linux/Documentation/DMA-API.txt
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   1============================================
   2Dynamic DMA mapping using the generic device
   3============================================
   4
   5:Author: James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
   6
   7This document describes the DMA API.  For a more gentle introduction
   8of the API (and actual examples), see Documentation/DMA-API-HOWTO.txt.
   9
  10This API is split into two pieces.  Part I describes the basic API.
  11Part II describes extensions for supporting non-consistent memory
  12machines.  Unless you know that your driver absolutely has to support
  13non-consistent platforms (this is usually only legacy platforms) you
  14should only use the API described in part I.
  15
  16Part I - dma_API
  17----------------
  18
  19To get the dma_API, you must #include <linux/dma-mapping.h>.  This
  20provides dma_addr_t and the interfaces described below.
  21
  22A dma_addr_t can hold any valid DMA address for the platform.  It can be
  23given to a device to use as a DMA source or target.  A CPU cannot reference
  24a dma_addr_t directly because there may be translation between its physical
  25address space and the DMA address space.
  26
  27Part Ia - Using large DMA-coherent buffers
  28------------------------------------------
  29
  30::
  31
  32        void *
  33        dma_alloc_coherent(struct device *dev, size_t size,
  34                           dma_addr_t *dma_handle, gfp_t flag)
  35
  36Consistent memory is memory for which a write by either the device or
  37the processor can immediately be read by the processor or device
  38without having to worry about caching effects.  (You may however need
  39to make sure to flush the processor's write buffers before telling
  40devices to read that memory.)
  41
  42This routine allocates a region of <size> bytes of consistent memory.
  43
  44It returns a pointer to the allocated region (in the processor's virtual
  45address space) or NULL if the allocation failed.
  46
  47It also returns a <dma_handle> which may be cast to an unsigned integer the
  48same width as the bus and given to the device as the DMA address base of
  49the region.
  50
  51Note: consistent memory can be expensive on some platforms, and the
  52minimum allocation length may be as big as a page, so you should
  53consolidate your requests for consistent memory as much as possible.
  54The simplest way to do that is to use the dma_pool calls (see below).
  55
  56The flag parameter (dma_alloc_coherent() only) allows the caller to
  57specify the ``GFP_`` flags (see kmalloc()) for the allocation (the
  58implementation may choose to ignore flags that affect the location of
  59the returned memory, like GFP_DMA).
  60
  61::
  62
  63        void *
  64        dma_zalloc_coherent(struct device *dev, size_t size,
  65                            dma_addr_t *dma_handle, gfp_t flag)
  66
  67Wraps dma_alloc_coherent() and also zeroes the returned memory if the
  68allocation attempt succeeded.
  69
  70::
  71
  72        void
  73        dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
  74                          dma_addr_t dma_handle)
  75
  76Free a region of consistent memory you previously allocated.  dev,
  77size and dma_handle must all be the same as those passed into
  78dma_alloc_coherent().  cpu_addr must be the virtual address returned by
  79the dma_alloc_coherent().
  80
  81Note that unlike their sibling allocation calls, these routines
  82may only be called with IRQs enabled.
  83
  84
  85Part Ib - Using small DMA-coherent buffers
  86------------------------------------------
  87
  88To get this part of the dma_API, you must #include <linux/dmapool.h>
  89
  90Many drivers need lots of small DMA-coherent memory regions for DMA
  91descriptors or I/O buffers.  Rather than allocating in units of a page
  92or more using dma_alloc_coherent(), you can use DMA pools.  These work
  93much like a struct kmem_cache, except that they use the DMA-coherent allocator,
  94not __get_free_pages().  Also, they understand common hardware constraints
  95for alignment, like queue heads needing to be aligned on N-byte boundaries.
  96
  97
  98::
  99
 100        struct dma_pool *
 101        dma_pool_create(const char *name, struct device *dev,
 102                        size_t size, size_t align, size_t alloc);
 103
 104dma_pool_create() initializes a pool of DMA-coherent buffers
 105for use with a given device.  It must be called in a context which
 106can sleep.
