linux/Documentation/DMA-API.txt
<<
>>
Prefs
   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 or bus address for the platform.  It
  22can be given to a device to use as a DMA source or target.  A CPU cannot
  23reference a dma_addr_t directly because there may be translation between
  24its physical address space and the bus 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 bus 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_alloc(struct dma_pool *pool, gfp_t gfp_flags,
 108                        dma_addr_t *dma_handle);
 109
 110This allocates memory from the pool; the returned memory will meet the
 111size and alignment requirements specified at creation time.  Pass
 112GFP_ATOMIC to prevent blocking, or if it's permitted (not
 113in_interrupt, not holding SMP locks), pass GFP_KERNEL to allow
 114blocking.  Like dma_alloc_coherent(), this returns two values:  an
 115address usable by the CPU, and the DMA address usable by the pool's
 116device.
 117
 118
 119        void dma_pool_free(struct dma_pool *pool, void *vaddr,
 120                        dma_addr_t addr);
 121
 122This puts memory back into the pool.  The pool is what was passed to
 123dma_pool_alloc(); the CPU (vaddr) and DMA addresses are what
 124were returned when that routine allocated the memory being freed.
 125
 126
 127        void dma_pool_destroy(struct dma_pool *pool);
 128
 129dma_pool_destroy() frees the resources of the pool.  It must be
 130called in a context which can sleep.  Make sure you've freed all allocated
 131memory back to the pool before you destroy it.
 132
 133
 134Part Ic - DMA addressing limitations
 135------------------------------------
 136
 137int
 138dma_supported(struct device *dev, u64 mask)
 139
 140Checks to see if the device can support DMA to the memory described by
 141mask.
 142
 143Returns: 1 if it can and 0 if it can't.
 144
 145Notes: This routine merely tests to see if the mask is possible.  It
 146won't change the current mask settings.  It is more intended as an
 147internal API for use by the platform than an external API for use by
 148driver writers.
 149
 150int
 151dma_set_mask_and_coherent(struct device *dev, u64 mask)
 152
 153Checks to see if the mask is possible and updates the device
 154streaming and coherent DMA mask parameters if it is.
 155
 156Returns: 0 if successful and a negative error if not.
 157
 158int
 159dma_set_mask(struct device *dev, u64 mask)
 160
 161Checks to see if the mask is possible and updates the device
 162parameters if it is.
 163
 164Returns: 0 if successful and a negative error if not.
 165
 166int
 167dma_set_coherent_mask(struct device *dev, u64 mask)
 168
 169Checks to see if the mask is possible and updates the device
 170parameters if it is.
 171
 172Returns: 0 if successful and a negative error if not.
 173
 174u64
 175dma_get_required_mask(struct device *dev)
 176
 177This API returns the mask that the platform requires to
 178operate efficiently.  Usually this means the returned mask
 179is the minimum required to cover all of memory.  Examining the
 180required mask gives drivers with variable descriptor sizes the
 181opportunity to use smaller descriptors as necessary.
 182
 183Requesting the required mask does not alter the current mask.  If you
 184wish to take advantage of it, you should issue a dma_set_mask()
 185call to set the mask to the value returned.
 186
 187
 188Part Id - Streaming DMA mappings
 189--------------------------------
 190
 191dma_addr_t
 192dma_map_single(struct device *dev, void *cpu_addr, size_t size,
 193                      enum dma_data_direction direction)
 194
 195Maps a piece of processor virtual memory so it can be accessed by the
 196device and returns the bus address of the memory.
 197
 198The direction for both APIs may be converted freely by casting.
 199However the dma_ API uses a strongly typed enumerator for its
 200direction:
 201
 202DMA_NONE                no direction (used for debugging)
 203DMA_TO_DEVICE           data is going from the memory to the device
 204DMA_FROM_DEVICE         data is coming from the device to the memory
 205DMA_BIDIRECTIONAL       direction isn't known
 206
 207Notes:  Not all memory regions in a machine can be mapped by this API.
 208Further, contiguous kernel virtual space may not be contiguous as
 209physical memory.  Since this API does not provide any scatter/gather
 210capability, it will fail if the user tries to map a non-physically
 211contiguous piece of memory.  For this reason, memory to be mapped by
 212this API should be obtained from sources which guarantee it to be
 213physically contiguous (like kmalloc).
