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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
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
32dma-buf operations for device dma only
33--------------------------------------
34
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. All these fields are filled in struct
53 dma_buf_export_info, defined via the DEFINE_DMA_BUF_EXPORT_INFO macro.
54
55 Interface:
56 DEFINE_DMA_BUF_EXPORT_INFO(exp_info)
57 struct dma_buf *dma_buf_export(struct dma_buf_export_info *exp_info)
58
59 If this succeeds, dma_buf_export allocates a dma_buf structure, and
60 returns a pointer to the same. It also associates an anonymous file with this
61 buffer, so it can be exported. On failure to allocate the dma_buf object,
62 it returns NULL.
63
64 'exp_name' in struct dma_buf_export_info is the name of exporter - to
65 facilitate information while debugging. It is set to KBUILD_MODNAME by
66 default, so exporters don't have to provide a specific name, if they don't
67 wish to.
68
69 DEFINE_DMA_BUF_EXPORT_INFO macro defines the struct dma_buf_export_info,
70 zeroes it out and pre-populates exp_name in it.
71
72
732. Userspace gets a handle to pass around to potential buffer-users
74
75 Userspace entity requests for a file-descriptor (fd) which is a handle to the
76 anonymous file associated with the buffer. It can then share the fd with other
77 drivers and/or processes.
78
79 Interface:
80 int dma_buf_fd(struct dma_buf *dmabuf, int flags)
81
82 This API installs an fd for the anonymous file associated with this buffer;
83 returns either 'fd', or error.
84
853. Each buffer-user 'connects' itself to the buffer
86
87 Each buffer-user now gets a reference to the buffer, using the fd passed to
88 it.
89
90 Interface:
91 struct dma_buf *dma_buf_get(int fd)
92
93 This API will return a reference to the dma_buf, and increment refcount for
94 it.
95
96 After this, the buffer-user needs to attach its device with the buffer, which
97 helps the exporter to know of device buffer constraints.
98
99 Interface:
100 struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
101 struct device *dev)
102
103 This API returns reference to an attachment structure, which is then used
104 for scatterlist operations. It will optionally call the 'attach' dma_buf
105 operation, if provided by the exporter.
106
107 The dma-buf sharing framework does the bookkeeping bits related to managing
108 the list of all attachments to a buffer.
109
110Until this stage, the buffer-exporter has the option to choose not to actually
111allocate the backing storage for this buffer, but wait for the first buffer-user
112to request use of buffer for allocation.
113
114
1154. When needed, buffer-user requests access to the buffer
116
117 Whenever a buffer-user wants to use the buffer for any DMA, it asks for
118 access to the buffer using dma_buf_map_attachment API. At least one attach to
119 the buffer must have happened before map_dma_buf can be called.
120
121 Interface:
122 struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *,
123 enum dma_data_direction);
124
125 This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the
126 "dma_buf->ops->" indirection from the users of this interface.
127
128 In struct dma_buf_ops, map_dma_buf is defined as
129 struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
130 enum dma_data_direction);
131
132 It is one of the buffer operations that must be implemented by the exporter.
133 It should return the sg_table containing scatterlist for this buffer, mapped
134 into caller's address space.
135
136 If this is being called for the first time, the exporter can now choose to
137 scan through the list of attachments for this buffer, collate the requirements
138 of the attached devices, and choose an appropriate backing storage for the
139 buffer.
140
141 Based on enum dma_data_direction, it might be possible to have multiple users
142 accessing at the same time (for reading, maybe), or any other kind of sharing
143 that the exporter might wish to make available to buffer-users.
144
145 map_dma_buf() operation can return -EINTR if it is interrupted by a signal.
146
147
1485. When finished, the buffer-user notifies end-of-DMA to exporter
149
150 Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to
151 the exporter using the dma_buf_unmap_attachment API.
152
153 Interface:
154 void dma_buf_unmap_attachment(struct dma_buf_attachment *,
155 struct sg_table *);
156
157 This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the
158 "dma_buf->ops->" indirection from the users of this interface.