 107
 108The "name" is for diagnostics (like a struct kmem_cache name); dev and size
 109are like what you'd pass to dma_alloc_coherent().  The device's hardware
 110alignment requirement for this type of data is "align" (which is expressed
 111in bytes, and must be a power of two).  If your device has no boundary
 112crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated
 113from this pool must not cross 4KByte boundaries.
 114
 115::
 116
 117        void *
 118        dma_pool_zalloc(struct dma_pool *pool, gfp_t mem_flags,
 119                        dma_addr_t *handle)
 120
 121Wraps dma_pool_alloc() and also zeroes the returned memory if the
 122allocation attempt succeeded.
 123
 124
 125::
 126
 127        void *
 128        dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
 129                       dma_addr_t *dma_handle);
 130
 131This allocates memory from the pool; the returned memory will meet the
 132size and alignment requirements specified at creation time.  Pass
 133GFP_ATOMIC to prevent blocking, or if it's permitted (not
 134in_interrupt, not holding SMP locks), pass GFP_KERNEL to allow
 135blocking.  Like dma_alloc_coherent(), this returns two values:  an
 136address usable by the CPU, and the DMA address usable by the pool's
 137device.
 138
 139::
 140
 141        void
 142        dma_pool_free(struct dma_pool *pool, void *vaddr,
 143                      dma_addr_t addr);
 144
 145This puts memory back into the pool.  The pool is what was passed to
 146dma_pool_alloc(); the CPU (vaddr) and DMA addresses are what
 147were returned when that routine allocated the memory being freed.
 148
 149::
 150
 151        void
 152        dma_pool_destroy(struct dma_pool *pool);
 153
 154dma_pool_destroy() frees the resources of the pool.  It must be
 155called in a context which can sleep.  Make sure you've freed all allocated
 156memory back to the pool before you destroy it.
 157
 158
 159Part Ic - DMA addressing limitations
 160------------------------------------
 161
 162::
 163
 164        int
 165        dma_set_mask_and_coherent(struct device *dev, u64 mask)
 166
 167Checks to see if the mask is possible and updates the device
 168streaming and coherent DMA mask parameters if it is.
 169
 170Returns: 0 if successful and a negative error if not.
 171
 172::
 173
 174        int
 175        dma_set_mask(struct device *dev, u64 mask)
 176
 177Checks to see if the mask is possible and updates the device
 178parameters if it is.
 179
 180Returns: 0 if successful and a negative error if not.
 181
 182::
 183
 184        int
 185        dma_set_coherent_mask(struct device *dev, u64 mask)
 186
 187Checks to see if the mask is possible and updates the device
 188parameters if it is.
 189
 190Returns: 0 if successful and a negative error if not.
 191
 192::
 193
 194        u64
 195        dma_get_required_mask(struct device *dev)
 196
 197This API returns the mask that the platform requires to
 198operate efficiently.  Usually this means the returned mask
 199is the minimum required to cover all of memory.  Examining the
 200required mask gives drivers with variable descriptor sizes the
 201opportunity to use smaller descriptors as necessary.
 202
 203Requesting the required mask does not alter the current mask.  If you
 204wish to take advantage of it, you should issue a dma_set_mask()
 205call to set the mask to the value returned.
 206
 207
 208Part Id - Streaming DMA mappings
 209--------------------------------
 210
 211::
 212
 213        dma_addr_t
 214        dma_map_single(struct device *dev, void *cpu_addr, size_t size,
 215                       enum dma_data_direction direction)
 216
 217Maps a piece of processor virtual memory so it can be accessed by the
 218device and returns the DMA address of the memory.
 219
 220The direction for both APIs may be converted freely by casting.