 214
 215Further, the bus address of the memory must be within the
 216dma_mask of the device (the dma_mask is a bit mask of the
 217addressable region for the device, i.e., if the bus address of
 218the memory ANDed with the dma_mask is still equal to the bus
 219address, then the device can perform DMA to the memory).  To
 220ensure that the memory allocated by kmalloc is within the dma_mask,
 221the driver may specify various platform-dependent flags to restrict
 222the bus address range of the allocation (e.g., on x86, GFP_DMA
 223guarantees to be within the first 16MB of available bus addresses,
 224as required by ISA devices).
 225
 226Note also that the above constraints on physical contiguity and
 227dma_mask may not apply if the platform has an IOMMU (a device which
 228maps an I/O bus address to a physical memory address).  However, to be
 229portable, device driver writers may *not* assume that such an IOMMU
 230exists.
 231
 232Warnings:  Memory coherency operates at a granularity called the cache
 233line width.  In order for memory mapped by this API to operate
 234correctly, the mapped region must begin exactly on a cache line
 235boundary and end exactly on one (to prevent two separately mapped
 236regions from sharing a single cache line).  Since the cache line size
 237may not be known at compile time, the API will not enforce this
 238requirement.  Therefore, it is recommended that driver writers who
 239don't take special care to determine the cache line size at run time
 240only map virtual regions that begin and end on page boundaries (which
 241are guaranteed also to be cache line boundaries).
 242
 243DMA_TO_DEVICE synchronisation must be done after the last modification
 244of the memory region by the software and before it is handed off to
 245the driver.  Once this primitive is used, memory covered by this
 246primitive should be treated as read-only by the device.  If the device
 247may write to it at any point, it should be DMA_BIDIRECTIONAL (see
 248below).
 249
 250DMA_FROM_DEVICE synchronisation must be done before the driver
 251accesses data that may be changed by the device.  This memory should
 252be treated as read-only by the driver.  If the driver needs to write
 253to it at any point, it should be DMA_BIDIRECTIONAL (see below).
 254
 255DMA_BIDIRECTIONAL requires special handling: it means that the driver
 256isn't sure if the memory was modified before being handed off to the
 257device and also isn't sure if the device will also modify it.  Thus,
 258you must always sync bidirectional memory twice: once before the
 259memory is handed off to the device (to make sure all memory changes
 260are flushed from the processor) and once before the data may be
 261accessed after being used by the device (to make sure any processor
 262cache lines are updated with data that the device may have changed).
 263
 264void
 265dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
 266                 enum dma_data_direction direction)
 267
 268Unmaps the region previously mapped.  All the parameters passed in
 269must be identical to those passed in (and returned) by the mapping
 270API.
 271
 272dma_addr_t
 273dma_map_page(struct device *dev, struct page *page,
 274                    unsigned long offset, size_t size,
 275                    enum dma_data_direction direction)
 276void
 277dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
 278               enum dma_data_direction direction)
 279
 280API for mapping and unmapping for pages.  All the notes and warnings
 281for the other mapping APIs apply here.  Also, although the <offset>
 282and <size> parameters are provided to do partial page mapping, it is
 283recommended that you never use these unless you really know what the
 284cache width is.
 285
 286int
 287dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
 288
 289In some circumstances dma_map_single() and dma_map_page() will fail to create
 290a mapping. A driver can check for these errors by testing the returned
 291DMA address with dma_mapping_error(). A non-zero return value means the mapping
 292could not be created and the driver should take appropriate action (e.g.
 293reduce current DMA mapping usage or delay and try again later).
 294
 295        int
 296        dma_map_sg(struct device *dev, struct scatterlist *sg,
 297                int nents, enum dma_data_direction direction)
 298
 299Returns: the number of bus address segments mapped (this may be shorter
 300than <nents> passed in if some elements of the scatter/gather list are
 301physically or virtually adjacent and an IOMMU maps them with a single
 302entry).
 303
 304Please note that the sg cannot be mapped again if it has been mapped once.
 305The mapping process is allowed to destroy information in the sg.
 306
 307As with the other mapping interfaces, dma_map_sg() can fail. When it
 308does, 0 is returned and a driver must take appropriate action. It is
 309critical that the driver do something, in the case of a block driver
 310aborting the request or even oopsing is better than doing nothing and
 311corrupting the filesystem.
