Documentation/DMA-API.txt

   1                Dynamic DMA mapping using the generic device
   2                ============================================
   3
   4         James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
   5
   6 This document describes the DMA API.  For a more gentle introduction
   7 of the API (and actual examples), see Documentation/DMA-API-HOWTO.txt.
   8
   9 This API is split into two pieces.  Part I describes the basic API.
  10 Part II describes extensions for supporting non-consistent memory
  11 machines.  Unless you know that your driver absolutely has to support
  12 non-consistent platforms (this is usually only legacy platforms) you
  13 should only use the API described in part I.
  14
  15 Part I - dma_ API
  16 -------------------------------------
  17
  18 To get the dma_ API, you must #include <linux/dma-mapping.h>.  This
  19 provides dma_addr_t and the interfaces described below.
  20
  21 A dma_addr_t can hold any valid DMA address for the platform.  It can be
  22 given to a device to use as a DMA source or target.  A CPU cannot reference
  23 a dma_addr_t directly because there may be translation between its physical
  24 address space and the DMA address space.
  25
  26 Part Ia - Using large DMA-coherent buffers
  27 ------------------------------------------
  28
  29 void *
  30 dma_alloc_coherent(struct device *dev, size_t size,
  31                              dma_addr_t *dma_handle, gfp_t flag)
  32
  33 Consistent memory is memory for which a write by either the device or
  34 the processor can immediately be read by the processor or device
  35 without having to worry about caching effects.  (You may however need
  36 to make sure to flush the processor's write buffers before telling
  37 devices to read that memory.)
  38
  39 This routine allocates a region of <size> bytes of consistent memory.
  40
  41 It returns a pointer to the allocated region (in the processor's virtual
  42 address space) or NULL if the allocation failed.
  43
  44 It also returns a <dma_handle> which may be cast to an unsigned integer the
  45 same width as the bus and given to the device as the DMA address base of
  46 the region.
  47
  48 Note: consistent memory can be expensive on some platforms, and the
  49 minimum allocation length may be as big as a page, so you should
  50 consolidate your requests for consistent memory as much as possible.
  51 The simplest way to do that is to use the dma_pool calls (see below).
  52
  53 The flag parameter (dma_alloc_coherent() only) allows the caller to
  54 specify the GFP_ flags (see kmalloc()) for the allocation (the
  55 implementation may choose to ignore flags that affect the location of
  56 the returned memory, like GFP_DMA).
  57
  58 void *
  59 dma_zalloc_coherent(struct device *dev, size_t size,
  60                              dma_addr_t *dma_handle, gfp_t flag)
  61
  62 Wraps dma_alloc_coherent() and also zeroes the returned memory if the
  63 allocation attempt succeeded.
  64
  65 void
  66 dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
  67                            dma_addr_t dma_handle)
  68
  69 Free a region of consistent memory you previously allocated.  dev,
  70 size and dma_handle must all be the same as those passed into
  71 dma_alloc_coherent().  cpu_addr must be the virtual address returned by
  72 the dma_alloc_coherent().
  73
  74 Note that unlike their sibling allocation calls, these routines
  75 may only be called with IRQs enabled.
  76
  77
  78 Part Ib - Using small DMA-coherent buffers
  79 ------------------------------------------
  80
  81 To get this part of the dma_ API, you must #include <linux/dmapool.h>
  82
  83 Many drivers need lots of small DMA-coherent memory regions for DMA
  84 descriptors or I/O buffers.  Rather than allocating in units of a page
  85 or more using dma_alloc_coherent(), you can use DMA pools.  These work
  86 much like a struct kmem_cache, except that they use the DMA-coherent allocator,
  87 not __get_free_pages().  Also, they understand common hardware constraints
  88 for alignment, like queue heads needing to be aligned on N-byte boundaries.
  89
  90
  91         struct dma_pool *
  92         dma_pool_create(const char *name, struct device *dev,
  93                         size_t size, size_t align, size_t alloc);
  94
  95 dma_pool_create() initializes a pool of DMA-coherent buffers
  96 for use with a given device.  It must be called in a context which
  97 can sleep.
