linux/Documentation/dma-buf-sharing.txt
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   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
   8This document serves as a guide to device-driver writers on what is the dma-buf
   9buffer sharing API, how to use it for exporting and using shared buffers.
  10
  11Any device driver which wishes to be a part of DMA buffer sharing, can do so as
  12either the 'exporter' of buffers, or the 'user' of buffers.
  13
  14Say a driver A wants to use buffers created by driver B, then we call B as the
  15exporter, and A as buffer-user.
  16
  17The 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
  25The 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
  32dma-buf operations for device dma only
  33--------------------------------------
  34
  35The dma_buf buffer sharing API usage contains the following steps:
  36
  371. Exporter announces that it wishes to export a buffer
  382. Userspace gets the file descriptor associated with the exported buffer, and
  39   passes it around to potential buffer-users based on use case
  403. Each buffer-user 'connects' itself to the buffer
  414. When needed, buffer-user requests access to the buffer from exporter
  425. When finished with its use, the buffer-user notifies end-of-DMA to exporter
  436. when buffer-user is done using this buffer completely, it 'disconnects'
  44   itself from the buffer.
  45
  46
  471. Exporter's announcement of buffer export
  48
  49   The buffer exporter announces its wish to export a buffer. In this, it
  50   connects its own private buffer data, provides implementation for operations
  51   that can be performed on the exported dma_buf, and flags for the file
  52   associated with this buffer.
  53
  54   Interface:
  55      struct dma_buf *dma_buf_export_named(void *priv, struct dma_buf_ops *ops,
  56                                     size_t size, int flags,
  57                                     const char *exp_name)
  58
  59   If this succeeds, dma_buf_export_named 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' is the name of exporter - to facilitate information while
  65   debugging.
  66
  67   Exporting modules which do not wish to provide any specific name may use the
  68   helper define 'dma_buf_export()', with the same arguments as above, but
  69   without the last argument; a KBUILD_MODNAME pre-processor directive will be
  70   inserted in place of 'exp_name' instead.
  71
  722. Userspace gets a handle to pass around to potential buffer-users
  73
  74   Userspace entity requests for a file-descriptor (fd) which is a handle to the
  75   anonymous file associated with the buffer. It can then share the fd with other
  76   drivers and/or processes.
  77
  78   Interface:
  79      int dma_buf_fd(struct dma_buf *dmabuf, int flags)
  80
  81   This API installs an fd for the anonymous file associated with this buffer;
  82   returns either 'fd', or error.
  83
  843. Each buffer-user 'connects' itself to the buffer
  85
  86   Each buffer-user now gets a reference to the buffer, using the fd passed to
  87   it.
  88
  89   Interface:
  90      struct dma_buf *dma_buf_get(int fd)
  91
  92   This API will return a reference to the dma_buf, and increment refcount for
  93   it.
  94
  95   After this, the buffer-user needs to attach its device with the buffer, which
  96   helps the exporter to know of device buffer constraints.
  97
  98   Interface:
  99      struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
 100                                                struct device *dev)
 101
 102   This API returns reference to an attachment structure, which is then used
 103   for scatterlist operations. It will optionally call the 'attach' dma_buf
 104   operation, if provided by the exporter.
 105
 106   The dma-buf sharing framework does the bookkeeping bits related to managing
 107   the list of all attachments to a buffer.
 108
 109Until this stage, the buffer-exporter has the option to choose not to actually
 110allocate the backing storage for this buffer, but wait for the first buffer-user
 111to request use of buffer for allocation.
 112
 113
 1144. When needed, buffer-user requests access to the buffer
 115
 116   Whenever a buffer-user wants to use the buffer for any DMA, it asks for
 117   access to the buffer using dma_buf_map_attachment API. At least one attach to
 118   the buffer must have happened before map_dma_buf can be called.
 119
 120   Interface:
 121      struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *,
 122                                         enum dma_data_direction);
 123
 124   This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the
 125   "dma_buf->ops->" indirection from the users of this interface.
 126
 127   In struct dma_buf_ops, map_dma_buf is defined as
 128      struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
 129                                                enum dma_data_direction);
 130
 131   It is one of the buffer operations that must be implemented by the exporter.
 132   It should return the sg_table containing scatterlist for this buffer, mapped
 133   into caller's address space.
 134
 135   If this is being called for the first time, the exporter can now choose to
 136   scan through the list of attachments for this buffer, collate the requirements
 137   of the attached devices, and choose an appropriate backing storage for the
 138   buffer.
