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a7df4719 SS |
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 | ||
b0b40f24 DV |
32 | dma-buf operations for device dma only |
33 | -------------------------------------- | |
a7df4719 SS |
34 | |
35 | The dma_buf buffer sharing API usage contains the following steps: | |
36 | ||
37 | 1. Exporter announces that it wishes to export a buffer | |
38 | 2. Userspace gets the file descriptor associated with the exported buffer, and | |
39 | passes it around to potential buffer-users based on use case | |
40 | 3. Each buffer-user 'connects' itself to the buffer | |
41 | 4. When needed, buffer-user requests access to the buffer from exporter | |
42 | 5. When finished with its use, the buffer-user notifies end-of-DMA to exporter | |
43 | 6. when buffer-user is done using this buffer completely, it 'disconnects' | |
44 | itself from the buffer. | |
45 | ||
46 | ||
47 | 1. Exporter's announcement of buffer export | |
48 | ||
49 | The buffer exporter announces its wish to export a buffer. In this, it | |
50 | connects its own private buffer data, provides implementation for operations | |
51 | that can be performed on the exported dma_buf, and flags for the file | |
52 | associated with this buffer. | |
53 | ||
54 | Interface: | |
78df9695 SS |
55 | struct dma_buf *dma_buf_export_named(void *priv, struct dma_buf_ops *ops, |
56 | size_t size, int flags, | |
57 | const char *exp_name) | |
a7df4719 SS |
58 | |
59 | If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a | |
60 | pointer to the same. It also associates an anonymous file with this buffer, | |
61 | so it can be exported. On failure to allocate the dma_buf object, it returns | |
62 | NULL. | |
63 | ||
78df9695 SS |
64 | 'exp_name' is the name of exporter - to facilitate information while |
65 | debugging. | |
66 | ||
67 | Exporting modules which do not wish to provide any specific name may use the | |
68 | helper define 'dma_buf_export()', with the same arguments as above, but | |
69 | without the last argument; a __FILE__ pre-processor directive will be | |
70 | inserted in place of 'exp_name' instead. | |
71 | ||
a7df4719 SS |
72 | 2. Userspace gets a handle to pass around to potential buffer-users |
73 | ||
74 | Userspace entity requests for a file-descriptor (fd) which is a handle to the | |
75 | anonymous file associated with the buffer. It can then share the fd with other | |
76 | drivers and/or processes. | |
77 | ||
78 | Interface: | |
79 | int dma_buf_fd(struct dma_buf *dmabuf) | |
80 | ||
81 | This API installs an fd for the anonymous file associated with this buffer; | |
82 | returns either 'fd', or error. | |
83 | ||
84 | 3. Each buffer-user 'connects' itself to the buffer | |
85 | ||
86 | Each buffer-user now gets a reference to the buffer, using the fd passed to | |
87 | it. | |
88 | ||
89 | Interface: | |
90 | struct dma_buf *dma_buf_get(int fd) | |
91 | ||
92 | This API will return a reference to the dma_buf, and increment refcount for | |
93 | it. | |
94 | ||
95 | After this, the buffer-user needs to attach its device with the buffer, which | |
96 | helps the exporter to know of device buffer constraints. | |
97 | ||
98 | Interface: | |
99 | struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf, | |
100 | struct device *dev) | |
101 | ||
102 | This API returns reference to an attachment structure, which is then used | |
103 | for scatterlist operations. It will optionally call the 'attach' dma_buf | |
104 | operation, if provided by the exporter. | |
105 | ||
106 | The dma-buf sharing framework does the bookkeeping bits related to managing | |
107 | the list of all attachments to a buffer. | |
108 | ||
109 | Until this stage, the buffer-exporter has the option to choose not to actually | |
110 | allocate the backing storage for this buffer, but wait for the first buffer-user | |
111 | to request use of buffer for allocation. | |
112 | ||
113 | ||
114 | 4. When needed, buffer-user requests access to the buffer | |
115 | ||
116 | Whenever a buffer-user wants to use the buffer for any DMA, it asks for | |
117 | access to the buffer using dma_buf_map_attachment API. At least one attach to | |
118 | the buffer must have happened before map_dma_buf can be called. | |
119 | ||
120 | Interface: | |
121 | struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *, | |
122 | enum dma_data_direction); | |
123 | ||
124 | This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the | |
125 | "dma_buf->ops->" indirection from the users of this interface. | |
126 | ||
