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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 | |
b0b40f24 DV |
35 | this framework. |
36 | - with this new iteration of the dma-buf api cpu access from the kernel has been | |
37 | enable, see below for the details. | |
38 | ||
39 | dma-buf operations for device dma only | |
40 | -------------------------------------- | |
a7df4719 SS |
41 | |
42 | The dma_buf buffer sharing API usage contains the following steps: | |
43 | ||
44 | 1. Exporter announces that it wishes to export a buffer | |
45 | 2. Userspace gets the file descriptor associated with the exported buffer, and | |
46 | passes it around to potential buffer-users based on use case | |
47 | 3. Each buffer-user 'connects' itself to the buffer | |
48 | 4. When needed, buffer-user requests access to the buffer from exporter | |
49 | 5. When finished with its use, the buffer-user notifies end-of-DMA to exporter | |
50 | 6. when buffer-user is done using this buffer completely, it 'disconnects' | |
51 | itself from the buffer. | |
52 | ||
53 | ||
54 | 1. Exporter's announcement of buffer export | |
55 | ||
56 | The buffer exporter announces its wish to export a buffer. In this, it | |
57 | connects its own private buffer data, provides implementation for operations | |
58 | that can be performed on the exported dma_buf, and flags for the file | |
59 | associated with this buffer. | |
60 | ||
61 | Interface: | |
62 | struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops, | |
63 | size_t size, int flags) | |
64 | ||
65 | If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a | |
66 | pointer to the same. It also associates an anonymous file with this buffer, | |
67 | so it can be exported. On failure to allocate the dma_buf object, it returns | |
68 | NULL. | |
69 | ||
70 | 2. Userspace gets a handle to pass around to potential buffer-users | |
71 | ||
72 | Userspace entity requests for a file-descriptor (fd) which is a handle to the | |
73 | anonymous file associated with the buffer. It can then share the fd with other | |
74 | drivers and/or processes. | |
75 | ||
76 | Interface: | |
77 | int dma_buf_fd(struct dma_buf *dmabuf) | |
78 | ||
79 | This API installs an fd for the anonymous file associated with this buffer; | |
80 | returns either 'fd', or error. | |
81 | ||
82 | 3. Each buffer-user 'connects' itself to the buffer | |
83 | ||
84 | Each buffer-user now gets a reference to the buffer, using the fd passed to | |
85 | it. | |
86 | ||
87 | Interface: | |
88 | struct dma_buf *dma_buf_get(int fd) | |
89 | ||
90 | This API will return a reference to the dma_buf, and increment refcount for | |
91 | it. | |
92 | ||
93 | After this, the buffer-user needs to attach its device with the buffer, which | |
94 | helps the exporter to know of device buffer constraints. | |
95 | ||
96 | Interface: | |
97 | struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf, | |
98 | struct device *dev) | |
99 | ||
100 | This API returns reference to an attachment structure, which is then used | |
101 | for scatterlist operations. It will optionally call the 'attach' dma_buf | |
102 | operation, if provided by the exporter. | |
103 | ||
104 | The dma-buf sharing framework does the bookkeeping bits related to managing | |
105 | the list of all attachments to a buffer. | |
106 | ||
107 | Until this stage, the buffer-exporter has the option to choose not to actually | |
108 | allocate the backing storage for this buffer, but wait for the first buffer-user | |
109 | to request use of buffer for allocation. | |
110 | ||
111 | ||
112 | 4. When needed, buffer-user requests access to the buffer | |
113 | ||
114 | Whenever a buffer-user wants to use the buffer for any DMA, it asks for | |
115 | access to the buffer using dma_buf_map_attachment API. At least one attach to | |
116 | the buffer must have happened before map_dma_buf can be called. | |
117 | ||
118 | Interface: | |
119 | struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *, | |
120 | enum dma_data_direction); | |
121 | ||
122 | This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the | |
123 | "dma_buf->ops->" indirection from the users of this interface. | |
124 | ||
125 | In struct dma_buf_ops, map_dma_buf is defined as | |
126 | struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *, | |
127 | enum dma_data_direction); | |
128 | ||
129 | It is one of the buffer operations that must be implemented by the exporter. | |
130 | It should return the sg_table containing scatterlist for this buffer, mapped | |
131 | into caller's address space. | |
132 | ||
133 | If this is being called for the first time, the exporter can now choose to | |
134 | scan through the list of attachments for this buffer, collate the requirements | |
135 | of the attached devices, and choose an appropriate backing storage for the | |
136 | buffer. | |
137 | ||
138 | Based on enum dma_data_direction, it might be possible to have multiple users | |
139 | accessing at the same time (for reading, maybe), or any other kind of sharing | |
140 | that the exporter might wish to make available to buffer-users. | |
