[PATCH RFC 01/26] dma-buf: Introduce DMA-buf mapping types

Jason Gunthorpe
2026-02-18T00:12:04Z

DMA-buf mapping types allow the importer and exporter to negotiate the
format of the map/unmap to be used during the attachment.

Currently DMA-buf only supports struct scatterlist as the attachment map
operation. This is not sufficient for all use cases as dma_addr_t is a
very specific and limited type.

With mapping types the importing driver can declare what it supports. For
example:

struct dma_buf_mapping_match imp_match[] = {
DMA_BUF_IMAPPING_MY_DRIVER(dev, ...),
DMA_BUF_IMAPPING_SGT(dev, false),
};
attach = dma_buf_mapping_attach(dmabuf, imp_match, ...)

And the exporting driver can declare what it supports:

int exporter_match_mapping(struct dma_buf_match_args *args)
{
struct dma_buf_mapping_match exp_match[] = {
DMA_BUF_EMAPPING_MY_DRIVER(my_ops, dev, ...),
DMA_BUF_EMAPPING_SGT(sgt_ops, dev, false),
DMA_BUF_EMAPPING_PAL(PAL_ops),
};
return dma_buf_match_mapping(args, exp_match, ...);
}

During dma_buf_mapping_attach() the core code will select a mutual match
between the importer and exporter and record it in the attach->map_type.

Add the basic types:

struct dma_buf_mapping_type
Type tag and ops for each mapping type.

struct dma_buf_mapping_match
Entry in a list of importer or exporter match specifications.
The match specification can be extended by the mapping type with
unique data.

dma_buf_match_mapping() / struct dma_buf_match_args
Helper to do the matching. Called by the exporting driver
via a dma_buf_ops callback.

struct dma_buf_mapping_exp_ops
Base type for the per-mapping type exporter provided functions. This
would be the map/unmap callbacks. Each mapping type can provide its own
functions for map/unmap type operations with optimal type signatures.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
---
drivers/dma-buf/dma-buf-mapping.c | 46 +++++++++++++++++++
include/linux/dma-buf-mapping.h | 76 +++++++++++++++++++++++++++++++
include/linux/dma-buf.h | 18 ++++++++
3 files changed, 140 insertions(+)

diff --git a/drivers/dma-buf/dma-buf-mapping.c b/drivers/dma-buf/dma-buf-mapping.c
index b7352e609fbdfa..459c204cabb803 100644
--- a/drivers/dma-buf/dma-buf-mapping.c
+++ b/drivers/dma-buf/dma-buf-mapping.c
@@ -5,6 +5,7 @@
*/
#include <linux/dma-buf-mapping.h>
#include <linux/dma-resv.h>
+#include <linux/dma-buf.h>

static struct scatterlist *fill_sg_entry(struct scatterlist *sgl, size_t length,
dma_addr_t addr)
@@ -246,3 +247,48 @@ void dma_buf_free_sgt(struct dma_buf_attachment *attach, struct sg_table *sgt,

}
EXPORT_SYMBOL_NS_GPL(dma_buf_free_sgt, "DMA_BUF");
+
+/**
+ * dma_buf_match_mapping - Select a mapping type agreed upon by exporter and
+ * importer
+ * @args: Match arguments from attach. On success this is updated with the
+ * matched exporter and importer entries.
+ * @exp: Array of mapping types supported by the exporter, in priority order
+ * @exp_len: Number of entries in @exp
+ *
+ * Iterate over the exporter's supported mapping types and for each one search
+ * the importer's list for a compatible matching type. args and args->attach are
+ * populated with the resulting match.
+ *
+ * Because the exporter list is walked in order, the exporter controls the
+ * priority of mapping types.
+ */
+int dma_buf_match_mapping(struct dma_buf_match_args *args,
+ const struct dma_buf_mapping_match *exp,
+ size_t exp_len)
+{
+ const struct dma_buf_mapping_match *exp_end = exp + exp_len;
+ const struct dma_buf_mapping_match *imp_end =
+ args->imp_matches + args->imp_len;
+ int ret;
+
+ for (; exp != exp_end; exp++) {
+ const struct dma_buf_mapping_match *imp = args->imp_matches;
+
+ for (; imp != imp_end; imp++) {
+ if (exp->type != imp->type)
+ continue;
+ if (exp->type->match) {
+ ret = exp->type->match(args->dmabuf, exp, imp);
+ if (ret == -EOPNOTSUPP)
+ continue;
+ if (ret != 0)
+ return ret;
+ }
+ exp->type->finish_match(args, exp, imp);
+ return 0;
+ }
+ }
+ return -EINVAL;
+}
+EXPORT_SYMBOL_NS_GPL(dma_buf_match_mapping, "DMA_BUF");
diff --git a/include/linux/dma-buf-mapping.h b/include/linux/dma-buf-mapping.h
index a3c0ce2d3a42fe..080ccbf3a3f8b8 100644
--- a/include/linux/dma-buf-mapping.h
+++ b/include/linux/dma-buf-mapping.h
@@ -7,6 +7,77 @@
#define __DMA_BUF_MAPPING_H__
#include <linux/dma-buf.h>