127 | In struct dma_buf_ops, map_dma_buf is defined as | |
128 | struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *, | |
129 | enum dma_data_direction); | |
130 | ||
131 | It is one of the buffer operations that must be implemented by the exporter. | |
132 | It should return the sg_table containing scatterlist for this buffer, mapped | |
133 | into caller's address space. | |
134 | ||
135 | If this is being called for the first time, the exporter can now choose to | |
136 | scan through the list of attachments for this buffer, collate the requirements | |
137 | of the attached devices, and choose an appropriate backing storage for the | |
138 | buffer. | |
139 | ||
140 | Based on enum dma_data_direction, it might be possible to have multiple users | |
141 | accessing at the same time (for reading, maybe), or any other kind of sharing | |
142 | that the exporter might wish to make available to buffer-users. | |
143 | ||
144 | map_dma_buf() operation can return -EINTR if it is interrupted by a signal. | |
145 | ||
146 | ||
147 | 5. When finished, the buffer-user notifies end-of-DMA to exporter | |
148 | ||
149 | Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to | |
150 | the exporter using the dma_buf_unmap_attachment API. | |
151 | ||
152 | Interface: | |
153 | void dma_buf_unmap_attachment(struct dma_buf_attachment *, | |
154 | struct sg_table *); | |
155 | ||
156 | This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the | |
157 | "dma_buf->ops->" indirection from the users of this interface. | |
158 | ||
159 | In struct dma_buf_ops, unmap_dma_buf is defined as | |
160 | void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *); | |
161 | ||
162 | unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like | |
163 | map_dma_buf, this API also must be implemented by the exporter. | |
164 | ||
165 | ||
166 | 6. when buffer-user is done using this buffer, it 'disconnects' itself from the | |
167 | buffer. | |
168 | ||
169 | After the buffer-user has no more interest in using this buffer, it should | |
170 | disconnect itself from the buffer: | |
171 | ||
172 | - it first detaches itself from the buffer. | |
173 | ||
174 | Interface: | |
175 | void dma_buf_detach(struct dma_buf *dmabuf, | |
176 | struct dma_buf_attachment *dmabuf_attach); | |
177 | ||
178 | This API removes the attachment from the list in dmabuf, and optionally calls | |
179 | dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits. | |
180 | ||
181 | - Then, the buffer-user returns the buffer reference to exporter. | |
182 | ||
183 | Interface: | |
184 | void dma_buf_put(struct dma_buf *dmabuf); | |
185 | ||
186 | This API then reduces the refcount for this buffer. | |
187 | ||
188 | If, as a result of this call, the refcount becomes 0, the 'release' file | |
189 | operation related to this fd is called. It calls the dmabuf->ops->release() | |
190 | operation in turn, and frees the memory allocated for dmabuf when exported. | |
191 | ||
192 | NOTES: | |
193 | - Importance of attach-detach and {map,unmap}_dma_buf operation pairs | |
194 | The attach-detach calls allow the exporter to figure out backing-storage | |
195 | constraints for the currently-interested devices. This allows preferential | |
196 | allocation, and/or migration of pages across different types of storage | |
197 | available, if possible. | |
198 | ||
199 | Bracketing of DMA access with {map,unmap}_dma_buf operations is essential | |
200 | to allow just-in-time backing of storage, and migration mid-way through a | |
201 | use-case. | |
202 | ||
203 | - Migration of backing storage if needed | |
204 | If after | |
205 | - at least one map_dma_buf has happened, | |
206 | - and the backing storage has been allocated for this buffer, | |
207 | another new buffer-user intends to attach itself to this buffer, it might | |
208 | be allowed, if possible for the exporter. | |
209 | ||
210 | In case it is allowed by the exporter: | |
211 | if the new buffer-user has stricter 'backing-storage constraints', and the | |
212 | exporter can handle these constraints, the exporter can just stall on the | |
213 | map_dma_buf until all outstanding access is completed (as signalled by | |
214 | unmap_dma_buf). | |
215 | Once all users have finished accessing and have unmapped this buffer, the | |
216 | exporter could potentially move the buffer to the stricter backing-storage, | |
217 | and then allow further {map,unmap}_dma_buf operations from any buffer-user | |
218 | from the migrated backing-storage. | |
219 | ||
220 | If the exporter cannot fulfil the backing-storage constraints of the new | |