 139
 140   Based on enum dma_data_direction, it might be possible to have multiple users
 141   accessing at the same time (for reading, maybe), or any other kind of sharing
 142   that the exporter might wish to make available to buffer-users.
 143
 144   map_dma_buf() operation can return -EINTR if it is interrupted by a signal.
 145
 146
 1475. When finished, the buffer-user notifies end-of-DMA to exporter
 148
 149   Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to
 150   the exporter using the dma_buf_unmap_attachment API.
 151
 152   Interface:
 153      void dma_buf_unmap_attachment(struct dma_buf_attachment *,
 154                                    struct sg_table *);
 155
 156   This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the
 157   "dma_buf->ops->" indirection from the users of this interface.
 158
 159   In struct dma_buf_ops, unmap_dma_buf is defined as
 160      void (*unmap_dma_buf)(struct dma_buf_attachment *,
 161                            struct sg_table *,
 162                            enum dma_data_direction);
 163
 164   unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like
 165   map_dma_buf, this API also must be implemented by the exporter.
 166
 167
 1686. when buffer-user is done using this buffer, it 'disconnects' itself from the
 169   buffer.
 170
 171   After the buffer-user has no more interest in using this buffer, it should
 172   disconnect itself from the buffer:
 173
 174   - it first detaches itself from the buffer.
 175
 176   Interface:
 177      void dma_buf_detach(struct dma_buf *dmabuf,
 178                          struct dma_buf_attachment *dmabuf_attach);
 179
 180   This API removes the attachment from the list in dmabuf, and optionally calls
 181   dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits.
 182
 183   - Then, the buffer-user returns the buffer reference to exporter.
 184
 185   Interface:
 186     void dma_buf_put(struct dma_buf *dmabuf);
 187
 188   This API then reduces the refcount for this buffer.
 189
 190   If, as a result of this call, the refcount becomes 0, the 'release' file
 191   operation related to this fd is called. It calls the dmabuf->ops->release()
 192   operation in turn, and frees the memory allocated for dmabuf when exported.
 193
 194NOTES:
 195- Importance of attach-detach and {map,unmap}_dma_buf operation pairs
 196   The attach-detach calls allow the exporter to figure out backing-storage
 197   constraints for the currently-interested devices. This allows preferential
 198   allocation, and/or migration of pages across different types of storage
 199   available, if possible.
 200
 201   Bracketing of DMA access with {map,unmap}_dma_buf operations is essential
 202   to allow just-in-time backing of storage, and migration mid-way through a
 203   use-case.
 204
 205- Migration of backing storage if needed
 206   If after
 207   - at least one map_dma_buf has happened,
 208   - and the backing storage has been allocated for this buffer,
 209   another new buffer-user intends to attach itself to this buffer, it might
 210   be allowed, if possible for the exporter.
 211
 212   In case it is allowed by the exporter:
 213    if the new buffer-user has stricter 'backing-storage constraints', and the
 214    exporter can handle these constraints, the exporter can just stall on the
 215    map_dma_buf until all outstanding access is completed (as signalled by
 216    unmap_dma_buf).
 217    Once all users have finished accessing and have unmapped this buffer, the
 218    exporter could potentially move the buffer to the stricter backing-storage,
 219    and then allow further {map,unmap}_dma_buf operations from any buffer-user
 220    from the migrated backing-storage.
 221
 222   If the exporter cannot fulfill the backing-storage constraints of the new
 223   buffer-user device as requested, dma_buf_attach() would return an error to
 224   denote non-compatibility of the new buffer-sharing request with the current
 225   buffer.
 226
 227   If the exporter chooses not to allow an attach() operation once a
 228   map_dma_buf() API has been called, it simply returns an error.
 229
 230Kernel cpu access to a dma-buf buffer object
 231--------------------------------------------
 232
 233The motivation to allow cpu access from the kernel to a dma-buf object from the
 234importers side are:
 235- fallback operations, e.g. if the devices is connected to a usb bus and the
 236  kernel needs to shuffle the data around first before sending it away.
 237- full transparency for existing users on the importer side, i.e. userspace
 238  should not notice the difference between a normal object from that subsystem
 239  and an imported one backed by a dma-buf. This is really important for drm
 240  opengl drivers that expect to still use all the existing upload/download
 241  paths.
 242
 243Access to a dma_buf from the kernel context involves three steps:
 244
 2451. Prepare access, which invalidate any necessary caches and make the object
 246   available for cpu access.
 2472. Access the object page-by-page with the dma_buf map apis
 2483. Finish access, which will flush any necessary cpu caches and free reserved
 249   resources.