141 | ||
142 | map_dma_buf() operation can return -EINTR if it is interrupted by a signal. | |
143 | ||
144 | ||
145 | 5. When finished, the buffer-user notifies end-of-DMA to exporter | |
146 | ||
147 | Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to | |
148 | the exporter using the dma_buf_unmap_attachment API. | |
149 | ||
150 | Interface: | |
151 | void dma_buf_unmap_attachment(struct dma_buf_attachment *, | |
152 | struct sg_table *); | |
153 | ||
154 | This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the | |
155 | "dma_buf->ops->" indirection from the users of this interface. | |
156 | ||
157 | In struct dma_buf_ops, unmap_dma_buf is defined as | |
158 | void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *); | |
159 | ||
160 | unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like | |
161 | map_dma_buf, this API also must be implemented by the exporter. | |
162 | ||
163 | ||
164 | 6. when buffer-user is done using this buffer, it 'disconnects' itself from the | |
165 | buffer. | |
166 | ||
167 | After the buffer-user has no more interest in using this buffer, it should | |
168 | disconnect itself from the buffer: | |
169 | ||
170 | - it first detaches itself from the buffer. | |
171 | ||
172 | Interface: | |
173 | void dma_buf_detach(struct dma_buf *dmabuf, | |
174 | struct dma_buf_attachment *dmabuf_attach); | |
175 | ||
176 | This API removes the attachment from the list in dmabuf, and optionally calls | |
177 | dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits. | |
178 | ||
179 | - Then, the buffer-user returns the buffer reference to exporter. | |
180 | ||
181 | Interface: | |
182 | void dma_buf_put(struct dma_buf *dmabuf); | |
183 | ||
184 | This API then reduces the refcount for this buffer. | |
185 | ||
186 | If, as a result of this call, the refcount becomes 0, the 'release' file | |
187 | operation related to this fd is called. It calls the dmabuf->ops->release() | |
188 | operation in turn, and frees the memory allocated for dmabuf when exported. | |
189 | ||
190 | NOTES: | |
191 | - Importance of attach-detach and {map,unmap}_dma_buf operation pairs | |
192 | The attach-detach calls allow the exporter to figure out backing-storage | |
193 | constraints for the currently-interested devices. This allows preferential | |
194 | allocation, and/or migration of pages across different types of storage | |
195 | available, if possible. | |
196 | ||
197 | Bracketing of DMA access with {map,unmap}_dma_buf operations is essential | |
198 | to allow just-in-time backing of storage, and migration mid-way through a | |
199 | use-case. | |
200 | ||
201 | - Migration of backing storage if needed | |
202 | If after | |
203 | - at least one map_dma_buf has happened, | |
204 | - and the backing storage has been allocated for this buffer, | |
205 | another new buffer-user intends to attach itself to this buffer, it might | |
206 | be allowed, if possible for the exporter. | |
207 | ||
208 | In case it is allowed by the exporter: | |
209 | if the new buffer-user has stricter 'backing-storage constraints', and the | |
210 | exporter can handle these constraints, the exporter can just stall on the | |
211 | map_dma_buf until all outstanding access is completed (as signalled by | |
212 | unmap_dma_buf). | |
213 | Once all users have finished accessing and have unmapped this buffer, the | |
214 | exporter could potentially move the buffer to the stricter backing-storage, | |
215 | and then allow further {map,unmap}_dma_buf operations from any buffer-user | |
216 | from the migrated backing-storage. | |
217 | ||
218 | If the exporter cannot fulfil the backing-storage constraints of the new | |
219 | buffer-user device as requested, dma_buf_attach() would return an error to | |
220 | denote non-compatibility of the new buffer-sharing request with the current | |
221 | buffer. | |
222 | ||
223 | If the exporter chooses not to allow an attach() operation once a | |
224 | map_dma_buf() API has been called, it simply returns an error. | |
225 | ||
b0b40f24 DV |
226 | Kernel cpu access to a dma-buf buffer object |
227 | -------------------------------------------- | |
228 | ||
229 | The motivation to allow cpu access from the kernel to a dma-buf object from the | |
230 | importers side are: | |
231 | - fallback operations, e.g. if the devices is connected to a usb bus and the | |
232 | kernel needs to shuffle the data around first before sending it away. | |
233 | - full transparency for existing users on the importer side, i.e. userspace | |
234 | should not notice the difference between a normal object from that subsystem | |
235 | and an imported one backed by a dma-buf. This is really important for drm | |
236 | opengl drivers that expect to still use all the existing upload/download | |
237 | paths. | |
238 | ||
239 | Access to a dma_buf from the kernel context involves three steps: | |
240 | ||
241 | 1. Prepare access, which invalidate any necessary caches and make the object | |
242 | available for cpu access. | |
243 | 2. Access the object page-by-page with the dma_buf map apis | |
