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Sumit Semwala7df47192011-12-26 14:53:16 +05301 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
8This document serves as a guide to device-driver writers on what is the dma-buf
9buffer sharing API, how to use it for exporting and using shared buffers.
10
11Any device driver which wishes to be a part of DMA buffer sharing, can do so as
12either the 'exporter' of buffers, or the 'user' of buffers.
13
14Say a driver A wants to use buffers created by driver B, then we call B as the
15exporter, and A as buffer-user.
16
17The 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
25The 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
Daniel Vetterb0b40f22012-03-19 00:34:27 +010032dma-buf operations for device dma only
33--------------------------------------
Sumit Semwala7df47192011-12-26 14:53:16 +053034
35The dma_buf buffer sharing API usage contains the following steps:
36
371. Exporter announces that it wishes to export a buffer
382. Userspace gets the file descriptor associated with the exported buffer, and
39 passes it around to potential buffer-users based on use case
403. Each buffer-user 'connects' itself to the buffer
414. When needed, buffer-user requests access to the buffer from exporter
425. When finished with its use, the buffer-user notifies end-of-DMA to exporter
436. when buffer-user is done using this buffer completely, it 'disconnects'
44 itself from the buffer.
45
46
471. 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:
Sumit Semwal78df9692013-03-22 18:22:16 +053055 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)
Sumit Semwala7df47192011-12-26 14:53:16 +053058
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
Sumit Semwal78df9692013-03-22 18:22:16 +053064 '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
Sumit Semwala7df47192011-12-26 14:53:16 +0530722. 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
843. 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
109Until this stage, the buffer-exporter has the option to choose not to actually
110allocate the backing storage for this buffer, but wait for the first buffer-user
111to request use of buffer for allocation.
112
113
1144. 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
1475. 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
1666. 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
192NOTES:
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
Daniel Vetterb0b40f22012-03-19 00:34:27 +0100228Kernel cpu access to a dma-buf buffer object
229--------------------------------------------
230
231The motivation to allow cpu access from the kernel to a dma-buf object from the
232importers 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
241Access to a dma_buf from the kernel context involves three steps:
242
2431. Prepare access, which invalidate any necessary caches and make the object
244 available for cpu access.
2452. Access the object page-by-page with the dma_buf map apis
2463. Finish access, which will flush any necessary cpu caches and free reserved
247 resources.
248
2491. 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
2712. 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
Dave Airlieb25b0862012-05-22 13:34:38 +0100305 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
Daniel Vetterf00b4da2012-12-20 14:14:23 +0100314 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.
Dave Airlieb25b0862012-05-22 13:34:38 +0100319
Daniel Vetterb0b40f22012-03-19 00:34:27 +01003203. 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
Masanari Iidac9f3f2d2013-07-18 01:29:12 +0900324 unpinning of any pinned resources). The result of any dma_buf kmap calls
Daniel Vetterb0b40f22012-03-19 00:34:27 +0100325 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
Daniel Vetter4c785132012-04-24 14:38:52 +0530333Direct Userspace Access/mmap Support
334------------------------------------
335
336Being 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
3401. 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
3552. 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
3803. 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
Masanari Iida4e79162a2012-11-08 21:57:35 +0900392 required. This is because the kernel uses the underlying inode's address_space
Daniel Vetter4c785132012-04-24 14:38:52 +0530393 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
Masanari Iida4e79162a2012-11-08 21:57:35 +0900404 buffer and hence should not be handled by dma-buf itself. This is explicitly
Daniel Vetter4c785132012-04-24 14:38:52 +0530405 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
Christopher James Halse Rogers19e8697b2013-09-10 11:36:45 +0530410Other 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
Daniel Vetterb0b40f22012-03-19 00:34:27 +0100422Miscellaneous notes
423-------------------
424
Sumit Semwal08179452012-01-13 15:15:05 +0530425- Any exporters or users of the dma-buf buffer sharing framework must have
426 a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
427
Rob Clarkfbb231e2012-03-19 16:42:49 -0500428- 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
Daniel Vetter4c785132012-04-24 14:38:52 +0530445- 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
Sumit Semwala7df47192011-12-26 14:53:16 +0530459References:
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
Rob Clarkfbb231e2012-03-19 16:42:49 -0500462[3] https://lwn.net/Articles/236486/