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