1 DMA Buffer Sharing API Guide
2 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
5 <sumit dot semwal at linaro dot org>
6 <sumit dot semwal at ti dot com>
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.
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.
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.
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
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
32 dma-buf operations for device dma only
33 --------------------------------------
35 The dma_buf buffer sharing API usage contains the following steps:
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.
47 1. Exporter's announcement of buffer export
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.
56 DEFINE_DMA_BUF_EXPORT_INFO(exp_info)
57 struct dma_buf *dma_buf_export(struct dma_buf_export_info *exp_info)
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,
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
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.
73 2. Userspace gets a handle to pass around to potential buffer-users
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.
80 int dma_buf_fd(struct dma_buf *dmabuf, int flags)
82 This API installs an fd for the anonymous file associated with this buffer;
83 returns either 'fd', or error.
85 3. Each buffer-user 'connects' itself to the buffer
87 Each buffer-user now gets a reference to the buffer, using the fd passed to
91 struct dma_buf *dma_buf_get(int fd)
93 This API will return a reference to the dma_buf, and increment refcount for
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.
100 struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
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.
107 The dma-buf sharing framework does the bookkeeping bits related to managing
108 the list of all attachments to a buffer.
110 Until this stage, the buffer-exporter has the option to choose not to actually
111 allocate the backing storage for this buffer, but wait for the first buffer-user
112 to request use of buffer for allocation.
115 4. When needed, buffer-user requests access to the buffer
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.
122 struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *,
123 enum dma_data_direction);
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.
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);
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.
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
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.
145 map_dma_buf() operation can return -EINTR if it is interrupted by a signal.
148 5. When finished, the buffer-user notifies end-of-DMA to exporter
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.
154 void dma_buf_unmap_attachment(struct dma_buf_attachment *,
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.
160 In struct dma_buf_ops, unmap_dma_buf is defined as
161 void (*unmap_dma_buf)(struct dma_buf_attachment *,
163 enum dma_data_direction);
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.
169 6. when buffer-user is done using this buffer, it 'disconnects' itself from the
172 After the buffer-user has no more interest in using this buffer, it should
173 disconnect itself from the buffer:
175 - it first detaches itself from the buffer.
178 void dma_buf_detach(struct dma_buf *dmabuf,
179 struct dma_buf_attachment *dmabuf_attach);
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.
184 - Then, the buffer-user returns the buffer reference to exporter.
187 void dma_buf_put(struct dma_buf *dmabuf);
189 This API then reduces the refcount for this buffer.
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.
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.
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
206 - Migration of backing storage if needed
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.
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
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.
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
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.
231 Kernel cpu access to a dma-buf buffer object
232 --------------------------------------------
234 The motivation to allow cpu access from the kernel to a dma-buf object from the
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
244 Access to a dma_buf from the kernel context involves three steps:
246 1. Prepare access, which invalidate any necessary caches and make the object
247 available for cpu access.
248 2. Access the object page-by-page with the dma_buf map apis
249 3. Finish access, which will flush any necessary cpu caches and free reserved
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
259 int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
260 size_t start, size_t len,
261 enum dma_data_direction direction)
263 This allows the exporter to ensure that the memory is actually available for
264 cpu access - the exporter might need to allocate or swap-in and pin the
265 backing storage. The exporter also needs to ensure that cpu access is
266 coherent for the given range and access direction. The range and access
267 direction can be used by the exporter to optimize the cache flushing, i.e.
268 access outside of the range or with a different direction (read instead of
269 write) might return stale or even bogus data (e.g. when the exporter needs to
270 copy the data to temporary storage).
272 This step might fail, e.g. in oom conditions.
274 2. Accessing the buffer
276 To support dma_buf objects residing in highmem cpu access is page-based using
277 an api similar to kmap. Accessing a dma_buf is done in aligned chunks of
278 PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns
279 a pointer in kernel virtual address space. Afterwards the chunk needs to be
280 unmapped again. There is no limit on how often a given chunk can be mapped
281 and unmapped, i.e. the importer does not need to call begin_cpu_access again
282 before mapping the same chunk again.
285 void *dma_buf_kmap(struct dma_buf *, unsigned long);
286 void dma_buf_kunmap(struct dma_buf *, unsigned long, void *);
288 There are also atomic variants of these interfaces. Like for kmap they
289 facilitate non-blocking fast-paths. Neither the importer nor the exporter (in
290 the callback) is allowed to block when using these.
293 void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long);
294 void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *);
296 For importers all the restrictions of using kmap apply, like the limited
297 supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2
298 atomic dma_buf kmaps at the same time (in any given process context).
300 dma_buf kmap calls outside of the range specified in begin_cpu_access are
301 undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on
302 the partial chunks at the beginning and end but may return stale or bogus
303 data outside of the range (in these partial chunks).
305 Note that these calls need to always succeed. The exporter needs to complete
306 any preparations that might fail in begin_cpu_access.
308 For some cases the overhead of kmap can be too high, a vmap interface
309 is introduced. This interface should be used very carefully, as vmalloc
310 space is a limited resources on many architectures.
313 void *dma_buf_vmap(struct dma_buf *dmabuf)
314 void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
316 The vmap call can fail if there is no vmap support in the exporter, or if it
317 runs out of vmalloc space. Fallback to kmap should be implemented. Note that
318 the dma-buf layer keeps a reference count for all vmap access and calls down
319 into the exporter's vmap function only when no vmapping exists, and only
320 unmaps it once. Protection against concurrent vmap/vunmap calls is provided
321 by taking the dma_buf->lock mutex.