+struct device;
+struct dma_buf;
+struct dma_buf_attachment;
+struct dma_buf_mapping_exp_ops;
+
+/* Type tag for all mapping operations */
+struct dma_buf_mapping_exp_ops {};
+
+/*
+ * Internal struct to pass arguments from the attach function to the matching
+ * function
+ */
+struct dma_buf_match_args {
+ struct dma_buf *dmabuf;
+ struct dma_buf_attachment *attach;
+ const struct dma_buf_mapping_match *imp_matches;
+ size_t imp_len;
+};
+
+/**
+ * struct dma_buf_mapping_type - Operations for a DMA-buf mapping type
+ *
+ * Each mapping type provides a singleton instance of this struct to describe
+ * the mapping type and its operations.
+ */
+struct dma_buf_mapping_type {
+ /**
+ * @name: Human-readable name for this mapping type, used in debugfs
+ * output
+ */
+ const char *name;
+
+ /**
+ * @match:
+ *
+ * Called during attach from dma_buf_match_mapping(). &exp and &imp are
+ * single items from the importer and exporter mapping match lists.
+ * Both will have the same instance of this struct as their type member.
+ *
+ * It determines if the exporter/importer are compatible.
+ *
+ * Returns: 0 on success
+ * -EOPNOTSUPP means ignore the failure and continue
+ * Everything else aborts the search and returns the -errno
+ */
+ int (*match)(struct dma_buf *dmabuf,
+ const struct dma_buf_mapping_match *exp,
+ const struct dma_buf_mapping_match *imp);
+
+ /**
+ * @finish_match:
+ *
+ * Called by dma_buf_match_mapping() after a successful match to store
+ * the negotiated result in @args->attach. The matched @exp and @imp
+ * entries are provided so the callback can copy type-specific data into
+ * the attachment.
+ */
+ void (*finish_match)(struct dma_buf_match_args *args,
+ const struct dma_buf_mapping_match *exp,
+ const struct dma_buf_mapping_match *imp);
+
+ /**
+ * @debugfs_dump:
+ *
+ * Optional callback to write mapping-type-specific diagnostic
+ * information about @attach to the debugfs seq_file @s.
+ */
+ void (*debugfs_dump)(struct seq_file *s,
+ struct dma_buf_attachment *attach);
+};
+
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,
@@ -14,4 +85,9 @@ struct sg_table *dma_buf_phys_vec_to_sgt(struct dma_buf_attachment *attach,
enum dma_data_direction dir);
void dma_buf_free_sgt(struct dma_buf_attachment *attach, struct sg_table *sgt,
enum dma_data_direction dir);
+
+int dma_buf_match_mapping(struct dma_buf_match_args *args,
+ const struct dma_buf_mapping_match *exp_mappings,
+ size_t exp_len);
+
#endif
diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index 0bc492090237ed..a2b01b13026810 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -27,6 +27,21 @@
struct device;
struct dma_buf;
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
+ * defines its own signature with its own data to make the match and attachment.
+ */
+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 */
+ };
+};

/**
* struct dma_buf_ops - operations possible on struct dma_buf
@@ -488,6 +503,8 @@ struct dma_buf_attach_ops {
* @importer_ops: importer operations for this attachment, if provided
* dma_buf_map/unmap_attachment() must be called with the dma_resv lock held.
* @importer_priv: importer specific attachment data.
+ * @map_type: The match that defines the mutually compatible mapping type to use
+ * for this attachment.
*
* This structure holds the attachment information between the dma_buf buffer
* and its user device(s). The list contains one attachment struct per device
@@ -506,6 +523,7 @@ struct dma_buf_attachment {
const struct dma_buf_attach_ops *importer_ops;
void *importer_priv;
void *priv;
+ struct dma_buf_mapping_match map_type;
};

/**
--
2.43.0