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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:44 EST.

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	
32	dma-buf operations for device dma only
33	--------------------------------------
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. 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	
73	2. 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	
85	3. 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	
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.
113	
114	
115	4. 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	
148	5. 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	
169	6. 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	
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.
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	
231	Kernel cpu access to a dma-buf buffer object
232	--------------------------------------------
233	
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.
243	
244	Access to a dma_buf from the kernel context involves three steps:
245	
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.
251	
252	1. 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	
272	2. 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	
321	3. 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	
333	Direct Userspace Access/mmap Support
334	------------------------------------
335	
336	Being 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	
340	1. 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	
375	2. 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	
400	3. 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	
430	Other 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	
442	Miscellaneous 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	
479	References:
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/
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