 250
 2511. Prepare access
 252
 253   Before an importer can access a dma_buf object with the cpu from the kernel
 254   context, it needs to notify the exporter of the access that is about to
 255   happen.
 256
 257   Interface:
 258      int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
 259                                   size_t start, size_t len,
 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 given range and access direction. The range and access
 266   direction can be used by the exporter to optimize the cache flushing, i.e.
 267   access outside of the range or with a different direction (read instead of
 268   write) might return stale or even bogus data (e.g. when the exporter needs to
 269   copy the data to temporary storage).
 270
 271   This step might fail, e.g. in oom conditions.
 272
 2732. Accessing the buffer
 274
 275   To support dma_buf objects residing in highmem cpu access is page-based using
 276   an api similar to kmap. Accessing a dma_buf is done in aligned chunks of
 277   PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns
 278   a pointer in kernel virtual address space. Afterwards the chunk needs to be
 279   unmapped again. There is no limit on how often a given chunk can be mapped
 280   and unmapped, i.e. the importer does not need to call begin_cpu_access again
 281   before mapping the same chunk again.
 282
 283   Interfaces:
 284      void *dma_buf_kmap(struct dma_buf *, unsigned long);
 285      void dma_buf_kunmap(struct dma_buf *, unsigned long, void *);
 286
 287   There are also atomic variants of these interfaces. Like for kmap they
 288   facilitate non-blocking fast-paths. Neither the importer nor the exporter (in
 289   the callback) is allowed to block when using these.
 290
 291   Interfaces:
 292      void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long);
 293      void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *);
 294
 295   For importers all the restrictions of using kmap apply, like the limited
 296   supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2
 297   atomic dma_buf kmaps at the same time (in any given process context).
 298
 299   dma_buf kmap calls outside of the range specified in begin_cpu_access are
 300   undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on
 301   the partial chunks at the beginning and end but may return stale or bogus
 302   data outside of the range (in these partial chunks).
 303
 304   Note that these calls need to always succeed. The exporter needs to complete
 305   any preparations that might fail in begin_cpu_access.
 306
 307   For some cases the overhead of kmap can be too high, a vmap interface
 308   is introduced. This interface should be used very carefully, as vmalloc
 309   space is a limited resources on many architectures.
 310
 311   Interfaces:
 312      void *dma_buf_vmap(struct dma_buf *dmabuf)
 313      void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
 314
 315   The vmap call can fail if there is no vmap support in the exporter, or if it
 316   runs out of vmalloc space. Fallback to kmap should be implemented. Note that
 317   the dma-buf layer keeps a reference count for all vmap access and calls down
 318   into the exporter's vmap function only when no vmapping exists, and only
 319   unmaps it once. Protection against concurrent vmap/vunmap calls is provided
 320   by taking the dma_buf->lock mutex.
 321
 3223. Finish access
 323
 324   When the importer is done accessing the range specified in begin_cpu_access,
 325   it needs to announce this to the exporter (to facilitate cache flushing and
 326   unpinning of any pinned resources). The result of any dma_buf kmap calls
 327   after end_cpu_access is undefined.
 328
 329   Interface:
 330      void dma_buf_end_cpu_access(struct dma_buf *dma_buf,
 331                                  size_t start, size_t len,
 332                                  enum dma_data_direction dir);
 333
 334
 335Direct Userspace Access/mmap Support
 336------------------------------------
 337
 338Being able to mmap an export dma-buf buffer object has 2 main use-cases:
 339- CPU fallback processing in a pipeline and
 340- supporting existing mmap interfaces in importers.
 341
 3421. CPU fallback processing in a pipeline
 343
 344   In many processing pipelines it is sometimes required that the cpu can access
 345   the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
 346   the need to handle this specially in userspace frameworks for buffer sharing
 347   it's ideal if the dma_buf fd itself can be used to access the backing storage
 348   from userspace using mmap.
 349
 350   Furthermore Android's ION framework already supports this (and is otherwise
 351   rather similar to dma-buf from a userspace consumer side with using fds as
 352   handles, too). So it's beneficial to support this in a similar fashion on
 353   dma-buf to have a good transition path for existing Android userspace.
 354
 355   No special interfaces, userspace simply calls mmap on the dma-buf fd.
 356
 3572. Supporting existing mmap interfaces in importers
 358
 359   Similar to the motivation for kernel cpu access it is again important that
 360   the userspace code of a given importing subsystem can use the same interfaces
 361   with a imported dma-buf buffer object as with a native buffer object. This is
 362   especially important for drm where the userspace part of contemporary OpenGL,
 363   X, and other drivers is huge, and reworking them to use a different way to
 364   mmap a buffer rather invasive.
