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1 | DMA Buffer Sharing API Guide |
2 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
3 | ||
4 | Sumit Semwal | |
5 | <sumit dot semwal at linaro dot org> | |
6 | <sumit dot semwal at ti dot com> | |
7 | ||
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. | |
10 | ||
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. | |
13 | ||
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. | |
16 | ||
17 | The exporter | |
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 | |
23 | buffer, | |
24 | ||
25 | The buffer-user | |
26 | - is one of (many) sharing users of the buffer. | |
27 | - doesn't need to worry about how the buffer is allocated, or where. | |
28 | - needs a mechanism to get access to the scatterlist that makes up this buffer | |
29 | in memory, mapped into its own address space, so it can access the same area | |
30 | of memory. | |
31 | ||
32 | *IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details] | |
33 | For this first version, A buffer shared using the dma_buf sharing API: | |
34 | - *may* be exported to user space using "mmap" *ONLY* by exporter, outside of | |
35 | this framework. | |
36 | - may be used *ONLY* by importers that do not need CPU access to the buffer. | |
37 | ||
38 | The dma_buf buffer sharing API usage contains the following steps: | |
39 | ||
40 | 1. Exporter announces that it wishes to export a buffer | |
41 | 2. Userspace gets the file descriptor associated with the exported buffer, and | |
42 | passes it around to potential buffer-users based on use case | |
43 | 3. Each buffer-user 'connects' itself to the buffer | |
44 | 4. When needed, buffer-user requests access to the buffer from exporter | |
45 | 5. When finished with its use, the buffer-user notifies end-of-DMA to exporter | |
46 | 6. when buffer-user is done using this buffer completely, it 'disconnects' | |
47 | itself from the buffer. | |
48 | ||
49 | ||
50 | 1. Exporter's announcement of buffer export | |
51 | ||
52 | The buffer exporter announces its wish to export a buffer. In this, it | |
53 | connects its own private buffer data, provides implementation for operations | |
54 | that can be performed on the exported dma_buf, and flags for the file | |
55 | associated with this buffer. | |
56 | ||
57 | Interface: | |
58 | struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops, | |
59 | size_t size, int flags) | |
60 | ||
61 | If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a | |
62 | pointer to the same. It also associates an anonymous file with this buffer, | |
63 | so it can be exported. On failure to allocate the dma_buf object, it returns | |
64 | NULL. | |
65 | ||
66 | 2. Userspace gets a handle to pass around to potential buffer-users | |
67 | ||
68 | Userspace entity requests for a file-descriptor (fd) which is a handle to the | |
69 | anonymous file associated with the buffer. It can then share the fd with other | |
70 | drivers and/or processes. | |
71 | ||
72 | Interface: | |
73 | int dma_buf_fd(struct dma_buf *dmabuf) | |
74 | ||
75 | This API installs an fd for the anonymous file associated with this buffer; | |
76 | returns either 'fd', or error. | |
77 | ||
78 | 3. Each buffer-user 'connects' itself to the buffer | |
79 | ||
80 | Each buffer-user now gets a reference to the buffer, using the fd passed to | |
81 | it. | |
82 | ||
83 | Interface: | |
84 | struct dma_buf *dma_buf_get(int fd) | |
85 | ||
86 | This API will return a reference to the dma_buf, and increment refcount for | |
87 | it. | |
88 | ||
89 | After this, the buffer-user needs to attach its device with the buffer, which | |
90 | helps the exporter to know of device buffer constraints. | |
91 | ||
92 | Interface: | |
93 | struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf, | |
94 | struct device *dev) | |
95 | ||
96 | This API returns reference to an attachment structure, which is then used | |
97 | for scatterlist operations. It will optionally call the 'attach' dma_buf | |
98 | operation, if provided by the exporter. | |
99 | ||
100 | The dma-buf sharing framework does the bookkeeping bits related to managing | |
101 | the list of all attachments to a buffer. | |
102 | ||
103 | Until this stage, the buffer-exporter has the option to choose not to actually | |
104 | allocate the backing storage for this buffer, but wait for the first buffer-user | |
105 | to request use of buffer for allocation. | |
106 | ||
107 | ||
108 | 4. When needed, buffer-user requests access to the buffer | |
109 | ||
110 | Whenever a buffer-user wants to use the buffer for any DMA, it asks for | |
111 | access to the buffer using dma_buf_map_attachment API. At least one attach to | |
112 | the buffer must have happened before map_dma_buf can be called. | |
113 | ||
114 | Interface: | |
115 | struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *, | |
116 | enum dma_data_direction); | |
117 | ||
118 | This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the | |
119 | "dma_buf->ops->" indirection from the users of this interface. | |
120 | ||
121 | In struct dma_buf_ops, map_dma_buf is defined as | |
122 | struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *, | |
123 | enum dma_data_direction); | |