159
160 In struct dma_buf_ops, unmap_dma_buf is defined as
161 void (*unmap_dma_buf)(struct dma_buf_attachment *,
162 struct sg_table *,
163 enum dma_data_direction);
164
165 unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like
166 map_dma_buf, this API also must be implemented by the exporter.
167
168
1696. when buffer-user is done using this buffer, it 'disconnects' itself from the
170 buffer.
171
172 After the buffer-user has no more interest in using this buffer, it should
173 disconnect itself from the buffer:
174
175 - it first detaches itself from the buffer.
176
177 Interface:
178 void dma_buf_detach(struct dma_buf *dmabuf,
179 struct dma_buf_attachment *dmabuf_attach);
180
181 This API removes the attachment from the list in dmabuf, and optionally calls
182 dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits.
183
184 - Then, the buffer-user returns the buffer reference to exporter.
185
186 Interface:
187 void dma_buf_put(struct dma_buf *dmabuf);
188
189 This API then reduces the refcount for this buffer.
190
191 If, as a result of this call, the refcount becomes 0, the 'release' file
192 operation related to this fd is called. It calls the dmabuf->ops->release()
193 operation in turn, and frees the memory allocated for dmabuf when exported.
194
195NOTES:
196- Importance of attach-detach and {map,unmap}_dma_buf operation pairs
197 The attach-detach calls allow the exporter to figure out backing-storage
198 constraints for the currently-interested devices. This allows preferential
199 allocation, and/or migration of pages across different types of storage
200 available, if possible.
201
202 Bracketing of DMA access with {map,unmap}_dma_buf operations is essential
203 to allow just-in-time backing of storage, and migration mid-way through a
204 use-case.
205
206- Migration of backing storage if needed
207 If after
208 - at least one map_dma_buf has happened,
209 - and the backing storage has been allocated for this buffer,
210 another new buffer-user intends to attach itself to this buffer, it might
211 be allowed, if possible for the exporter.
212
213 In case it is allowed by the exporter:
214 if the new buffer-user has stricter 'backing-storage constraints', and the
215 exporter can handle these constraints, the exporter can just stall on the
216 map_dma_buf until all outstanding access is completed (as signalled by
217 unmap_dma_buf).
218 Once all users have finished accessing and have unmapped this buffer, the
219 exporter could potentially move the buffer to the stricter backing-storage,
220 and then allow further {map,unmap}_dma_buf operations from any buffer-user
221 from the migrated backing-storage.
222
223 If the exporter cannot fulfill the backing-storage constraints of the new
224 buffer-user device as requested, dma_buf_attach() would return an error to
225 denote non-compatibility of the new buffer-sharing request with the current
226 buffer.
227
228 If the exporter chooses not to allow an attach() operation once a
229 map_dma_buf() API has been called, it simply returns an error.
230
231Kernel cpu access to a dma-buf buffer object
232--------------------------------------------
233
234The motivation to allow cpu access from the kernel to a dma-buf object from the
235importers side are:
236- fallback operations, e.g. if the devices is connected to a usb bus and the
237 kernel needs to shuffle the data around first before sending it away.
238- full transparency for existing users on the importer side, i.e. userspace
239 should not notice the difference between a normal object from that subsystem
240 and an imported one backed by a dma-buf. This is really important for drm
241 opengl drivers that expect to still use all the existing upload/download
242 paths.
243
244Access to a dma_buf from the kernel context involves three steps:
245
2461. Prepare access, which invalidate any necessary caches and make the object
247 available for cpu access.
2482. Access the object page-by-page with the dma_buf map apis
2493. Finish access, which will flush any necessary cpu caches and free reserved
250 resources.
251
2521. Prepare access
253
254 Before an importer can access a dma_buf object with the cpu from the kernel
255 context, it needs to notify the exporter of the access that is about to
256 happen.
257
258 Interface:
259 int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
260 enum dma_data_direction direction)
261
262 This allows the exporter to ensure that the memory is actually available for
263 cpu access - the exporter might need to allocate or swap-in and pin the
264 backing storage. The exporter also needs to ensure that cpu access is
265 coherent for the access direction. The direction can be used by the exporter
266 to optimize the cache flushing, i.e. access with a different direction (read
267 instead of write) might return stale or even bogus data (e.g. when the
268 exporter needs to copy the data to temporary storage).
