-mapping.h b/include/linux/dma-buf-mapping.h
index 080ccbf3a3f8b8..360a7fe0b098be 100644
--- a/include/linux/dma-buf-mapping.h
+++ b/include/linux/dma-buf-mapping.h
@@ -12,6 +12,12 @@ struct dma_buf;
struct dma_buf_attachment;
struct dma_buf_mapping_exp_ops;

+enum dma_sgt_requires_p2p {
+ DMA_SGT_NO_P2P = 0,
+ DMA_SGT_EXPORTER_REQUIRES_P2P_DISTANCE,
+ DMA_SGT_IMPORTER_ACCEPTS_P2P,
+};
+
/* Type tag for all mapping operations */
struct dma_buf_mapping_exp_ops {};

@@ -90,4 +96,99 @@ int dma_buf_match_mapping(struct dma_buf_match_args *args,
const struct dma_buf_mapping_match *exp_mappings,
size_t exp_len);

+/*
+ * DMA Mapped Scatterlist Type
+ *
+ * When this type is matched the map/unmap functions are:
+ *
+ * dma_buf_map_attachment()
+ * dma_buf_unmap_attachment()
+ *
+ * The struct sg_table returned by those functions has only the DMA portions
+ * available. The caller must not try to use the struct page * information.
+ *
+ * importing_dma_device is passed to the DMA API to provide the dma_addr_t's.
+ */
+extern struct dma_buf_mapping_type dma_buf_mapping_sgt_type;
+
+struct dma_buf_mapping_sgt_exp_ops {
+ struct dma_buf_mapping_exp_ops ops;
+ struct sg_table *(*map_dma_buf)(struct dma_buf_attachment *attach,
+ enum dma_data_direction dir);
+ void (*unmap_dma_buf)(struct dma_buf_attachment *attach,
+ struct sg_table *sgt,
+ enum dma_data_direction dir);
+};
+
+/**
+ * dma_buf_sgt_dma_device - Return the device to use for DMA mapping
+ * @attach: sgt mapping type attachment
+ *
+ * Called by the exporter to get the struct device to pass to the DMA API
+ * during map and unmap callbacks.
+ */
+static inline struct device *
+dma_buf_sgt_dma_device(struct dma_buf_attachment *attach)
+{
+ if (attach->map_type.type != &dma_buf_mapping_sgt_type)
+ return NULL;
+ return attach->map_type.sgt_data.importing_dma_device;
+}
+
+/**
+ * dma_buf_sgt_p2p_allowed - True if MMIO memory can be used peer to peer
+ * @attach: sgt mapping type attachment
+ *
+ * Should be called by exporters, returns true if the exporter's
+ * DMA_SGT_EXPORTER_REQUIRES_P2P_DISTANCE was matched.
+ */
+static inline bool dma_buf_sgt_p2p_allowed(struct dma_buf_attachment *attach)
+{
+ if (attach->map_type.type != &dma_buf_mapping_sgt_type)
+ return false;
+ return attach->map_type.sgt_data.exporter_requires_p2p ==
+ DMA_SGT_EXPORTER_REQUIRES_P2P_DISTANCE;
+}
+
+static inline const struct dma_buf_mapping_sgt_exp_ops *
+dma_buf_get_sgt_ops(struct dma_buf_attachment *attach)
+{
+ if (attach->map_type.type != &dma_buf_mapping_sgt_type)
+ return NULL;
+ return container_of(attach->map_type.exp_ops,
+ struct dma_buf_mapping_sgt_exp_ops, ops);
+}
+
+static inline struct dma_buf_mapping_match
+DMA_BUF_IMAPPING_SGT(struct device *importing_dma_device,
+ enum dma_sgt_requires_p2p importer_accepts_p2p)
+{
+ return (struct dma_buf_mapping_match){
+ .type = &dma_buf_mapping_sgt_type,
+ .sgt_data = { .importing_dma_device = importing_dma_device,
+ .importer_accepts_p2p = importer_accepts_p2p },
+ };
+}
+#define DMA_BUF_EMAPPING_SGT(_exp_ops) \
+ ((struct dma_buf_mapping_match){ .type = &dma_buf_mapping_sgt_type, \
+ .exp_ops = &((_exp_ops)->ops) })
+
+/*
+ * Only matches if the importing device is P2P capable and the P2P subsystem
+ * says P2P is possible from p2p_device.
+ */
+static inline struct dma_buf_mapping_match
+DMA_BUF_EMAPPING_SGT_P2P(const struct dma_buf_mapping_sgt_exp_ops *exp_ops,
+ struct pci_dev *p2p_device)
+{
+ struct dma_buf_mapping_match match = DMA_BUF_EMAPPING_SGT(exp_ops);
+
+ match.sgt_data.exporter_requires_p2p =
+ DMA_SGT_EXPORTER_REQUIRES_P2P_DISTANCE;
+ match.sgt_data.exporting_p2p_device = p2p_device;
+ return match;
+}
+
+extern const struct dma_buf_mapping_match dma_buf_sgt_exp_compat_match;
+
#endif
diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index a2b01b13026810..3bcd1d6d150188 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -30,6 +30,7 @@ struct dma_buf_attachment;
struct dma_buf_mapping_type;
struct dma_buf_mapping_exp_ops;

+
/*
* Match items are generated by the importer using the DMA_BUF_IMAPPING_*() and
* the exporter using the DMA_BUF_EMAPPING_*() functions. Each mapping type
@@ -39,7 +40,22 @@ struct dma_buf_mapping_match {
const struct dma_buf_mapping_type *type;
const struct dma_buf_mapping_exp_ops *exp_ops;
union {
- /* Each mapping_type has unique match parameters here */
+ /* For dma_buf_mapping_sgt_type */
+ struct {
+ struct device *importing_dma_device;
+ /* Only used if DMA_SGT_EXPORTER_REQUIRES_P2P_DISTANCE */
+ struct pci_dev *exporting_p2p_device;
+ /*
+ * These p2p flags are used to support the hard coded
+ * mechanism for p2p. If an exporting device knows it
+ * will put MMIO into the sgt then it should set
+ * exporter_requires_p2p. Importers should set
+ * importer_accepts_p2p unless it is known that the
+ * importing HW never supports P2P because of HW issues.
+ */
+ u8 importer_accepts_p2p;
+ u8 exporter_requires_p2p;
+ } sgt_data;
};
};

--
2.43.0