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a7df471 dma-buf: Documentation for buffer sharing framework
Sumit Semwal authored
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
b0b40f2 @danvet dma_buf: Add documentation for the new cpu access support
danvet authored
32 dma-buf operations for device dma only
33 --------------------------------------
a7df471 dma-buf: Documentation for buffer sharing framework
Sumit Semwal authored
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
b0b40f2 @danvet dma_buf: Add documentation for the new cpu access support
danvet authored
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
b25b086 dma-buf: add initial vmap documentation
Dave Airlie authored
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
b0b40f2 @danvet dma_buf: Add documentation for the new cpu access support
danvet authored
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
4c78513 @danvet dma-buf: mmap support
danvet authored
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
b0b40f2 @danvet dma_buf: Add documentation for the new cpu access support
danvet authored
397 Miscellaneous notes
398 -------------------
399
0817945 dma-buf: Documentation update for Kconfig select
Sumit Semwal authored
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
fbb231e dma-buf: document fd flags and O_CLOEXEC requirement
Rob Clark authored
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
4c78513 @danvet dma-buf: mmap support
danvet authored
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
a7df471 dma-buf: Documentation for buffer sharing framework
Sumit Semwal authored
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
fbb231e dma-buf: document fd flags and O_CLOEXEC requirement
Rob Clark authored
437 [3] https://lwn.net/Articles/236486/
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