221 | buffer-user device as requested, dma_buf_attach() would return an error to | |
222 | denote non-compatibility of the new buffer-sharing request with the current | |
223 | buffer. | |
224 | ||
225 | If the exporter chooses not to allow an attach() operation once a | |
226 | map_dma_buf() API has been called, it simply returns an error. | |
227 | ||
b0b40f24 DV |
228 | Kernel cpu access to a dma-buf buffer object |
229 | -------------------------------------------- | |
230 | ||
231 | The motivation to allow cpu access from the kernel to a dma-buf object from the | |
232 | importers side are: | |
233 | - fallback operations, e.g. if the devices is connected to a usb bus and the | |
234 | kernel needs to shuffle the data around first before sending it away. | |
235 | - full transparency for existing users on the importer side, i.e. userspace | |
236 | should not notice the difference between a normal object from that subsystem | |
237 | and an imported one backed by a dma-buf. This is really important for drm | |
238 | opengl drivers that expect to still use all the existing upload/download | |
239 | paths. | |
240 | ||
241 | Access to a dma_buf from the kernel context involves three steps: | |
242 | ||
243 | 1. Prepare access, which invalidate any necessary caches and make the object | |
244 | available for cpu access. | |
245 | 2. Access the object page-by-page with the dma_buf map apis | |
246 | 3. Finish access, which will flush any necessary cpu caches and free reserved | |
247 | resources. | |
248 | ||
249 | 1. Prepare access | |
250 | ||
251 | Before an importer can access a dma_buf object with the cpu from the kernel | |
252 | context, it needs to notify the exporter of the access that is about to | |
253 | happen. | |
254 | ||
255 | Interface: | |
256 | int dma_buf_begin_cpu_access(struct dma_buf *dmabuf, | |
257 | size_t start, size_t len, | |
258 | enum dma_data_direction direction) | |
259 | ||
260 | This allows the exporter to ensure that the memory is actually available for | |
261 | cpu access - the exporter might need to allocate or swap-in and pin the | |
262 | backing storage. The exporter also needs to ensure that cpu access is | |
263 | coherent for the given range and access direction. The range and access | |
264 | direction can be used by the exporter to optimize the cache flushing, i.e. | |
265 | access outside of the range or with a different direction (read instead of | |
266 | write) might return stale or even bogus data (e.g. when the exporter needs to | |
267 | copy the data to temporary storage). | |
268 | ||
269 | This step might fail, e.g. in oom conditions. | |
270 | ||
271 | 2. Accessing the buffer | |
272 | ||
273 | To support dma_buf objects residing in highmem cpu access is page-based using | |
274 | an api similar to kmap. Accessing a dma_buf is done in aligned chunks of | |
275 | PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns | |
276 | a pointer in kernel virtual address space. Afterwards the chunk needs to be | |
277 | unmapped again. There is no limit on how often a given chunk can be mapped | |
278 | and unmapped, i.e. the importer does not need to call begin_cpu_access again | |
279 | before mapping the same chunk again. | |
280 | ||
281 | Interfaces: | |
282 | void *dma_buf_kmap(struct dma_buf *, unsigned long); | |
283 | void dma_buf_kunmap(struct dma_buf *, unsigned long, void *); | |
284 | ||
285 | There are also atomic variants of these interfaces. Like for kmap they | |
286 | facilitate non-blocking fast-paths. Neither the importer nor the exporter (in | |
287 | the callback) is allowed to block when using these. | |
288 | ||
289 | Interfaces: | |
290 | void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long); | |
291 | void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *); | |
292 | ||
293 | For importers all the restrictions of using kmap apply, like the limited | |
294 | supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2 | |
295 | atomic dma_buf kmaps at the same time (in any given process context). | |
296 | ||
297 | dma_buf kmap calls outside of the range specified in begin_cpu_access are | |
298 | undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on | |
299 | the partial chunks at the beginning and end but may return stale or bogus | |
300 | data outside of the range (in these partial chunks). | |
301 | ||
302 | Note that these calls need to always succeed. The exporter needs to complete | |
303 | any preparations that might fail in begin_cpu_access. | |
304 | ||
b25b086d DA |
305 | For some cases the overhead of kmap can be too high, a vmap interface |