 365
 366   The assumption in the current dma-buf interfaces is that redirecting the
 367   initial mmap is all that's needed. A survey of some of the existing
 368   subsystems shows that no driver seems to do any nefarious thing like syncing
 369   up with outstanding asynchronous processing on the device or allocating
 370   special resources at fault time. So hopefully this is good enough, since
 371   adding interfaces to intercept pagefaults and allow pte shootdowns would
 372   increase the complexity quite a bit.
 373
 374   Interface:
 375      int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
 376                       unsigned long);
 377
 378   If the importing subsystem simply provides a special-purpose mmap call to set
 379   up a mapping in userspace, calling do_mmap with dma_buf->file will equally
 380   achieve that for a dma-buf object.
 381
 3823. Implementation notes for exporters
 383
 384   Because dma-buf buffers have invariant size over their lifetime, the dma-buf
 385   core checks whether a vma is too large and rejects such mappings. The
 386   exporter hence does not need to duplicate this check.
 387
 388   Because existing importing subsystems might presume coherent mappings for
 389   userspace, the exporter needs to set up a coherent mapping. If that's not
 390   possible, it needs to fake coherency by manually shooting down ptes when
 391   leaving the cpu domain and flushing caches at fault time. Note that all the
 392   dma_buf files share the same anon inode, hence the exporter needs to replace
 393   the dma_buf file stored in vma->vm_file with it's own if pte shootdown is
 394   required. This is because the kernel uses the underlying inode's address_space
 395   for vma tracking (and hence pte tracking at shootdown time with
 396   unmap_mapping_range).
 397
 398   If the above shootdown dance turns out to be too expensive in certain
 399   scenarios, we can extend dma-buf with a more explicit cache tracking scheme
 400   for userspace mappings. But the current assumption is that using mmap is
 401   always a slower path, so some inefficiencies should be acceptable.
 402
 403   Exporters that shoot down mappings (for any reasons) shall not do any
 404   synchronization at fault time with outstanding device operations.
 405   Synchronization is an orthogonal issue to sharing the backing storage of a
 406   buffer and hence should not be handled by dma-buf itself. This is explicitly
 407   mentioned here because many people seem to want something like this, but if
 408   different exporters handle this differently, buffer sharing can fail in
 409   interesting ways depending upong the exporter (if userspace starts depending
 410   upon this implicit synchronization).
 411
 412Other Interfaces Exposed to Userspace on the dma-buf FD
 413------------------------------------------------------
 414
 415- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
 416  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
 417  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
 418  llseek operation will report -EINVAL.
 419
 420  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
 421  cases. Userspace can use this to detect support for discovering the dma-buf
 422  size using llseek.
 423
 424Miscellaneous notes
 425-------------------
 426
 427- Any exporters or users of the dma-buf buffer sharing framework must have
 428  a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
 429
 430- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
 431  on the file descriptor.  This is not just a resource leak, but a
 432  potential security hole.  It could give the newly exec'd application
 433  access to buffers, via the leaked fd, to which it should otherwise
 434  not be permitted access.
 435
 436  The problem with doing this via a separate fcntl() call, versus doing it
 437  atomically when the fd is created, is that this is inherently racy in a
 438  multi-threaded app[3].  The issue is made worse when it is library code
 439  opening/creating the file descriptor, as the application may not even be
 440  aware of the fd's.
 441
 442  To avoid this problem, userspace must have a way to request O_CLOEXEC
 443  flag be set when the dma-buf fd is created.  So any API provided by
 444  the exporting driver to create a dmabuf fd must provide a way to let
 445  userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
 446
 447- If an exporter needs to manually flush caches and hence needs to fake
 448  coherency for mmap support, it needs to be able to zap all the ptes pointing
 449  at the backing storage. Now linux mm needs a struct address_space associated
 450  with the struct file stored in vma->vm_file to do that with the function
 451  unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd
 452  with the anon_file struct file, i.e. all dma_bufs share the same file.
 453
 454  Hence exporters need to setup their own file (and address_space) association
 455  by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap
 456  callback. In the specific case of a gem driver the exporter could use the
 457  shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then
 458  zap ptes by unmapping the corresponding range of the struct address_space
 459  associated with their own file.
 460
 461References:
 462[1] struct dma_buf_ops in include/linux/dma-buf.h
 463[2] All interfaces mentioned above defined in include/linux/dma-buf.h
 464[3] https://lwn.net/Articles/236486/
 465
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