124 | ||
125 | It is one of the buffer operations that must be implemented by the exporter. | |
126 | It should return the sg_table containing scatterlist for this buffer, mapped | |
127 | into caller's address space. | |
128 | ||
129 | If this is being called for the first time, the exporter can now choose to | |
130 | scan through the list of attachments for this buffer, collate the requirements | |
131 | of the attached devices, and choose an appropriate backing storage for the | |
132 | buffer. | |
133 | ||
134 | Based on enum dma_data_direction, it might be possible to have multiple users | |
135 | accessing at the same time (for reading, maybe), or any other kind of sharing | |
136 | that the exporter might wish to make available to buffer-users. | |
137 | ||
138 | map_dma_buf() operation can return -EINTR if it is interrupted by a signal. | |
139 | ||
140 | ||
141 | 5. When finished, the buffer-user notifies end-of-DMA to exporter | |
142 | ||
143 | Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to | |
144 | the exporter using the dma_buf_unmap_attachment API. | |
145 | ||
146 | Interface: | |
147 | void dma_buf_unmap_attachment(struct dma_buf_attachment *, | |
148 | struct sg_table *); | |
149 | ||
150 | This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the | |
151 | "dma_buf->ops->" indirection from the users of this interface. | |
152 | ||
153 | In struct dma_buf_ops, unmap_dma_buf is defined as | |
154 | void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *); | |
155 | ||
156 | unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like | |
157 | map_dma_buf, this API also must be implemented by the exporter. | |
158 | ||
159 | ||
160 | 6. when buffer-user is done using this buffer, it 'disconnects' itself from the | |
161 | buffer. | |
162 | ||
163 | After the buffer-user has no more interest in using this buffer, it should | |
164 | disconnect itself from the buffer: | |
165 | ||
166 | - it first detaches itself from the buffer. | |
167 | ||
168 | Interface: | |
169 | void dma_buf_detach(struct dma_buf *dmabuf, | |
170 | struct dma_buf_attachment *dmabuf_attach); | |
171 | ||
172 | This API removes the attachment from the list in dmabuf, and optionally calls | |
173 | dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits. | |
174 | ||
175 | - Then, the buffer-user returns the buffer reference to exporter. | |
176 | ||
177 | Interface: | |
178 | void dma_buf_put(struct dma_buf *dmabuf); | |
179 | ||
180 | This API then reduces the refcount for this buffer. | |
181 | ||
182 | If, as a result of this call, the refcount becomes 0, the 'release' file | |
183 | operation related to this fd is called. It calls the dmabuf->ops->release() | |
184 | operation in turn, and frees the memory allocated for dmabuf when exported. | |
185 | ||
186 | NOTES: | |
187 | - Importance of attach-detach and {map,unmap}_dma_buf operation pairs | |
188 | The attach-detach calls allow the exporter to figure out backing-storage | |
189 | constraints for the currently-interested devices. This allows preferential | |
190 | allocation, and/or migration of pages across different types of storage | |
191 | available, if possible. | |
192 | ||
193 | Bracketing of DMA access with {map,unmap}_dma_buf operations is essential | |
194 | to allow just-in-time backing of storage, and migration mid-way through a | |
195 | use-case. | |
196 | ||
197 | - Migration of backing storage if needed | |
198 | If after | |
199 | - at least one map_dma_buf has happened, | |
200 | - and the backing storage has been allocated for this buffer, | |
201 | another new buffer-user intends to attach itself to this buffer, it might | |
202 | be allowed, if possible for the exporter. | |
203 | ||
204 | In case it is allowed by the exporter: | |
205 | if the new buffer-user has stricter 'backing-storage constraints', and the | |
206 | exporter can handle these constraints, the exporter can just stall on the | |
207 | map_dma_buf until all outstanding access is completed (as signalled by | |
208 | unmap_dma_buf). | |
209 | Once all users have finished accessing and have unmapped this buffer, the | |
210 | exporter could potentially move the buffer to the stricter backing-storage, | |
211 | and then allow further {map,unmap}_dma_buf operations from any buffer-user | |
212 | from the migrated backing-storage. | |
213 | ||
214 | If the exporter cannot fulfil the backing-storage constraints of the new | |
215 | buffer-user device as requested, dma_buf_attach() would return an error to | |
216 | denote non-compatibility of the new buffer-sharing request with the current | |
217 | buffer. | |
218 | ||
219 | If the exporter chooses not to allow an attach() operation once a | |
220 | map_dma_buf() API has been called, it simply returns an error. | |
221 | ||
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222 | Miscellaneous notes: |
223 | - Any exporters or users of the dma-buf buffer sharing framework must have | |
224 | a 'select DMA_SHARED_BUFFER' in their respective Kconfigs. | |
225 | ||
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226 | References: |
227 | [1] struct dma_buf_ops in include/linux/dma-buf.h | |
228 | [2] All interfaces mentioned above defined in include/linux/dma-buf.h |