244 | 3. Finish access, which will flush any necessary cpu caches and free reserved | |
245 | resources. | |
246 | ||
247 | 1. Prepare access | |
248 | ||
249 | Before an importer can access a dma_buf object with the cpu from the kernel | |
250 | context, it needs to notify the exporter of the access that is about to | |
251 | happen. | |
252 | ||
253 | Interface: | |
254 | int dma_buf_begin_cpu_access(struct dma_buf *dmabuf, | |
255 | size_t start, size_t len, | |
256 | enum dma_data_direction direction) | |
257 | ||
258 | This allows the exporter to ensure that the memory is actually available for | |
259 | cpu access - the exporter might need to allocate or swap-in and pin the | |
260 | backing storage. The exporter also needs to ensure that cpu access is | |
261 | coherent for the given range and access direction. The range and access | |
262 | direction can be used by the exporter to optimize the cache flushing, i.e. | |
263 | access outside of the range or with a different direction (read instead of | |
264 | write) might return stale or even bogus data (e.g. when the exporter needs to | |
265 | copy the data to temporary storage). | |
266 | ||
267 | This step might fail, e.g. in oom conditions. | |
268 | ||
269 | 2. Accessing the buffer | |
270 | ||
271 | To support dma_buf objects residing in highmem cpu access is page-based using | |
272 | an api similar to kmap. Accessing a dma_buf is done in aligned chunks of | |
273 | PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns | |
274 | a pointer in kernel virtual address space. Afterwards the chunk needs to be | |
275 | unmapped again. There is no limit on how often a given chunk can be mapped | |
276 | and unmapped, i.e. the importer does not need to call begin_cpu_access again | |
277 | before mapping the same chunk again. | |
278 | ||
279 | Interfaces: | |
280 | void *dma_buf_kmap(struct dma_buf *, unsigned long); | |
281 | void dma_buf_kunmap(struct dma_buf *, unsigned long, void *); | |
282 | ||
283 | There are also atomic variants of these interfaces. Like for kmap they | |
284 | facilitate non-blocking fast-paths. Neither the importer nor the exporter (in | |
285 | the callback) is allowed to block when using these. | |
286 | ||
287 | Interfaces: | |
288 | void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long); | |
289 | void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *); | |
290 | ||
291 | For importers all the restrictions of using kmap apply, like the limited | |
292 | supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2 | |
293 | atomic dma_buf kmaps at the same time (in any given process context). | |
294 | ||
295 | dma_buf kmap calls outside of the range specified in begin_cpu_access are | |
296 | undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on | |
297 | the partial chunks at the beginning and end but may return stale or bogus | |
298 | data outside of the range (in these partial chunks). | |
299 | ||
300 | Note that these calls need to always succeed. The exporter needs to complete | |
301 | any preparations that might fail in begin_cpu_access. | |
302 | ||
303 | 3. Finish access | |
304 | ||
305 | When the importer is done accessing the range specified in begin_cpu_access, | |
306 | it needs to announce this to the exporter (to facilitate cache flushing and | |
307 | unpinning of any pinned resources). The result of of any dma_buf kmap calls | |
308 | after end_cpu_access is undefined. | |
309 | ||
310 | Interface: | |
311 | void dma_buf_end_cpu_access(struct dma_buf *dma_buf, | |
312 | size_t start, size_t len, | |
313 | enum dma_data_direction dir); | |
314 | ||
315 | ||
316 | Miscellaneous notes | |
317 | ------------------- | |
318 | ||
08179456 SS |
319 | - Any exporters or users of the dma-buf buffer sharing framework must have |
320 | a 'select DMA_SHARED_BUFFER' in their respective Kconfigs. | |
321 | ||
fbb231e1 RC |
322 | - In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set |
323 | on the file descriptor. This is not just a resource leak, but a | |
324 | potential security hole. It could give the newly exec'd application | |
325 | access to buffers, via the leaked fd, to which it should otherwise | |
326 | not be permitted access. | |
327 | ||
328 | The problem with doing this via a separate fcntl() call, versus doing it | |
329 | atomically when the fd is created, is that this is inherently racy in a | |
330 | multi-threaded app[3]. The issue is made worse when it is library code | |
331 | opening/creating the file descriptor, as the application may not even be | |
332 | aware of the fd's. | |
333 | ||
334 | To avoid this problem, userspace must have a way to request O_CLOEXEC | |
335 | flag be set when the dma-buf fd is created. So any API provided by | |
336 | the exporting driver to create a dmabuf fd must provide a way to let | |
337 | userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd(). | |
338 | ||
a7df4719 SS |
339 | References: |
340 | [1] struct dma_buf_ops in include/linux/dma-buf.h | |
341 | [2] All interfaces mentioned above defined in include/linux/dma-buf.h | |
fbb231e1 | 342 | [3] https://lwn.net/Articles/236486/ |