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  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                                    enum dma_data_direction direction)
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).
270    This step might fail, e.g. in oom conditions.
272 2. Accessing the buffer
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.
282    Interfaces:
283       void *dma_buf_kmap(struct dma_buf *, unsigned long);
284       void dma_buf_kunmap(struct dma_buf *, unsigned long, void *);
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.
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 *);
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).
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).
303    Note that these calls need to always succeed. The exporter needs to complete
304    any preparations that might fail in begin_cpu_access.
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.
310    Interfaces:
311       void *dma_buf_vmap(struct dma_buf *dmabuf)
312       void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
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.
321 3. Finish access
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.
328    Interface:
329       void dma_buf_end_cpu_access(struct dma_buf *dma_buf,
330                                   enum dma_data_direction dir);
333 Direct Userspace Access/mmap Support
334 ------------------------------------
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.
340 1. CPU fallback processing in a pipeline
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.
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.
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.
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
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.
375 2. Supporting existing mmap interfaces in importers
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.
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.
392    Interface:
393       int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
394                        unsigned long);
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.
400 3. Implementation notes for exporters
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.
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).
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.
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).
430 Other Interfaces Exposed to Userspace on the dma-buf FD
431 ------------------------------------------------------
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.
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.
442 Miscellaneous notes
443 -------------------
445 - Any exporters or users of the dma-buf buffer sharing framework must have
446   a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
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.
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.
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().
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.
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.
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]

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