[PATCH v2 02/23] dma-buf: Introduce DMA-buf mapping types
Alistair Popple
apopple at nvidia.com
Wed Nov 4 22:40:37 PST 2026
From: Jason Gunthorpe <jgg at nvidia.com>
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:
static const struct dma_buf_mapping_match imp_match[] = {
DMA_BUF_IMAPPING_MY_DRIVER(...),
DMA_BUF_IMAPPING_SGT(DMA_SGT_NO_P2P),
};
static const struct dma_buf_attach_ops importer_ops = {
.supported_mapping_types = imp_match,
.nr_supported_mapping_types = ARRAY_SIZE(imp_match),
...
};
attach = dma_buf_dynamic_attach(dmabuf, dev, &importer_ops, ...)
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),
};
return dma_buf_match_mapping(args, exp_match, ...);
}
During dma_buf_dynamic_attach() the core code records every mutually
acceptable (exporter entry, importer entry) pair in attach->matches, a
small fixed-size array in the attachment, in the exporter's order of
preference. The first entry is the default and is what
dma_buf_attach_map_type() returns; later patches let the exporter choose a different entry of the set
at pin or map time, when the buffer's placement and the other attachments
are known. Importers and exporters that list a single type see exactly one
entry.
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 at nvidia.com>
Co-developed-by: Alistair Popple <apopple at nvidia.com>
Signed-off-by: Alistair Popple <apopple at nvidia.com>
---
drivers/dma-buf/dma-buf-mapping.c | 65 +++++++++++++++++++++++++++
include/linux/dma-buf-mapping.h | 95 +++++++++++++++++++++++++++++++++++++++
include/linux/dma-buf.h | 29 ++++++++++++
3 files changed, 189 insertions(+)
diff --git a/drivers/dma-buf/dma-buf-mapping.c b/drivers/dma-buf/dma-buf-mapping.c
index 833be519e1e6..d27f02610c2a 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>
#include <linux/overflow.h>
#include <linux/align.h>
@@ -257,3 +258,67 @@ 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 - Record the mapping types agreed upon by exporter
+ * and importer
+ * @args: Match arguments from attach
+ * @exp: Array of mapping types supported by the exporter, in priority order
+ * @exp_len: Number of entries in @exp
+ *
+ * Walk the exporter's list in order and, for each entry, find the first
+ * importer entry of the same type that the type's match() callback accepts.
+ * Every such pair is recorded in @args->attach->matches, so the match set
+ * preserves the exporter's priority. The first entry is the default returned
+ * by dma_buf_attach_map_type().
+ *
+ * Because the exporter list is walked in order, the exporter controls the
+ * priority of mapping types.
+ *
+ * Returns: 0 if at least one type matched, -EOPNOTSUPP if the exporter and
+ * importer have no type in common, -EINVAL if either list is empty, -E2BIG if
+ * more than DMA_BUF_MAX_MATCHES types matched, or the error a match() callback
+ * aborted the search with.
+ */
+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;
+ struct dma_buf_attachment *attach = args->attach;
+ unsigned int nr = 0;
+ int ret;
+
+ if (!exp_len || !args->imp_len)
+ return -EINVAL;
+
+ 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, exp, imp);
+ if (ret == -EOPNOTSUPP)
+ continue;
+ if (ret != 0)
+ return ret;
+ }
+ if (nr == ARRAY_SIZE(attach->matches))
+ return -E2BIG;
+ exp->type->finish_match(args, exp, imp,
+ &attach->matches[nr++]);
+ break;
+ }
+ }
+
+ if (!nr)
+ return -EOPNOTSUPP;
+
+ attach->nr_matches = nr;
+ return 0;
+}
+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 09bde3f748e4..28b3a65c0672 100644
--- a/include/linux/dma-buf-mapping.h
+++ b/include/linux/dma-buf-mapping.h
@@ -7,6 +7,83 @@
#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;
+ /* The importing device passed to dma_buf_dynamic_attach() */
+ struct device *dev;
+ 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.
+ * The importing device is @args->dev; importer entries do not carry
+ * it, so they can be static tables shared by every attachment.
+ *
+ * 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_match_args *args,
+ 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 fill
+ * in @match, the entry being added to @args->attach->matches. The
+ * matched @exp and @imp entries are provided so the callback can copy
+ * type-specific data into it, along with @args->dev if the type needs
+ * the importing device later.
+ */
+ void (*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_mapping_match *match);
+
+ /**
+ * @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 phys_vec *phys_vec,
@@ -14,4 +91,22 @@ 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);
+
+/**
+ * dma_buf_attach_map_type - The match this attachment is mapped with
+ * @attach: the attachment
+ *
+ * Returns the entry of @attach->matches in force for the attachment: the
+ * exporter's default, which is the first entry.
+ */
+static inline const struct dma_buf_mapping_match *
+dma_buf_attach_map_type(const struct dma_buf_attachment *attach)
+{
+ return &attach->matches[0];
+}
+
#endif
diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index cedeb8ebc99e..81bbcd420d60 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -27,6 +27,28 @@
struct device;
struct dma_buf;
struct dma_buf_attachment;
+struct dma_buf_mapping_type;
+struct dma_buf_mapping_exp_ops;
+
+/*
+ * Upper bound on the number of mapping types an attachment can record as
+ * mutually acceptable. An exporter offering more compatible types than this
+ * to one importer fails the attach with -E2BIG.
+ */
+#define DMA_BUF_MAX_MATCHES 4
+
+/*
+ * 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
@@ -472,6 +494,11 @@ struct dma_buf_attach_ops {
* @importer_ops: importer operations for this attachment, if provided
* dma_buf_map/unmap_sgt() must be called with the dma_resv lock held.
* @importer_priv: importer specific attachment data.
+ * @matches: The match set negotiated at attach time: every mapping type both
+ * sides accept, in the exporter's order of preference. The first
+ * entry is the exporter's default; dma_buf_attach_map_type()
+ * returns the entry the attachment is mapped with.
+ * @nr_matches: Number of valid entries in @matches.
*
* This structure holds the attachment information between the dma_buf buffer
* and its user device(s). The list contains one attachment struct per device
@@ -490,6 +517,8 @@ struct dma_buf_attachment {
const struct dma_buf_attach_ops *importer_ops;
void *importer_priv;
void *priv;
+ struct dma_buf_mapping_match matches[DMA_BUF_MAX_MATCHES];
+ unsigned int nr_matches;
};
/**
--
2.54.0
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