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