 221However the dma_API uses a strongly typed enumerator for its
 222direction:
 223
 224======================= =============================================
 225DMA_NONE                no direction (used for debugging)
 226DMA_TO_DEVICE           data is going from the memory to the device
 227DMA_FROM_DEVICE         data is coming from the device to the memory
 228DMA_BIDIRECTIONAL       direction isn't known
 229======================= =============================================
 230
 231.. note::
 232
 233        Not all memory regions in a machine can be mapped by this API.
 234        Further, contiguous kernel virtual space may not be contiguous as
 235        physical memory.  Since this API does not provide any scatter/gather
 236        capability, it will fail if the user tries to map a non-physically
 237        contiguous piece of memory.  For this reason, memory to be mapped by
 238        this API should be obtained from sources which guarantee it to be
 239        physically contiguous (like kmalloc).
 240
 241        Further, the DMA address of the memory must be within the
 242        dma_mask of the device (the dma_mask is a bit mask of the
 243        addressable region for the device, i.e., if the DMA address of
 244        the memory ANDed with the dma_mask is still equal to the DMA
 245        address, then the device can perform DMA to the memory).  To
 246        ensure that the memory allocated by kmalloc is within the dma_mask,
 247        the driver may specify various platform-dependent flags to restrict
 248        the DMA address range of the allocation (e.g., on x86, GFP_DMA
 249        guarantees to be within the first 16MB of available DMA addresses,
 250        as required by ISA devices).
 251
 252        Note also that the above constraints on physical contiguity and
 253        dma_mask may not apply if the platform has an IOMMU (a device which
 254        maps an I/O DMA address to a physical memory address).  However, to be
 255        portable, device driver writers may *not* assume that such an IOMMU
 256        exists.
 257
 258.. warning::
 259
 260        Memory coherency operates at a granularity called the cache
 261        line width.  In order for memory mapped by this API to operate
 262        correctly, the mapped region must begin exactly on a cache line
 263        boundary and end exactly on one (to prevent two separately mapped
 264        regions from sharing a single cache line).  Since the cache line size
 265        may not be known at compile time, the API will not enforce this
 266        requirement.  Therefore, it is recommended that driver writers who
 267        don't take special care to determine the cache line size at run time
 268        only map virtual regions that begin and end on page boundaries (which
 269        are guaranteed also to be cache line boundaries).
 270
 271        DMA_TO_DEVICE synchronisation must be done after the last modification
 272        of the memory region by the software and before it is handed off to
 273        the device.  Once this primitive is used, memory covered by this
 274        primitive should be treated as read-only by the device.  If the device
 275        may write to it at any point, it should be DMA_BIDIRECTIONAL (see
 276        below).
 277
 278        DMA_FROM_DEVICE synchronisation must be done before the driver
 279        accesses data that may be changed by the device.  This memory should
 280        be treated as read-only by the driver.  If the driver needs to write
 281        to it at any point, it should be DMA_BIDIRECTIONAL (see below).
 282
 283        DMA_BIDIRECTIONAL requires special handling: it means that the driver
 284        isn't sure if the memory was modified before being handed off to the
 285        device and also isn't sure if the device will also modify it.  Thus,
 286        you must always sync bidirectional memory twice: once before the
 287        memory is handed off to the device (to make sure all memory changes
 288        are flushed from the processor) and once before the data may be
 289        accessed after being used by the device (to make sure any processor
 290        cache lines are updated with data that the device may have changed).
 291
 292::
 293
 294        void
 295        dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
 296                         enum dma_data_direction direction)
 297
 298Unmaps the region previously mapped.  All the parameters passed in
 299must be identical to those passed in (and returned) by the mapping
 300API.
 301
 302::
 303
 304        dma_addr_t
 305        dma_map_page(struct device *dev, struct page *page,
 306                     unsigned long offset, size_t size,
 307                     enum dma_data_direction direction)
 308
 309        void
 310        dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
 311                       enum dma_data_direction direction)
 312
 313API for mapping and unmapping for pages.  All the notes and warnings
 314for the other mapping APIs apply here.  Also, although the <offset>
 315and <size> parameters are provided to do partial page mapping, it is
 316recommended that you never use these unless you really know what the
 317cache width is.