306 | is introduced. This interface should be used very carefully, as vmalloc | |
307 | space is a limited resources on many architectures. | |
308 | ||
309 | Interfaces: | |
310 | void *dma_buf_vmap(struct dma_buf *dmabuf) | |
311 | void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr) | |
312 | ||
313 | The vmap call can fail if there is no vmap support in the exporter, or if it | |
f00b4dad DV |
314 | runs out of vmalloc space. Fallback to kmap should be implemented. Note that |
315 | the dma-buf layer keeps a reference count for all vmap access and calls down | |
316 | into the exporter's vmap function only when no vmapping exists, and only | |
317 | unmaps it once. Protection against concurrent vmap/vunmap calls is provided | |
318 | by taking the dma_buf->lock mutex. | |
b25b086d | 319 | |
b0b40f24 DV |
320 | 3. Finish access |
321 | ||
322 | When the importer is done accessing the range specified in begin_cpu_access, | |
323 | it needs to announce this to the exporter (to facilitate cache flushing and | |
c9f3f2d8 | 324 | unpinning of any pinned resources). The result of any dma_buf kmap calls |
b0b40f24 DV |
325 | after end_cpu_access is undefined. |
326 | ||
327 | Interface: | |
328 | void dma_buf_end_cpu_access(struct dma_buf *dma_buf, | |
329 | size_t start, size_t len, | |
330 | enum dma_data_direction dir); | |
331 | ||
332 | ||
4c78513e DV |
333 | Direct Userspace Access/mmap Support |
334 | ------------------------------------ | |
335 | ||
336 | Being able to mmap an export dma-buf buffer object has 2 main use-cases: | |
337 | - CPU fallback processing in a pipeline and | |
338 | - supporting existing mmap interfaces in importers. | |
339 | ||
340 | 1. CPU fallback processing in a pipeline | |
341 | ||
342 | In many processing pipelines it is sometimes required that the cpu can access | |
343 | the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid | |
344 | the need to handle this specially in userspace frameworks for buffer sharing | |
345 | it's ideal if the dma_buf fd itself can be used to access the backing storage | |
346 | from userspace using mmap. | |
347 | ||
348 | Furthermore Android's ION framework already supports this (and is otherwise | |
349 | rather similar to dma-buf from a userspace consumer side with using fds as | |
350 | handles, too). So it's beneficial to support this in a similar fashion on | |
351 | dma-buf to have a good transition path for existing Android userspace. | |
352 | ||
353 | No special interfaces, userspace simply calls mmap on the dma-buf fd. | |
354 | ||
355 | 2. Supporting existing mmap interfaces in exporters | |
356 | ||
357 | Similar to the motivation for kernel cpu access it is again important that | |
358 | the userspace code of a given importing subsystem can use the same interfaces | |
359 | with a imported dma-buf buffer object as with a native buffer object. This is | |
360 | especially important for drm where the userspace part of contemporary OpenGL, | |
361 | X, and other drivers is huge, and reworking them to use a different way to | |
362 | mmap a buffer rather invasive. | |
363 | ||
364 | The assumption in the current dma-buf interfaces is that redirecting the | |
365 | initial mmap is all that's needed. A survey of some of the existing | |
366 | subsystems shows that no driver seems to do any nefarious thing like syncing | |
367 | up with outstanding asynchronous processing on the device or allocating | |
368 | special resources at fault time. So hopefully this is good enough, since | |
369 | adding interfaces to intercept pagefaults and allow pte shootdowns would | |
370 | increase the complexity quite a bit. | |
371 | ||
372 | Interface: | |
373 | int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *, | |
374 | unsigned long); | |
375 | ||
376 | If the importing subsystem simply provides a special-purpose mmap call to set | |
377 | up a mapping in userspace, calling do_mmap with dma_buf->file will equally | |
378 | achieve that for a dma-buf object. | |
379 | ||
380 | 3. Implementation notes for exporters | |
381 | ||
382 | Because dma-buf buffers have invariant size over their lifetime, the dma-buf | |
383 | core checks whether a vma is too large and rejects such mappings. The | |
384 | exporter hence does not need to duplicate this check. | |
385 | ||
386 | Because existing importing subsystems might presume coherent mappings for | |
387 | userspace, the exporter needs to set up a coherent mapping. If that's not | |
388 | possible, it needs to fake coherency by manually shooting down ptes when | |
