1 DMA Buffer Sharing API Guide
2 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
5 <sumit dot semwal at linaro dot org>
6 <sumit dot semwal at ti dot com>
8 This document serves as a guide to device-driver writers on what is the dma-buf
9 buffer sharing API, how to use it for exporting and using shared buffers.
11 Any device driver which wishes to be a part of DMA buffer sharing, can do so as
12 either the 'exporter' of buffers, or the 'user' of buffers.
14 Say a driver A wants to use buffers created by driver B, then we call B as the
15 exporter, and A as buffer-user.
18 - implements and manages operations[1] for the buffer
19 - allows other users to share the buffer by using dma_buf sharing APIs,
20 - manages the details of buffer allocation,
21 - decides about the actual backing storage where this allocation happens,
22 - takes care of any migration of scatterlist - for all (shared) users of this
24 - optionally, provides mmap capability for drivers that need it.
27 - is one of (many) sharing users of the buffer.
28 - doesn't need to worry about how the buffer is allocated, or where.
29 - needs a mechanism to get access to the scatterlist that makes up this buffer
30 in memory, mapped into its own address space, so it can access the same area
34 The dma_buf buffer sharing API usage contains the following steps:
36 1. Exporter announces that it wishes to export a buffer
37 2. Userspace gets the file descriptor associated with the exported buffer, and
38 passes it around to potential buffer-users based on use case
39 3. Each buffer-user 'connects' itself to the buffer
40 4. When needed, buffer-user requests access to the buffer from exporter
41 5. When finished with its use, the buffer-user notifies end-of-DMA to exporter
42 6. when buffer-user is done using this buffer completely, it 'disconnects'
43 itself from the buffer.
46 1. Exporter's announcement of buffer export
48 The buffer exporter announces its wish to export a buffer. In this, it
49 connects its own private buffer data, provides implementation for operations
50 that can be performed on the exported dma_buf, and flags for the file
51 associated with this buffer.
54 struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops,
57 If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a
58 pointer to the same. It also associates an anonymous file with this buffer,
59 so it can be exported. On failure to allocate the dma_buf object, it returns
62 2. Userspace gets a handle to pass around to potential buffer-users
64 Userspace entity requests for a file-descriptor (fd) which is a handle to the
65 anonymous file associated with the buffer. It can then share the fd with other
66 drivers and/or processes.
69 int dma_buf_fd(struct dma_buf *dmabuf)
71 This API installs an fd for the anonymous file associated with this buffer;
72 returns either 'fd', or error.
74 3. Each buffer-user 'connects' itself to the buffer
76 Each buffer-user now gets a reference to the buffer, using the fd passed to
80 struct dma_buf *dma_buf_get(int fd)
82 This API will return a reference to the dma_buf, and increment refcount for
85 After this, the buffer-user needs to attach its device with the buffer, which
86 helps the exporter to know of device buffer constraints.
89 struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
92 This API returns reference to an attachment structure, which is then used
93 for scatterlist operations. It will optionally call the 'attach' dma_buf
94 operation, if provided by the exporter.
96 The dma-buf sharing framework does the bookkeeping bits related to managing
97 the list of all attachments to a buffer.
99 Until this stage, the buffer-exporter has the option to choose not to actually
100 allocate the backing storage for this buffer, but wait for the first buffer-user
101 to request use of buffer for allocation.
104 4. When needed, buffer-user requests access to the buffer
106 Whenever a buffer-user wants to use the buffer for any DMA, it asks for
107 access to the buffer using dma_buf_map_attachment API. At least one attach to
108 the buffer must have happened before map_dma_buf can be called.
111 struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *,
112 enum dma_data_direction);
114 This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the
115 "dma_buf->ops->" indirection from the users of this interface.
117 In struct dma_buf_ops, map_dma_buf is defined as
118 struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
119 enum dma_data_direction);
121 It is one of the buffer operations that must be implemented by the exporter.
122 It should return the sg_table containing scatterlist for this buffer, mapped
123 into caller's address space.
125 If this is being called for the first time, the exporter can now choose to
126 scan through the list of attachments for this buffer, collate the requirements
127 of the attached devices, and choose an appropriate backing storage for the
130 Based on enum dma_data_direction, it might be possible to have multiple users
131 accessing at the same time (for reading, maybe), or any other kind of sharing
132 that the exporter might wish to make available to buffer-users.
135 5. When finished, the buffer-user notifies end-of-DMA to exporter
137 Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to
138 the exporter using the dma_buf_unmap_attachment API.
141 void dma_buf_unmap_attachment(struct dma_buf_attachment *,
144 This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the
145 "dma_buf->ops->" indirection from the users of this interface.
147 In struct dma_buf_ops, unmap_dma_buf is defined as
148 void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *);
150 unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like
151 map_dma_buf, this API also must be implemented by the exporter.
154 6. when buffer-user is done using this buffer, it 'disconnects' itself from the
157 After the buffer-user has no more interest in using this buffer, it should
158 disconnect itself from the buffer:
160 - it first detaches itself from the buffer.
163 void dma_buf_detach(struct dma_buf *dmabuf,
164 struct dma_buf_attachment *dmabuf_attach);
166 This API removes the attachment from the list in dmabuf, and optionally calls
167 dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits.
169 - Then, the buffer-user returns the buffer reference to exporter.
172 void dma_buf_put(struct dma_buf *dmabuf);
174 This API then reduces the refcount for this buffer.
176 If, as a result of this call, the refcount becomes 0, the 'release' file
177 operation related to this fd is called. It calls the dmabuf->ops->release()
178 operation in turn, and frees the memory allocated for dmabuf when exported.
181 - Importance of attach-detach and {map,unmap}_dma_buf operation pairs
182 The attach-detach calls allow the exporter to figure out backing-storage
183 constraints for the currently-interested devices. This allows preferential
184 allocation, and/or migration of pages across different types of storage
185 available, if possible.
187 Bracketing of DMA access with {map,unmap}_dma_buf operations is essential
188 to allow just-in-time backing of storage, and migration mid-way through a
191 - Migration of backing storage if needed
193 - at least one map_dma_buf has happened,
194 - and the backing storage has been allocated for this buffer,
195 another new buffer-user intends to attach itself to this buffer, it might
196 be allowed, if possible for the exporter.
198 In case it is allowed by the exporter:
199 if the new buffer-user has stricter 'backing-storage constraints', and the
200 exporter can handle these constraints, the exporter can just stall on the
201 map_dma_buf until all outstanding access is completed (as signalled by
203 Once all ongoing access is completed, the exporter could potentially move
204 the buffer to the stricter backing-storage, and then allow further
205 {map,unmap}_dma_buf operations from any buffer-user from the migrated
208 If the exporter cannot fulfil the backing-storage constraints of the new
209 buffer-user device as requested, dma_buf_attach() would return an error to
210 denote non-compatibility of the new buffer-sharing request with the current
213 If the exporter chooses not to allow an attach() operation once a
214 map_dma_buf() API has been called, it simply returns an error.
216 - mmap file operation
217 An mmap() file operation is provided for the fd associated with the buffer.
218 If the exporter defines an mmap operation, the mmap() fop calls this to allow
219 mmap for devices that might need it; if not, it returns an error.
222 [1] struct dma_buf_ops in include/linux/dma-buf.h
223 [2] All interfaces mentioned above defined in include/linux/dma-buf.h