269
270 This step might fail, e.g. in oom conditions.
271
2722. Accessing the buffer
273
274 To support dma_buf objects residing in highmem cpu access is page-based using
275 an api similar to kmap. Accessing a dma_buf is done in aligned chunks of
276 PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns
277 a pointer in kernel virtual address space. Afterwards the chunk needs to be
278 unmapped again. There is no limit on how often a given chunk can be mapped
279 and unmapped, i.e. the importer does not need to call begin_cpu_access again
280 before mapping the same chunk again.
281
282 Interfaces:
283 void *dma_buf_kmap(struct dma_buf *, unsigned long);
284 void dma_buf_kunmap(struct dma_buf *, unsigned long, void *);
285
286 There are also atomic variants of these interfaces. Like for kmap they
287 facilitate non-blocking fast-paths. Neither the importer nor the exporter (in
288 the callback) is allowed to block when using these.
289
290 Interfaces:
291 void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long);
292 void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *);
293
294 For importers all the restrictions of using kmap apply, like the limited
295 supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2
296 atomic dma_buf kmaps at the same time (in any given process context).
297
298 dma_buf kmap calls outside of the range specified in begin_cpu_access are
299 undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on
300 the partial chunks at the beginning and end but may return stale or bogus
301 data outside of the range (in these partial chunks).
302
303 Note that these calls need to always succeed. The exporter needs to complete
304 any preparations that might fail in begin_cpu_access.
305
306 For some cases the overhead of kmap can be too high, a vmap interface
307 is introduced. This interface should be used very carefully, as vmalloc
308 space is a limited resources on many architectures.
309
310 Interfaces:
311 void *dma_buf_vmap(struct dma_buf *dmabuf)
312 void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
313
314 The vmap call can fail if there is no vmap support in the exporter, or if it
315 runs out of vmalloc space. Fallback to kmap should be implemented. Note that
316 the dma-buf layer keeps a reference count for all vmap access and calls down
317 into the exporter's vmap function only when no vmapping exists, and only
318 unmaps it once. Protection against concurrent vmap/vunmap calls is provided
319 by taking the dma_buf->lock mutex.
320
3213. Finish access
322
323 When the importer is done accessing the CPU, it needs to announce this to
324 the exporter (to facilitate cache flushing and unpinning of any pinned
325 resources). The result of any dma_buf kmap calls after end_cpu_access is
326 undefined.
327
328 Interface:
329 void dma_buf_end_cpu_access(struct dma_buf *dma_buf,
330 enum dma_data_direction dir);
331
332
333Direct 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, making
354 sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always*
355 used when the access happens. Note that DMA_BUF_IOCTL_SYNC can fail with
356 -EAGAIN or -EINTR, in which case it must be restarted.
357
358 Some systems might need some sort of cache coherency management e.g. when
359 CPU and GPU domains are being accessed through dma-buf at the same time. To
360 circumvent this problem there are begin/end coherency markers, that forward
361 directly to existing dma-buf device drivers vfunc hooks. Userspace can make
362 use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence
363 would be used like following:
364 - mmap dma-buf fd
365 - for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write
366 to mmap area 3. SYNC_END ioctl. This can be repeated as often as you
367 want (with the new data being consumed by the GPU or say scanout device)
368 - munmap once you don't need the buffer any more
369
370 For correctness and optimal performance, it is always required to use
371 SYNC_START and SYNC_END before and after, respectively, when accessing the
372 mapped address. Userspace cannot rely on coherent access, even when there
373 are systems where it just works without calling these ioctls.
374
3752. Supporting existing mmap interfaces in importers
376
377 Similar to the motivation for kernel cpu access it is again important that
378 the userspace code of a given importing subsystem can use the same interfaces
379 with a imported dma-buf buffer object as with a native buffer object. This is
380 especially important for drm where the userspace part of contemporary OpenGL,
381 X, and other drivers is huge, and reworking them to use a different way to
382 mmap a buffer rather invasive.