389 | leaving the cpu domain and flushing caches at fault time. Note that all the | |
390 | dma_buf files share the same anon inode, hence the exporter needs to replace | |
391 | the dma_buf file stored in vma->vm_file with it's own if pte shootdown is | |
4e79162a | 392 | required. This is because the kernel uses the underlying inode's address_space |
4c78513e DV |
393 | for vma tracking (and hence pte tracking at shootdown time with |
394 | unmap_mapping_range). | |
395 | ||
396 | If the above shootdown dance turns out to be too expensive in certain | |
397 | scenarios, we can extend dma-buf with a more explicit cache tracking scheme | |
398 | for userspace mappings. But the current assumption is that using mmap is | |
399 | always a slower path, so some inefficiencies should be acceptable. | |
400 | ||
401 | Exporters that shoot down mappings (for any reasons) shall not do any | |
402 | synchronization at fault time with outstanding device operations. | |
403 | Synchronization is an orthogonal issue to sharing the backing storage of a | |
4e79162a | 404 | buffer and hence should not be handled by dma-buf itself. This is explicitly |
4c78513e DV |
405 | mentioned here because many people seem to want something like this, but if |
406 | different exporters handle this differently, buffer sharing can fail in | |
407 | interesting ways depending upong the exporter (if userspace starts depending | |
408 | upon this implicit synchronization). | |
409 | ||
19e8697b CJHR |
410 | Other Interfaces Exposed to Userspace on the dma-buf FD |
411 | ------------------------------------------------------ | |
412 | ||
413 | - Since kernel 3.12 the dma-buf FD supports the llseek system call, but only | |
414 | with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow | |
415 | the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other | |
416 | llseek operation will report -EINVAL. | |
417 | ||
418 | If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all | |
419 | cases. Userspace can use this to detect support for discovering the dma-buf | |
420 | size using llseek. | |
421 | ||
b0b40f24 DV |
422 | Miscellaneous notes |
423 | ------------------- | |
424 | ||
08179456 SS |
425 | - Any exporters or users of the dma-buf buffer sharing framework must have |
426 | a 'select DMA_SHARED_BUFFER' in their respective Kconfigs. | |
427 | ||
fbb231e1 RC |
428 | - In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set |
429 | on the file descriptor. This is not just a resource leak, but a | |
430 | potential security hole. It could give the newly exec'd application | |
431 | access to buffers, via the leaked fd, to which it should otherwise | |
432 | not be permitted access. | |
433 | ||
434 | The problem with doing this via a separate fcntl() call, versus doing it | |
435 | atomically when the fd is created, is that this is inherently racy in a | |
436 | multi-threaded app[3]. The issue is made worse when it is library code | |
437 | opening/creating the file descriptor, as the application may not even be | |
438 | aware of the fd's. | |
439 | ||
440 | To avoid this problem, userspace must have a way to request O_CLOEXEC | |
441 | flag be set when the dma-buf fd is created. So any API provided by | |
442 | the exporting driver to create a dmabuf fd must provide a way to let | |
443 | userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd(). | |
444 | ||
4c78513e DV |
445 | - If an exporter needs to manually flush caches and hence needs to fake |
446 | coherency for mmap support, it needs to be able to zap all the ptes pointing | |
447 | at the backing storage. Now linux mm needs a struct address_space associated | |
448 | with the struct file stored in vma->vm_file to do that with the function | |
449 | unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd | |
450 | with the anon_file struct file, i.e. all dma_bufs share the same file. | |
451 | ||
452 | Hence exporters need to setup their own file (and address_space) association | |
453 | by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap | |
454 | callback. In the specific case of a gem driver the exporter could use the | |
455 | shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then | |
456 | zap ptes by unmapping the corresponding range of the struct address_space | |
457 | associated with their own file. | |
458 | ||
a7df4719 SS |
459 | References: |
460 | [1] struct dma_buf_ops in include/linux/dma-buf.h | |
461 | [2] All interfaces mentioned above defined in include/linux/dma-buf.h | |
fbb231e1 | 462 | [3] https://lwn.net/Articles/236486/ |