 312
 313With scatterlists, you use the resulting mapping like this:
 314
 315        int i, count = dma_map_sg(dev, sglist, nents, direction);
 316        struct scatterlist *sg;
 317
 318        for_each_sg(sglist, sg, count, i) {
 319                hw_address[i] = sg_dma_address(sg);
 320                hw_len[i] = sg_dma_len(sg);
 321        }
 322
 323where nents is the number of entries in the sglist.
 324
 325The implementation is free to merge several consecutive sglist entries
 326into one (e.g. with an IOMMU, or if several pages just happen to be
 327physically contiguous) and returns the actual number of sg entries it
 328mapped them to. On failure 0, is returned.
 329
 330Then you should loop count times (note: this can be less than nents times)
 331and use sg_dma_address() and sg_dma_len() macros where you previously
 332accessed sg->address and sg->length as shown above.
 333
 334        void
 335        dma_unmap_sg(struct device *dev, struct scatterlist *sg,
 336                int nhwentries, enum dma_data_direction direction)
 337
 338Unmap the previously mapped scatter/gather list.  All the parameters
 339must be the same as those and passed in to the scatter/gather mapping
 340API.
 341
 342Note: <nents> must be the number you passed in, *not* the number of
 343bus address entries returned.
 344
 345void
 346dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size,
 347                        enum dma_data_direction direction)
 348void
 349dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size,
 350                           enum dma_data_direction direction)
 351void
 352dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nelems,
 353                    enum dma_data_direction direction)
 354void
 355dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nelems,
 356                       enum dma_data_direction direction)
 357
 358Synchronise a single contiguous or scatter/gather mapping for the CPU
 359and device. With the sync_sg API, all the parameters must be the same
 360as those passed into the single mapping API. With the sync_single API,
 361you can use dma_handle and size parameters that aren't identical to
 362those passed into the single mapping API to do a partial sync.
 363
 364Notes:  You must do this:
 365
 366- Before reading values that have been written by DMA from the device
 367  (use the DMA_FROM_DEVICE direction)
 368- After writing values that will be written to the device using DMA
 369  (use the DMA_TO_DEVICE) direction
 370- before *and* after handing memory to the device if the memory is
 371  DMA_BIDIRECTIONAL
 372
 373See also dma_map_single().
 374
 375dma_addr_t
 376dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
 377                     enum dma_data_direction dir,
 378                     struct dma_attrs *attrs)
 379
 380void
 381dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
 382                       size_t size, enum dma_data_direction dir,
 383                       struct dma_attrs *attrs)
 384
 385int
 386dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
 387                 int nents, enum dma_data_direction dir,
 388                 struct dma_attrs *attrs)
 389
 390void
 391dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
 392                   int nents, enum dma_data_direction dir,
 393                   struct dma_attrs *attrs)
 394
 395The four functions above are just like the counterpart functions
 396without the _attrs suffixes, except that they pass an optional
 397struct dma_attrs*.
 398
 399struct dma_attrs encapsulates a set of "DMA attributes". For the
 400definition of struct dma_attrs see linux/dma-attrs.h.
 401
 402The interpretation of DMA attributes is architecture-specific, and
 403each attribute should be documented in Documentation/DMA-attributes.txt.
 404
 405If struct dma_attrs* is NULL, the semantics of each of these
 406functions is identical to those of the corresponding function
 407without the _attrs suffix. As a result dma_map_single_attrs()
 408can generally replace dma_map_single(), etc.
 409
 410As an example of the use of the *_attrs functions, here's how
 411you could pass an attribute DMA_ATTR_FOO when mapping memory
 412for DMA:
 413
 414#include <linux/dma-attrs.h>
 415/* DMA_ATTR_FOO should be defined in linux/dma-attrs.h and
 416 * documented in Documentation/DMA-attributes.txt */
 417...
 418
 419        DEFINE_DMA_ATTRS(attrs);
 420        dma_set_attr(DMA_ATTR_FOO, &attrs);
 421        ....
 422        n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, &attr);
 423        ....
 424
 425Architectures that care about DMA_ATTR_FOO would check for its
 426presence in their implementations of the mapping and unmapping
 427routines, e.g.:
 428
 429void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
 430                             size_t size, enum dma_data_direction dir,
 431                             struct dma_attrs *attrs)
 432{
 433        ....
