Based on kernel version 4.10.8. Page generated on 2017-04-01 14:43 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/