 318
 319::
 320
 321        dma_addr_t
 322        dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size,
 323                         enum dma_data_direction dir, unsigned long attrs)
 324
 325        void
 326        dma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size,
 327                           enum dma_data_direction dir, unsigned long attrs)
 328
 329API for mapping and unmapping for MMIO resources. All the notes and
 330warnings for the other mapping APIs apply here. The API should only be
 331used to map device MMIO resources, mapping of RAM is not permitted.
 332
 333::
 334
 335        int
 336        dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
 337
 338In some circumstances dma_map_single(), dma_map_page() and dma_map_resource()
 339will fail to create a mapping. A driver can check for these errors by testing
 340the returned DMA address with dma_mapping_error(). A non-zero return value
 341means the mapping could not be created and the driver should take appropriate
 342action (e.g. reduce current DMA mapping usage or delay and try again later).
 343
 344::
 345
 346        int
 347        dma_map_sg(struct device *dev, struct scatterlist *sg,
 348                   int nents, enum dma_data_direction direction)
 349
 350Returns: the number of DMA address segments mapped (this may be shorter
 351than <nents> passed in if some elements of the scatter/gather list are
 352physically or virtually adjacent and an IOMMU maps them with a single
 353entry).
 354
 355Please note that the sg cannot be mapped again if it has been mapped once.
 356The mapping process is allowed to destroy information in the sg.
 357
 358As with the other mapping interfaces, dma_map_sg() can fail. When it
 359does, 0 is returned and a driver must take appropriate action. It is
 360critical that the driver do something, in the case of a block driver
 361aborting the request or even oopsing is better than doing nothing and
 362corrupting the filesystem.
 363
 364With scatterlists, you use the resulting mapping like this::
 365
 366        int i, count = dma_map_sg(dev, sglist, nents, direction);
 367        struct scatterlist *sg;
 368
 369        for_each_sg(sglist, sg, count, i) {
 370                hw_address[i] = sg_dma_address(sg);
 371                hw_len[i] = sg_dma_len(sg);
 372        }
 373
 374where nents is the number of entries in the sglist.
 375
 376The implementation is free to merge several consecutive sglist entries
 377into one (e.g. with an IOMMU, or if several pages just happen to be
 378physically contiguous) and returns the actual number of sg entries it
 379mapped them to. On failure 0, is returned.
 380
 381Then you should loop count times (note: this can be less than nents times)
 382and use sg_dma_address() and sg_dma_len() macros where you previously
 383accessed sg->address and sg->length as shown above.
 384
 385::
 386
 387        void
 388        dma_unmap_sg(struct device *dev, struct scatterlist *sg,
 389                     int nents, enum dma_data_direction direction)
 390
 391Unmap the previously mapped scatter/gather list.  All the parameters
 392must be the same as those and passed in to the scatter/gather mapping
 393API.
 394
 395Note: <nents> must be the number you passed in, *not* the number of
 396DMA address entries returned.
 397
 398::
 399
 400        void
 401        dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle,
 402                                size_t size,
 403                                enum dma_data_direction direction)
 404
 405        void
 406        dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle,
 407                                   size_t size,
 408                                   enum dma_data_direction direction)
 409
 410        void
 411        dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
 412                            int nents,
 413                            enum dma_data_direction direction)
 414
 415        void
 416        dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
 417                               int nents,
 418                               enum dma_data_direction direction)
 419
 420Synchronise a single contiguous or scatter/gather mapping for the CPU
 421and device. With the sync_sg API, all the parameters must be the same
 422as those passed into the single mapping API. With the sync_single API,
 423you can use dma_handle and size parameters that aren't identical to
 424those passed into the single mapping API to do a partial sync.
 425
 426
 427.. note::
 428
 429   You must do this:
 430
 431   - Before reading values that have been written by DMA from the device
 432     (use the DMA_FROM_DEVICE direction)
 433   - After writing values that will be written to the device using DMA
 434     (use the DMA_TO_DEVICE) direction
 435   - before *and* after handing memory to the device if the memory is
 436     DMA_BIDIRECTIONAL
 437
 438See also dma_map_single().