  98
  99 The "name" is for diagnostics (like a struct kmem_cache name); dev and size
 100 are like what you'd pass to dma_alloc_coherent().  The device's hardware
 101 alignment requirement for this type of data is "align" (which is expressed
 102 in bytes, and must be a power of two).  If your device has no boundary
 103 crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated
 104 from this pool must not cross 4KByte boundaries.
 105
 106
 107         void *dma_pool_zalloc(struct dma_pool *pool, gfp_t mem_flags,
 108                               dma_addr_t *handle)
 109
 110 Wraps dma_pool_alloc() and also zeroes the returned memory if the
 111 allocation attempt succeeded.
 112
 113
 114         void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
 115                         dma_addr_t *dma_handle);
 116
 117 This allocates memory from the pool; the returned memory will meet the
 118 size and alignment requirements specified at creation time.  Pass
 119 GFP_ATOMIC to prevent blocking, or if it's permitted (not
 120 in_interrupt, not holding SMP locks), pass GFP_KERNEL to allow
 121 blocking.  Like dma_alloc_coherent(), this returns two values:  an
 122 address usable by the CPU, and the DMA address usable by the pool's
 123 device.
 124
 125
 126         void dma_pool_free(struct dma_pool *pool, void *vaddr,
 127                         dma_addr_t addr);
 128
 129 This puts memory back into the pool.  The pool is what was passed to
 130 dma_pool_alloc(); the CPU (vaddr) and DMA addresses are what
 131 were returned when that routine allocated the memory being freed.
 132
 133
 134         void dma_pool_destroy(struct dma_pool *pool);
 135
 136 dma_pool_destroy() frees the resources of the pool.  It must be
 137 called in a context which can sleep.  Make sure you've freed all allocated
 138 memory back to the pool before you destroy it.
 139
 140
 141 Part Ic - DMA addressing limitations
 142 ------------------------------------
 143
 144 int
 145 dma_set_mask_and_coherent(struct device *dev, u64 mask)
 146
 147 Checks to see if the mask is possible and updates the device
 148 streaming and coherent DMA mask parameters if it is.
 149
 150 Returns: 0 if successful and a negative error if not.
 151
 152 int
 153 dma_set_mask(struct device *dev, u64 mask)
 154
 155 Checks to see if the mask is possible and updates the device
 156 parameters if it is.
 157
 158 Returns: 0 if successful and a negative error if not.
 159
 160 int
 161 dma_set_coherent_mask(struct device *dev, u64 mask)
 162
 163 Checks to see if the mask is possible and updates the device
 164 parameters if it is.
 165
 166 Returns: 0 if successful and a negative error if not.
 167
 168 u64
 169 dma_get_required_mask(struct device *dev)
 170
 171 This API returns the mask that the platform requires to
 172 operate efficiently.  Usually this means the returned mask
 173 is the minimum required to cover all of memory.  Examining the
 174 required mask gives drivers with variable descriptor sizes the
 175 opportunity to use smaller descriptors as necessary.
 176
 177 Requesting the required mask does not alter the current mask.  If you
 178 wish to take advantage of it, you should issue a dma_set_mask()
 179 call to set the mask to the value returned.
 180
 181
 182 Part Id - Streaming DMA mappings
 183 --------------------------------
 184
 185 dma_addr_t
 186 dma_map_single(struct device *dev, void *cpu_addr, size_t size,
 187                       enum dma_data_direction direction)
 188
 189 Maps a piece of processor virtual memory so it can be accessed by the
 190 device and returns the DMA address of the memory.
 191
 192 The direction for both APIs may be converted freely by casting.
 193 However the dma_ API uses a strongly typed enumerator for its
 194 direction:
 195
 196 DMA_NONE                no direction (used for debugging)
 197 DMA_TO_DEVICE           data is going from the memory to the device
 198 DMA_FROM_DEVICE         data is coming from the device to the memory
 199 DMA_BIDIRECTIONAL       direction isn't known
 200
 201 Notes:  Not all memory regions in a machine can be mapped by this API.
 202 Further, contiguous kernel virtual space may not be contiguous as
 203 physical memory.  Since this API does not provide any scatter/gather
 204 capability, it will fail if the user tries to map a non-physically
 205 contiguous piece of memory.  For this reason, memory to be mapped by
 206 this API should be obtained from sources which guarantee it to be
 207 physically contiguous (like kmalloc).