383
384 The assumption in the current dma-buf interfaces is that redirecting the
385 initial mmap is all that's needed. A survey of some of the existing
386 subsystems shows that no driver seems to do any nefarious thing like syncing
387 up with outstanding asynchronous processing on the device or allocating
388 special resources at fault time. So hopefully this is good enough, since
389 adding interfaces to intercept pagefaults and allow pte shootdowns would
390 increase the complexity quite a bit.
391
392 Interface:
393 int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
394 unsigned long);
395
396 If the importing subsystem simply provides a special-purpose mmap call to set
397 up a mapping in userspace, calling do_mmap with dma_buf->file will equally
398 achieve that for a dma-buf object.
399
4003. Implementation notes for exporters
401
402 Because dma-buf buffers have invariant size over their lifetime, the dma-buf
403 core checks whether a vma is too large and rejects such mappings. The
404 exporter hence does not need to duplicate this check.
405
406 Because existing importing subsystems might presume coherent mappings for
407 userspace, the exporter needs to set up a coherent mapping. If that's not
408 possible, it needs to fake coherency by manually shooting down ptes when
409 leaving the cpu domain and flushing caches at fault time. Note that all the
410 dma_buf files share the same anon inode, hence the exporter needs to replace
411 the dma_buf file stored in vma->vm_file with it's own if pte shootdown is
412 required. This is because the kernel uses the underlying inode's address_space
413 for vma tracking (and hence pte tracking at shootdown time with
414 unmap_mapping_range).
415
416 If the above shootdown dance turns out to be too expensive in certain
417 scenarios, we can extend dma-buf with a more explicit cache tracking scheme
418 for userspace mappings. But the current assumption is that using mmap is
419 always a slower path, so some inefficiencies should be acceptable.
420
421 Exporters that shoot down mappings (for any reasons) shall not do any
422 synchronization at fault time with outstanding device operations.
423 Synchronization is an orthogonal issue to sharing the backing storage of a
424 buffer and hence should not be handled by dma-buf itself. This is explicitly
425 mentioned here because many people seem to want something like this, but if
426 different exporters handle this differently, buffer sharing can fail in
427 interesting ways depending upong the exporter (if userspace starts depending
428 upon this implicit synchronization).
429
430Other Interfaces Exposed to Userspace on the dma-buf FD
431------------------------------------------------------
432
433- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
434 with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
435 the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
436 llseek operation will report -EINVAL.
437
438 If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
439 cases. Userspace can use this to detect support for discovering the dma-buf
440 size using llseek.
441
442Miscellaneous notes
443-------------------
444
445- Any exporters or users of the dma-buf buffer sharing framework must have
446 a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
447
448- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
449 on the file descriptor. This is not just a resource leak, but a
450 potential security hole. It could give the newly exec'd application
451 access to buffers, via the leaked fd, to which it should otherwise
452 not be permitted access.
453
454 The problem with doing this via a separate fcntl() call, versus doing it
455 atomically when the fd is created, is that this is inherently racy in a
456 multi-threaded app[3]. The issue is made worse when it is library code
457 opening/creating the file descriptor, as the application may not even be
458 aware of the fd's.
459
460 To avoid this problem, userspace must have a way to request O_CLOEXEC
461 flag be set when the dma-buf fd is created. So any API provided by
462 the exporting driver to create a dmabuf fd must provide a way to let
463 userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
464
465- If an exporter needs to manually flush caches and hence needs to fake
466 coherency for mmap support, it needs to be able to zap all the ptes pointing
467 at the backing storage. Now linux mm needs a struct address_space associated
468 with the struct file stored in vma->vm_file to do that with the function
469 unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd
470 with the anon_file struct file, i.e. all dma_bufs share the same file.
471
472 Hence exporters need to setup their own file (and address_space) association
473 by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap
474 callback. In the specific case of a gem driver the exporter could use the
475 shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then
476 zap ptes by unmapping the corresponding range of the struct address_space
477 associated with their own file.
478
479References:
480[1] struct dma_buf_ops in include/linux/dma-buf.h
481[2] All interfaces mentioned above defined in include/linux/dma-buf.h
482[3] https://lwn.net/Articles/236486/