 439
 440::
 441
 442        dma_addr_t
 443        dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
 444                             enum dma_data_direction dir,
 445                             unsigned long attrs)
 446
 447        void
 448        dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
 449                               size_t size, enum dma_data_direction dir,
 450                               unsigned long attrs)
 451
 452        int
 453        dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
 454                         int nents, enum dma_data_direction dir,
 455                         unsigned long attrs)
 456
 457        void
 458        dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
 459                           int nents, enum dma_data_direction dir,
 460                           unsigned long attrs)
 461
 462The four functions above are just like the counterpart functions
 463without the _attrs suffixes, except that they pass an optional
 464dma_attrs.
 465
 466The interpretation of DMA attributes is architecture-specific, and
 467each attribute should be documented in Documentation/DMA-attributes.txt.
 468
 469If dma_attrs are 0, the semantics of each of these functions
 470is identical to those of the corresponding function
 471without the _attrs suffix. As a result dma_map_single_attrs()
 472can generally replace dma_map_single(), etc.
 473
 474As an example of the use of the ``*_attrs`` functions, here's how
 475you could pass an attribute DMA_ATTR_FOO when mapping memory
 476for DMA::
 477
 478        #include <linux/dma-mapping.h>
 479        /* DMA_ATTR_FOO should be defined in linux/dma-mapping.h and
 480        * documented in Documentation/DMA-attributes.txt */
 481        ...
 482
 483                unsigned long attr;
 484                attr |= DMA_ATTR_FOO;
 485                ....
 486                n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, attr);
 487                ....
 488
 489Architectures that care about DMA_ATTR_FOO would check for its
 490presence in their implementations of the mapping and unmapping
 491routines, e.g.:::
 492
 493        void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
 494                                     size_t size, enum dma_data_direction dir,
 495                                     unsigned long attrs)
 496        {
 497                ....
 498                if (attrs & DMA_ATTR_FOO)
 499                        /* twizzle the frobnozzle */
 500                ....
 501        }
 502
 503
 504Part II - Advanced dma usage
 505----------------------------
 506
 507Warning: These pieces of the DMA API should not be used in the
 508majority of cases, since they cater for unlikely corner cases that
 509don't belong in usual drivers.
 510
 511If you don't understand how cache line coherency works between a
 512processor and an I/O device, you should not be using this part of the
 513API at all.
 514
 515::
 516
 517        void *
 518        dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
 519                        gfp_t flag, unsigned long attrs)
 520
 521Identical to dma_alloc_coherent() except that when the
 522DMA_ATTR_NON_CONSISTENT flags is passed in the attrs argument, the
 523platform will choose to return either consistent or non-consistent memory
 524as it sees fit.  By using this API, you are guaranteeing to the platform
 525that you have all the correct and necessary sync points for this memory
 526in the driver should it choose to return non-consistent memory.
 527
 528Note: where the platform can return consistent memory, it will
 529guarantee that the sync points become nops.
 530
 531Warning:  Handling non-consistent memory is a real pain.  You should
 532only use this API if you positively know your driver will be
 533required to work on one of the rare (usually non-PCI) architectures
 534that simply cannot make consistent memory.
 535
 536::
 537
 538        void
 539        dma_free_attrs(struct device *dev, size_t size, void *cpu_addr,
 540                       dma_addr_t dma_handle, unsigned long attrs)
 541
 542Free memory allocated by the dma_alloc_attrs().  All parameters common
 543parameters must identical to those otherwise passed to dma_fre_coherent,
 544and the attrs argument must be identical to the attrs passed to
 545dma_alloc_attrs().
 546
 547::
 548
 549        int
 550        dma_get_cache_alignment(void)
 551
 552Returns the processor cache alignment.  This is the absolute minimum
 553alignment *and* width that you must observe when either mapping
 554memory or doing partial flushes.
