About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / dma-buf-sharing.txt

Custom Search

Based on kernel version 4.3. Page generated on 2015-11-02 12:48 EST.

1	                    DMA Buffer Sharing API Guide
2	                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4	                            Sumit Semwal
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.
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,
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.
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.
55	   Interface:
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,
62	   it returns NULL.
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.
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.
79	   Interface:
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
88	   it.
90	   Interface:
91	      struct dma_buf *dma_buf_get(int fd)
93	   This API will return a reference to the dma_buf, and increment refcount for
94	   it.
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.
99	   Interface:
100	      struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
101	                                                struct device *dev)
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.
121	   Interface:
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
139	   buffer.
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.
153	   Interface:
154	      void dma_buf_unmap_attachment(struct dma_buf_attachment *,
155	                                    struct sg_table *);
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 *,
162	                            struct sg_table *,
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
170	   buffer.
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.
177	   Interface:
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.
186	   Interface:
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.
195	NOTES:
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
204	   use-case.
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.
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.
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.
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
235	importers 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.
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
250	   resources.
252	1. Prepare access
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.
258	   Interface:
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.
284	   Interfaces:
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.
292	   Interfaces:
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.
312	   Interfaces:
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.
323	3. Finish access
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.
330	   Interface:
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.
375	   Interface:
376	      int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
377			       unsigned long);
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
423	  size using llseek.
425	Miscellaneous notes
426	-------------------
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
441	  aware of the fd's.
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.
462	References:
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/
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.