325 When the importer is done accessing the range specified in begin_cpu_access,
326 it needs to announce this to the exporter (to facilitate cache flushing and
327 unpinning of any pinned resources). The result of any dma_buf kmap calls
328 after end_cpu_access is undefined.
331 void dma_buf_end_cpu_access(struct dma_buf *dma_buf,
332 size_t start, size_t len,
333 enum dma_data_direction dir);
336 Direct Userspace Access/mmap Support
337 ------------------------------------
339 Being able to mmap an export dma-buf buffer object has 2 main use-cases:
340 - CPU fallback processing in a pipeline and
341 - supporting existing mmap interfaces in importers.
343 1. CPU fallback processing in a pipeline
345 In many processing pipelines it is sometimes required that the cpu can access
346 the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
347 the need to handle this specially in userspace frameworks for buffer sharing
348 it's ideal if the dma_buf fd itself can be used to access the backing storage
349 from userspace using mmap.
351 Furthermore Android's ION framework already supports this (and is otherwise
352 rather similar to dma-buf from a userspace consumer side with using fds as
353 handles, too). So it's beneficial to support this in a similar fashion on
354 dma-buf to have a good transition path for existing Android userspace.
356 No special interfaces, userspace simply calls mmap on the dma-buf fd.
358 2. Supporting existing mmap interfaces in importers
360 Similar to the motivation for kernel cpu access it is again important that
361 the userspace code of a given importing subsystem can use the same interfaces
362 with a imported dma-buf buffer object as with a native buffer object. This is
363 especially important for drm where the userspace part of contemporary OpenGL,
364 X, and other drivers is huge, and reworking them to use a different way to
365 mmap a buffer rather invasive.
367 The assumption in the current dma-buf interfaces is that redirecting the
368 initial mmap is all that's needed. A survey of some of the existing
369 subsystems shows that no driver seems to do any nefarious thing like syncing
370 up with outstanding asynchronous processing on the device or allocating
371 special resources at fault time. So hopefully this is good enough, since
372 adding interfaces to intercept pagefaults and allow pte shootdowns would
373 increase the complexity quite a bit.
376 int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
379 If the importing subsystem simply provides a special-purpose mmap call to set
380 up a mapping in userspace, calling do_mmap with dma_buf->file will equally
381 achieve that for a dma-buf object.
383 3. Implementation notes for exporters
385 Because dma-buf buffers have invariant size over their lifetime, the dma-buf
386 core checks whether a vma is too large and rejects such mappings. The
387 exporter hence does not need to duplicate this check.
389 Because existing importing subsystems might presume coherent mappings for
390 userspace, the exporter needs to set up a coherent mapping. If that's not
391 possible, it needs to fake coherency by manually shooting down ptes when
392 leaving the cpu domain and flushing caches at fault time. Note that all the
393 dma_buf files share the same anon inode, hence the exporter needs to replace
394 the dma_buf file stored in vma->vm_file with it's own if pte shootdown is
395 required. This is because the kernel uses the underlying inode's address_space
396 for vma tracking (and hence pte tracking at shootdown time with
397 unmap_mapping_range).
399 If the above shootdown dance turns out to be too expensive in certain
400 scenarios, we can extend dma-buf with a more explicit cache tracking scheme
401 for userspace mappings. But the current assumption is that using mmap is
402 always a slower path, so some inefficiencies should be acceptable.
404 Exporters that shoot down mappings (for any reasons) shall not do any
405 synchronization at fault time with outstanding device operations.
406 Synchronization is an orthogonal issue to sharing the backing storage of a
407 buffer and hence should not be handled by dma-buf itself. This is explicitly
408 mentioned here because many people seem to want something like this, but if
409 different exporters handle this differently, buffer sharing can fail in
410 interesting ways depending upong the exporter (if userspace starts depending
411 upon this implicit synchronization).
413 Other Interfaces Exposed to Userspace on the dma-buf FD
414 ------------------------------------------------------
416 - Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
417 with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
418 the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
419 llseek operation will report -EINVAL.
421 If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
422 cases. Userspace can use this to detect support for discovering the dma-buf
428 - Any exporters or users of the dma-buf buffer sharing framework must have
429 a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
431 - In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
432 on the file descriptor. This is not just a resource leak, but a
433 potential security hole. It could give the newly exec'd application
434 access to buffers, via the leaked fd, to which it should otherwise
435 not be permitted access.
437 The problem with doing this via a separate fcntl() call, versus doing it
438 atomically when the fd is created, is that this is inherently racy in a
439 multi-threaded app[3]. The issue is made worse when it is library code
440 opening/creating the file descriptor, as the application may not even be
443 To avoid this problem, userspace must have a way to request O_CLOEXEC
444 flag be set when the dma-buf fd is created. So any API provided by
445 the exporting driver to create a dmabuf fd must provide a way to let
446 userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
448 - If an exporter needs to manually flush caches and hence needs to fake
449 coherency for mmap support, it needs to be able to zap all the ptes pointing
450 at the backing storage. Now linux mm needs a struct address_space associated
451 with the struct file stored in vma->vm_file to do that with the function
452 unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd
453 with the anon_file struct file, i.e. all dma_bufs share the same file.
455 Hence exporters need to setup their own file (and address_space) association
456 by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap
457 callback. In the specific case of a gem driver the exporter could use the
458 shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then
459 zap ptes by unmapping the corresponding range of the struct address_space
460 associated with their own file.
463 [1] struct dma_buf_ops in include/linux/dma-buf.h
464 [2] All interfaces mentioned above defined in include/linux/dma-buf.h
465 [3] https://lwn.net/Articles/236486/