 555
 556.. note::
 557
 558        This API may return a number *larger* than the actual cache
 559        line, but it will guarantee that one or more cache lines fit exactly
 560        into the width returned by this call.  It will also always be a power
 561        of two for easy alignment.
 562
 563::
 564
 565        void
 566        dma_cache_sync(struct device *dev, void *vaddr, size_t size,
 567                       enum dma_data_direction direction)
 568
 569Do a partial sync of memory that was allocated by dma_alloc_attrs() with
 570the DMA_ATTR_NON_CONSISTENT flag starting at virtual address vaddr and
 571continuing on for size.  Again, you *must* observe the cache line
 572boundaries when doing this.
 573
 574::
 575
 576        int
 577        dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
 578                                    dma_addr_t device_addr, size_t size, int
 579                                    flags)
 580
 581Declare region of memory to be handed out by dma_alloc_coherent() when
 582it's asked for coherent memory for this device.
 583
 584phys_addr is the CPU physical address to which the memory is currently
 585assigned (this will be ioremapped so the CPU can access the region).
 586
 587device_addr is the DMA address the device needs to be programmed
 588with to actually address this memory (this will be handed out as the
 589dma_addr_t in dma_alloc_coherent()).
 590
 591size is the size of the area (must be multiples of PAGE_SIZE).
 592
 593flags can be ORed together and are:
 594
 595- DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions.
 596  Do not allow dma_alloc_coherent() to fall back to system memory when
 597  it's out of memory in the declared region.
 598
 599As a simplification for the platforms, only *one* such region of
 600memory may be declared per device.
 601
 602For reasons of efficiency, most platforms choose to track the declared
 603region only at the granularity of a page.  For smaller allocations,
 604you should use the dma_pool() API.
 605
 606::
 607
 608        void
 609        dma_release_declared_memory(struct device *dev)
 610
 611Remove the memory region previously declared from the system.  This
 612API performs *no* in-use checking for this region and will return
 613unconditionally having removed all the required structures.  It is the
 614driver's job to ensure that no parts of this memory region are
 615currently in use.
 616
 617::
 618
 619        void *
 620        dma_mark_declared_memory_occupied(struct device *dev,
 621                                          dma_addr_t device_addr, size_t size)
 622
 623This is used to occupy specific regions of the declared space
 624(dma_alloc_coherent() will hand out the first free region it finds).
 625
 626device_addr is the *device* address of the region requested.
 627
 628size is the size (and should be a page-sized multiple).
 629
 630The return value will be either a pointer to the processor virtual
 631address of the memory, or an error (via PTR_ERR()) if any part of the
 632region is occupied.
 633
 634Part III - Debug drivers use of the DMA-API
 635-------------------------------------------
 636
 637The DMA-API as described above has some constraints. DMA addresses must be
 638released with the corresponding function with the same size for example. With
 639the advent of hardware IOMMUs it becomes more and more important that drivers
 640do not violate those constraints. In the worst case such a violation can
 641result in data corruption up to destroyed filesystems.
 642
 643To debug drivers and find bugs in the usage of the DMA-API checking code can
 644be compiled into the kernel which will tell the developer about those
 645violations. If your architecture supports it you can select the "Enable
 646debugging of DMA-API usage" option in your kernel configuration. Enabling this
 647option has a performance impact. Do not enable it in production kernels.