 208
 209 Further, the DMA address of the memory must be within the
 210 dma_mask of the device (the dma_mask is a bit mask of the
 211 addressable region for the device, i.e., if the DMA address of
 212 the memory ANDed with the dma_mask is still equal to the DMA
 213 address, then the device can perform DMA to the memory).  To
 214 ensure that the memory allocated by kmalloc is within the dma_mask,
 215 the driver may specify various platform-dependent flags to restrict
 216 the DMA address range of the allocation (e.g., on x86, GFP_DMA
 217 guarantees to be within the first 16MB of available DMA addresses,
 218 as required by ISA devices).
 219
 220 Note also that the above constraints on physical contiguity and
 221 dma_mask may not apply if the platform has an IOMMU (a device which
 222 maps an I/O DMA address to a physical memory address).  However, to be
 223 portable, device driver writers may *not* assume that such an IOMMU
 224 exists.
 225
 226 Warnings:  Memory coherency operates at a granularity called the cache
 227 line width.  In order for memory mapped by this API to operate
 228 correctly, the mapped region must begin exactly on a cache line
 229 boundary and end exactly on one (to prevent two separately mapped
 230 regions from sharing a single cache line).  Since the cache line size
 231 may not be known at compile time, the API will not enforce this
 232 requirement.  Therefore, it is recommended that driver writers who
 233 don't take special care to determine the cache line size at run time
 234 only map virtual regions that begin and end on page boundaries (which
 235 are guaranteed also to be cache line boundaries).
 236
 237 DMA_TO_DEVICE synchronisation must be done after the last modification
 238 of the memory region by the software and before it is handed off to
 239 the device.  Once this primitive is used, memory covered by this
 240 primitive should be treated as read-only by the device.  If the device
 241 may write to it at any point, it should be DMA_BIDIRECTIONAL (see
 242 below).
 243
 244 DMA_FROM_DEVICE synchronisation must be done before the driver
 245 accesses data that may be changed by the device.  This memory should
 246 be treated as read-only by the driver.  If the driver needs to write
 247 to it at any point, it should be DMA_BIDIRECTIONAL (see below).
 248
 249 DMA_BIDIRECTIONAL requires special handling: it means that the driver
 250 isn't sure if the memory was modified before being handed off to the
 251 device and also isn't sure if the device will also modify it.  Thus,
 252 you must always sync bidirectional memory twice: once before the
 253 memory is handed off to the device (to make sure all memory changes
 254 are flushed from the processor) and once before the data may be
 255 accessed after being used by the device (to make sure any processor
 256 cache lines are updated with data that the device may have changed).
 257
 258 void
 259 dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
 260                  enum dma_data_direction direction)
 261
 262 Unmaps the region previously mapped.  All the parameters passed in
 263 must be identical to those passed in (and returned) by the mapping
 264 API.
 265
 266 dma_addr_t
 267 dma_map_page(struct device *dev, struct page *page,
 268                     unsigned long offset, size_t size,
 269                     enum dma_data_direction direction)
 270 void
 271 dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
 272                enum dma_data_direction direction)
 273
 274 API for mapping and unmapping for pages.  All the notes and warnings
 275 for the other mapping APIs apply here.  Also, although the <offset>
 276 and <size> parameters are provided to do partial page mapping, it is
 277 recommended that you never use these unless you really know what the
 278 cache width is.
 279
 280 int
 281 dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
 282
 283 In some circumstances dma_map_single() and dma_map_page() will fail to create
 284 a mapping. A driver can check for these errors by testing the returned
 285 DMA address with dma_mapping_error(). A non-zero return value means the mapping
 286 could not be created and the driver should take appropriate action (e.g.
 287 reduce current DMA mapping usage or delay and try again later).
