[RFC v2 01/11] file: add callback for pre-mapping dmabuf

[Pavel Begunkov <asml.silence@gmail.com>]

Add a file callback that maps a dmabuf for the given file and returns
an opaque token of type struct dma_token representing the mapping. The
implementation details are hidden from the caller, and the implementors
are normally expected to extend the structure.

The callback callers will be able to pass the token with an IO request,
which implemented in following patches as a new iterator type. The user
should release the token once it's not needed by calling the provided
release callback via appropriate helpers.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 include/linux/dma_token.h | 35 +++++++++++++++++++++++++++++++++++
 include/linux/fs.h        |  4 ++++
 2 files changed, 39 insertions(+)
 create mode 100644 include/linux/dma_token.h

diff --git a/include/linux/dma_token.h b/include/linux/dma_token.h
new file mode 100644
index 000000000000..9194b34282c2
--- /dev/null
+++ b/include/linux/dma_token.h
@@ -0,0 +1,35 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_DMA_TOKEN_H
+#define _LINUX_DMA_TOKEN_H
+
+#include <linux/dma-buf.h>
+
+struct dma_token_params {
+	struct dma_buf			*dmabuf;
+	enum dma_data_direction		dir;
+};
+
+struct dma_token {
+	void (*release)(struct dma_token *);
+};
+
+static inline void dma_token_release(struct dma_token *token)
+{
+	token->release(token);
+}
+
+static inline struct dma_token *
+dma_token_create(struct file *file, struct dma_token_params *params)
+{
+	struct dma_token *res;
+
+	if (!file->f_op->dma_map)
+		return ERR_PTR(-EOPNOTSUPP);
+	res = file->f_op->dma_map(file, params);
+
+	WARN_ON_ONCE(!IS_ERR(res) && !res->release);
+
+	return res;
+}
+
+#endif
diff --git a/include/linux/fs.h b/include/linux/fs.h
index c895146c1444..0ce9a53fabec 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2262,6 +2262,8 @@ struct dir_context {
 struct iov_iter;
 struct io_uring_cmd;
 struct offset_ctx;
+struct dma_token;
+struct dma_token_params;
 
 typedef unsigned int __bitwise fop_flags_t;
 
@@ -2309,6 +2311,8 @@ struct file_operations {
 	int (*uring_cmd_iopoll)(struct io_uring_cmd *, struct io_comp_batch *,
 				unsigned int poll_flags);
 	int (*mmap_prepare)(struct vm_area_desc *);
+	struct dma_token *(*dma_map)(struct file *,
+				     struct dma_token_params *);
 } __randomize_layout;
 
 /* Supports async buffered reads */
-- 
2.52.0