 648
 649If you boot the resulting kernel will contain code which does some bookkeeping
 650about what DMA memory was allocated for which device. If this code detects an
 651error it prints a warning message with some details into your kernel log. An
 652example warning message may look like this::
 653
 654        WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
 655                check_unmap+0x203/0x490()
 656        Hardware name:
 657        forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
 658                function [device address=0x00000000640444be] [size=66 bytes] [mapped as
 659        single] [unmapped as page]
 660        Modules linked in: nfsd exportfs bridge stp llc r8169
 661        Pid: 0, comm: swapper Tainted: G        W  2.6.28-dmatest-09289-g8bb99c0 #1
 662        Call Trace:
 663        <IRQ>  [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
 664        [<ffffffff80647b70>] _spin_unlock+0x10/0x30
 665        [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
 666        [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
 667        [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
 668        [<ffffffff80252f96>] queue_work+0x56/0x60
 669        [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
 670        [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
 671        [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
 672        [<ffffffff80235177>] find_busiest_group+0x207/0x8a0
 673        [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
 674        [<ffffffff803c7ea3>] check_unmap+0x203/0x490
 675        [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
 676        [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
 677        [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
 678        [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
 679        [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
 680        [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
 681        [<ffffffff8020c093>] ret_from_intr+0x0/0xa
 682        <EOI> <4>---[ end trace f6435a98e2a38c0e ]---
 683
 684The driver developer can find the driver and the device including a stacktrace
 685of the DMA-API call which caused this warning.
 686
 687Per default only the first error will result in a warning message. All other
 688errors will only silently counted. This limitation exist to prevent the code
 689from flooding your kernel log. To support debugging a device driver this can
 690be disabled via debugfs. See the debugfs interface documentation below for
 691details.
 692
 693The debugfs directory for the DMA-API debugging code is called dma-api/. In
 694this directory the following files can currently be found:
 695
 696=============================== ===============================================
 697dma-api/all_errors              This file contains a numeric value. If this
 698                                value is not equal to zero the debugging code
 699                                will print a warning for every error it finds
 700                                into the kernel log. Be careful with this
 701                                option, as it can easily flood your logs.
 702
 703dma-api/disabled                This read-only file contains the character 'Y'
 704                                if the debugging code is disabled. This can
 705                                happen when it runs out of memory or if it was
 706                                disabled at boot time
 707
 708dma-api/error_count             This file is read-only and shows the total
 709                                numbers of errors found.
 710
 711dma-api/num_errors              The number in this file shows how many
 712                                warnings will be printed to the kernel log
 713                                before it stops. This number is initialized to
 714                                one at system boot and be set by writing into
 715                                this file
 716
 717dma-api/min_free_entries        This read-only file can be read to get the
 718                                minimum number of free dma_debug_entries the
 719                                allocator has ever seen. If this value goes
 720                                down to zero the code will disable itself
 721                                because it is not longer reliable.
 722
 723dma-api/num_free_entries        The current number of free dma_debug_entries
 724                                in the allocator.
 725
 726dma-api/driver-filter           You can write a name of a driver into this file
 727                                to limit the debug output to requests from that
 728                                particular driver. Write an empty string to
 729                                that file to disable the filter and see
 730                                all errors again.
 731=============================== ===============================================
 732
 733If you have this code compiled into your kernel it will be enabled by default.
 734If you want to boot without the bookkeeping anyway you can provide
 735'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
 736Notice that you can not enable it again at runtime. You have to reboot to do
 737so.
 738
 739If you want to see debug messages only for a special device driver you can
 740specify the dma_debug_driver=<drivername> parameter. This will enable the
 741driver filter at boot time. The debug code will only print errors for that
 742driver afterwards. This filter can be disabled or changed later using debugfs.
 743
 744When the code disables itself at runtime this is most likely because it ran
 745out of dma_debug_entries. These entries are preallocated at boot. The number
 746of preallocated entries is defined per architecture. If it is too low for you
 747boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
 748architectural default.
 749
 750::
 751
 752        void
 753        debug_dma_mapping_error(struct device *dev, dma_addr_t dma_addr);
 754
 755dma-debug interface debug_dma_mapping_error() to debug drivers that fail
 756to check DMA mapping errors on addresses returned by dma_map_single() and
 757dma_map_page() interfaces. This interface clears a flag set by
 758debug_dma_map_page() to indicate that dma_mapping_error() has been called by
 759the driver. When driver does unmap, debug_dma_unmap() checks the flag and if
 760this flag is still set, prints warning message that includes call trace that
 761leads up to the unmap. This interface can be called from dma_mapping_error()
 762routines to enable DMA mapping error check debugging.
 763