 288
 289         int
 290         dma_map_sg(struct device *dev, struct scatterlist *sg,
 291                 int nents, enum dma_data_direction direction)
 292
 293 Returns: the number of DMA address segments mapped (this may be shorter
 294 than <nents> passed in if some elements of the scatter/gather list are
 295 physically or virtually adjacent and an IOMMU maps them with a single
 296 entry).
 297
 298 Please note that the sg cannot be mapped again if it has been mapped once.
 299 The mapping process is allowed to destroy information in the sg.
 300
 301 As with the other mapping interfaces, dma_map_sg() can fail. When it
 302 does, 0 is returned and a driver must take appropriate action. It is
 303 critical that the driver do something, in the case of a block driver
 304 aborting the request or even oopsing is better than doing nothing and
 305 corrupting the filesystem.
 306
 307 With scatterlists, you use the resulting mapping like this:
 308
 309         int i, count = dma_map_sg(dev, sglist, nents, direction);
 310         struct scatterlist *sg;
 311
 312         for_each_sg(sglist, sg, count, i) {
 313                 hw_address[i] = sg_dma_address(sg);
 314                 hw_len[i] = sg_dma_len(sg);
 315         }
 316
 317 where nents is the number of entries in the sglist.
 318
 319 The implementation is free to merge several consecutive sglist entries
 320 into one (e.g. with an IOMMU, or if several pages just happen to be
 321 physically contiguous) and returns the actual number of sg entries it
 322 mapped them to. On failure 0, is returned.
 323
 324 Then you should loop count times (note: this can be less than nents times)
 325 and use sg_dma_address() and sg_dma_len() macros where you previously
 326 accessed sg->address and sg->length as shown above.
 327
 328         void
 329         dma_unmap_sg(struct device *dev, struct scatterlist *sg,
 330                 int nents, enum dma_data_direction direction)
 331
 332 Unmap the previously mapped scatter/gather list.  All the parameters
 333 must be the same as those and passed in to the scatter/gather mapping
 334 API.
 335
 336 Note: <nents> must be the number you passed in, *not* the number of
 337 DMA address entries returned.
 338
 339 void
 340 dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size,
 341                         enum dma_data_direction direction)
 342 void
 343 dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size,
 344                            enum dma_data_direction direction)
 345 void
 346 dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nents,
 347                     enum dma_data_direction direction)
 348 void
 349 dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nents,
 350                        enum dma_data_direction direction)
 351
 352 Synchronise a single contiguous or scatter/gather mapping for the CPU
 353 and device. With the sync_sg API, all the parameters must be the same
 354 as those passed into the single mapping API. With the sync_single API,
 355 you can use dma_handle and size parameters that aren't identical to
 356 those passed into the single mapping API to do a partial sync.
 357
 358 Notes:  You must do this:
 359
 360 - Before reading values that have been written by DMA from the device
 361   (use the DMA_FROM_DEVICE direction)
 362 - After writing values that will be written to the device using DMA
 363   (use the DMA_TO_DEVICE) direction
 364 - before *and* after handing memory to the device if the memory is
 365   DMA_BIDIRECTIONAL
 366
 367 See also dma_map_single().
 368
 369 dma_addr_t
 370 dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
 371                      enum dma_data_direction dir,
 372                      struct dma_attrs *attrs)
 373
 374 void
 375 dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
 376                        size_t size, enum dma_data_direction dir,
 377                        struct dma_attrs *attrs)
 378
 379 int
 380 dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
 381                  int nents, enum dma_data_direction dir,
 382                  struct dma_attrs *attrs)
 383
 384 void
 385 dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
 386                    int nents, enum dma_data_direction dir,
 387                    struct dma_attrs *attrs)
 388
 389 The four functions above are just like the counterpart functions
 390 without the _attrs suffixes, except that they pass an optional
 391 struct dma_attrs*.
 392
 393 struct dma_attrs encapsulates a set of "DMA attributes". For the
 394 definition of struct dma_attrs see linux/dma-attrs.h.
 395
 396 The interpretation of DMA attributes is architecture-specific, and
 397 each attribute should be documented in Documentation/DMA-attributes.txt.
 398
 399 If struct dma_attrs* is NULL, the semantics of each of these
 400 functions is identical to those of the corresponding function
 401 without the _attrs suffix. As a result dma_map_single_attrs()
 402 can generally replace dma_map_single(), etc.
 403
 404 As an example of the use of the *_attrs functions, here's how
 405 you could pass an attribute DMA_ATTR_FOO when mapping memory
 406 for DMA:
 407
 408 #include <linux/dma-attrs.h>
 409 /* DMA_ATTR_FOO should be defined in linux/dma-attrs.h and
 410  * documented in Documentation/DMA-attributes.txt */
 411 ...
 412
 413         DEFINE_DMA_ATTRS(attrs);
 414         dma_set_attr(DMA_ATTR_FOO, &attrs);
 415         ....
 416         n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, &attr);
 417         ....
 418
 419 Architectures that care about DMA_ATTR_FOO would check for its
 420 presence in their implementations of the mapping and unmapping
 421 routines, e.g.:
 422
 423 void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
 424                              size_t size, enum dma_data_direction dir,
 425                              struct dma_attrs *attrs)
 426 {
 427         ....
 428         int foo =  dma_get_attr(DMA_ATTR_FOO, attrs);
 429         ....
 430         if (foo)
 431                 /* twizzle the frobnozzle */
 432         ....
 433
 434
 435 Part II - Advanced dma_ usage
 436 -----------------------------
 437
 438 Warning: These pieces of the DMA API should not be used in the
 439 majority of cases, since they cater for unlikely corner cases that
 440 don't belong in usual drivers.
 441
 442 If you don't understand how cache line coherency works between a
 443 processor and an I/O device, you should not be using this part of the
 444 API at all.
 445
 446 void *
 447 dma_alloc_noncoherent(struct device *dev, size_t size,
 448                                dma_addr_t *dma_handle, gfp_t flag)
 449
 450 Identical to dma_alloc_coherent() except that the platform will
 451 choose to return either consistent or non-consistent memory as it sees
 452 fit.  By using this API, you are guaranteeing to the platform that you
 453 have all the correct and necessary sync points for this memory in the
 454 driver should it choose to return non-consistent memory.
 455
 456 Note: where the platform can return consistent memory, it will
 457 guarantee that the sync points become nops.
 458
 459 Warning:  Handling non-consistent memory is a real pain.  You should
 460 only use this API if you positively know your driver will be
 461 required to work on one of the rare (usually non-PCI) architectures
 462 that simply cannot make consistent memory.
 463
 464 void
 465 dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
 466                               dma_addr_t dma_handle)
 467
 468 Free memory allocated by the nonconsistent API.  All parameters must
 469 be identical to those passed in (and returned by
 470 dma_alloc_noncoherent()).
 471
 472 int
 473 dma_get_cache_alignment(void)
 474
 475 Returns the processor cache alignment.  This is the absolute minimum
 476 alignment *and* width that you must observe when either mapping
 477 memory or doing partial flushes.
 478
 479 Notes: This API may return a number *larger* than the actual cache
 480 line, but it will guarantee that one or more cache lines fit exactly
 481 into the width returned by this call.  It will also always be a power
 482 of two for easy alignment.
 483
 484 void
 485 dma_cache_sync(struct device *dev, void *vaddr, size_t size,
 486                enum dma_data_direction direction)
 487
 488 Do a partial sync of memory that was allocated by
 489 dma_alloc_noncoherent(), starting at virtual address vaddr and
 490 continuing on for size.  Again, you *must* observe the cache line
 491 boundaries when doing this.
 492
 493 int
 494 dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
 495                             dma_addr_t device_addr, size_t size, int
 496                             flags)
 497
 498 Declare region of memory to be handed out by dma_alloc_coherent() when
 499 it's asked for coherent memory for this device.
 500
 501 phys_addr is the CPU physical address to which the memory is currently
 502 assigned (this will be ioremapped so the CPU can access the region).
 503
 504 device_addr is the DMA address the device needs to be programmed
 505 with to actually address this memory (this will be handed out as the
 506 dma_addr_t in dma_alloc_coherent()).
 507
 508 size is the size of the area (must be multiples of PAGE_SIZE).
 509
 510 flags can be ORed together and are:
 511
 512 DMA_MEMORY_MAP - request that the memory returned from
 513 dma_alloc_coherent() be directly writable.
 514
 515 DMA_MEMORY_IO - request that the memory returned from
 516 dma_alloc_coherent() be addressable using read()/write()/memcpy_toio() etc.
 517
 518 One or both of these flags must be present.
 519
 520 DMA_MEMORY_INCLUDES_CHILDREN - make the declared memory be allocated by
 521 dma_alloc_coherent of any child devices of this one (for memory residing
 522 on a bridge).
 523
 524 DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions.
 525 Do not allow dma_alloc_coherent() to fall back to system memory when
 526 it's out of memory in the declared region.
 527
 528 The return value will be either DMA_MEMORY_MAP or DMA_MEMORY_IO and
 529 must correspond to a passed in flag (i.e. no returning DMA_MEMORY_IO
 530 if only DMA_MEMORY_MAP were passed in) for success or zero for
 531 failure.
 532
 533 Note, for DMA_MEMORY_IO returns, all subsequent memory returned by
 534 dma_alloc_coherent() may no longer be accessed directly, but instead
 535 must be accessed using the correct bus functions.  If your driver
 536 isn't prepared to handle this contingency, it should not specify
 537 DMA_MEMORY_IO in the input flags.
 538
 539 As a simplification for the platforms, only *one* such region of
 540 memory may be declared per device.
 541
 542 For reasons of efficiency, most platforms choose to track the declared
 543 region only at the granularity of a page.  For smaller allocations,
 544 you should use the dma_pool() API.
 545
 546 void
 547 dma_release_declared_memory(struct device *dev)
 548
 549 Remove the memory region previously declared from the system.  This
 550 API performs *no* in-use checking for this region and will return
 551 unconditionally having removed all the required structures.  It is the
 552 driver's job to ensure that no parts of this memory region are
 553 currently in use.
 554
 555 void *
 556 dma_mark_declared_memory_occupied(struct device *dev,
 557                                   dma_addr_t device_addr, size_t size)
 558
 559 This is used to occupy specific regions of the declared space
 560 (dma_alloc_coherent() will hand out the first free region it finds).
 561
 562 device_addr is the *device* address of the region requested.
 563
 564 size is the size (and should be a page-sized multiple).
 565
 566 The return value will be either a pointer to the processor virtual
 567 address of the memory, or an error (via PTR_ERR()) if any part of the
 568 region is occupied.
 569
 570 Part III - Debug drivers use of the DMA-API
 571 -------------------------------------------
 572
 573 The DMA-API as described above has some constraints. DMA addresses must be
 574 released with the corresponding function with the same size for example. With
 575 the advent of hardware IOMMUs it becomes more and more important that drivers
 576 do not violate those constraints. In the worst case such a violation can
 577 result in data corruption up to destroyed filesystems.
 578
 579 To debug drivers and find bugs in the usage of the DMA-API checking code can
 580 be compiled into the kernel which will tell the developer about those
 581 violations. If your architecture supports it you can select the "Enable
 582 debugging of DMA-API usage" option in your kernel configuration. Enabling this
 583 option has a performance impact. Do not enable it in production kernels.
 584
 585 If you boot the resulting kernel will contain code which does some bookkeeping
 586 about what DMA memory was allocated for which device. If this code detects an
 587 error it prints a warning message with some details into your kernel log. An
 588 example warning message may look like this:
 589
 590 ------------[ cut here ]------------
 591 WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
 592         check_unmap+0x203/0x490()
 593 Hardware name:
 594 forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
 595         function [device address=0x00000000640444be] [size=66 bytes] [mapped as
 596 single] [unmapped as page]
 597 Modules linked in: nfsd exportfs bridge stp llc r8169
 598 Pid: 0, comm: swapper Tainted: G        W  2.6.28-dmatest-09289-g8bb99c0 #1
 599 Call Trace:
 600  <IRQ>  [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
 601  [<ffffffff80647b70>] _spin_unlock+0x10/0x30
 602  [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
 603  [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
 604  [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
 605  [<ffffffff80252f96>] queue_work+0x56/0x60
 606  [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
 607  [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
 608  [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
 609  [<ffffffff80235177>] find_busiest_group+0x207/0x8a0
 610  [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
 611  [<ffffffff803c7ea3>] check_unmap+0x203/0x490
 612  [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
 613  [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
 614  [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
 615  [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
 616  [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
 617  [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
 618  [<ffffffff8020c093>] ret_from_intr+0x0/0xa
 619  <EOI> <4>---[ end trace f6435a98e2a38c0e ]---
 620
 621 The driver developer can find the driver and the device including a stacktrace
 622 of the DMA-API call which caused this warning.
 623
 624 Per default only the first error will result in a warning message. All other
 625 errors will only silently counted. This limitation exist to prevent the code
 626 from flooding your kernel log. To support debugging a device driver this can
 627 be disabled via debugfs. See the debugfs interface documentation below for
 628 details.
 629
 630 The debugfs directory for the DMA-API debugging code is called dma-api/. In
 631 this directory the following files can currently be found:
 632
 633         dma-api/all_errors      This file contains a numeric value. If this
 634                                 value is not equal to zero the debugging code
 635                                 will print a warning for every error it finds
 636                                 into the kernel log. Be careful with this
 637                                 option, as it can easily flood your logs.
 638
 639         dma-api/disabled        This read-only file contains the character 'Y'
 640                                 if the debugging code is disabled. This can
 641                                 happen when it runs out of memory or if it was
 642                                 disabled at boot time
 643
 644         dma-api/error_count     This file is read-only and shows the total
 645                                 numbers of errors found.
 646
 647         dma-api/num_errors      The number in this file shows how many
 648                                 warnings will be printed to the kernel log
 649                                 before it stops. This number is initialized to
 650                                 one at system boot and be set by writing into
 651                                 this file
 652
 653         dma-api/min_free_entries
 654                                 This read-only file can be read to get the
 655                                 minimum number of free dma_debug_entries the
 656                                 allocator has ever seen. If this value goes
 657                                 down to zero the code will disable itself
 658                                 because it is not longer reliable.
 659
 660         dma-api/num_free_entries
 661                                 The current number of free dma_debug_entries
 662                                 in the allocator.
 663
 664         dma-api/driver-filter
 665                                 You can write a name of a driver into this file
 666                                 to limit the debug output to requests from that
 667                                 particular driver. Write an empty string to
 668                                 that file to disable the filter and see
 669                                 all errors again.
 670
 671 If you have this code compiled into your kernel it will be enabled by default.
 672 If you want to boot without the bookkeeping anyway you can provide
 673 'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
 674 Notice that you can not enable it again at runtime. You have to reboot to do
 675 so.
 676
 677 If you want to see debug messages only for a special device driver you can
 678 specify the dma_debug_driver=<drivername> parameter. This will enable the
 679 driver filter at boot time. The debug code will only print errors for that
 680 driver afterwards. This filter can be disabled or changed later using debugfs.
 681
 682 When the code disables itself at runtime this is most likely because it ran
 683 out of dma_debug_entries. These entries are preallocated at boot. The number
 684 of preallocated entries is defined per architecture. If it is too low for you
 685 boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
 686 architectural default.
 687
 688 void debug_dmap_mapping_error(struct device *dev, dma_addr_t dma_addr);
 689
 690 dma-debug interface debug_dma_mapping_error() to debug drivers that fail
 691 to check DMA mapping errors on addresses returned by dma_map_single() and
 692 dma_map_page() interfaces. This interface clears a flag set by
 693 debug_dma_map_page() to indicate that dma_mapping_error() has been called by
 694 the driver. When driver does unmap, debug_dma_unmap() checks the flag and if
 695 this flag is still set, prints warning message that includes call trace that
 696 leads up to the unmap. This interface can be called from dma_mapping_error()
 697 routines to enable DMA mapping error check debugging.
 698