 434        int foo =  dma_get_attr(DMA_ATTR_FOO, attrs);
 435        ....
 436        if (foo)
 437                /* twizzle the frobnozzle */
 438        ....
 439
 440
 441Part II - Advanced dma_ usage
 442-----------------------------
 443
 444Warning: These pieces of the DMA API should not be used in the
 445majority of cases, since they cater for unlikely corner cases that
 446don't belong in usual drivers.
 447
 448If you don't understand how cache line coherency works between a
 449processor and an I/O device, you should not be using this part of the
 450API at all.
 451
 452void *
 453dma_alloc_noncoherent(struct device *dev, size_t size,
 454                               dma_addr_t *dma_handle, gfp_t flag)
 455
 456Identical to dma_alloc_coherent() except that the platform will
 457choose to return either consistent or non-consistent memory as it sees
 458fit.  By using this API, you are guaranteeing to the platform that you
 459have all the correct and necessary sync points for this memory in the
 460driver should it choose to return non-consistent memory.
 461
 462Note: where the platform can return consistent memory, it will
 463guarantee that the sync points become nops.
 464
 465Warning:  Handling non-consistent memory is a real pain.  You should
 466only use this API if you positively know your driver will be
 467required to work on one of the rare (usually non-PCI) architectures
 468that simply cannot make consistent memory.
 469
 470void
 471dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
 472                              dma_addr_t dma_handle)
 473
 474Free memory allocated by the nonconsistent API.  All parameters must
 475be identical to those passed in (and returned by
 476dma_alloc_noncoherent()).
 477
 478int
 479dma_get_cache_alignment(void)
 480
 481Returns the processor cache alignment.  This is the absolute minimum
 482alignment *and* width that you must observe when either mapping
 483memory or doing partial flushes.
 484
 485Notes: This API may return a number *larger* than the actual cache
 486line, but it will guarantee that one or more cache lines fit exactly
 487into the width returned by this call.  It will also always be a power
 488of two for easy alignment.
 489
 490void
 491dma_cache_sync(struct device *dev, void *vaddr, size_t size,
 492               enum dma_data_direction direction)
 493
 494Do a partial sync of memory that was allocated by
 495dma_alloc_noncoherent(), starting at virtual address vaddr and
 496continuing on for size.  Again, you *must* observe the cache line
 497boundaries when doing this.
 498
 499int
 500dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
 501                            dma_addr_t device_addr, size_t size, int
 502                            flags)
 503
 504Declare region of memory to be handed out by dma_alloc_coherent() when
 505it's asked for coherent memory for this device.
 506
 507phys_addr is the CPU physical address to which the memory is currently
 508assigned (this will be ioremapped so the CPU can access the region).
 509
 510device_addr is the bus address the device needs to be programmed
 511with to actually address this memory (this will be handed out as the
 512dma_addr_t in dma_alloc_coherent()).
 513
 514size is the size of the area (must be multiples of PAGE_SIZE).
 515
 516flags can be ORed together and are:
 517
 518DMA_MEMORY_MAP - request that the memory returned from
 519dma_alloc_coherent() be directly writable.
 520
 521DMA_MEMORY_IO - request that the memory returned from
 522dma_alloc_coherent() be addressable using read()/write()/memcpy_toio() etc.
 523
 524One or both of these flags must be present.
 525
 526DMA_MEMORY_INCLUDES_CHILDREN - make the declared memory be allocated by
 527dma_alloc_coherent of any child devices of this one (for memory residing
 528on a bridge).
 529
 530DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions. 
 531Do not allow dma_alloc_coherent() to fall back to system memory when
 532it's out of memory in the declared region.
 533
 534The return value will be either DMA_MEMORY_MAP or DMA_MEMORY_IO and
 535must correspond to a passed in flag (i.e. no returning DMA_MEMORY_IO
 536if only DMA_MEMORY_MAP were passed in) for success or zero for
 537failure.
 538
 539Note, for DMA_MEMORY_IO returns, all subsequent memory returned by
 540dma_alloc_coherent() may no longer be accessed directly, but instead
 541must be accessed using the correct bus functions.  If your driver
 542isn't prepared to handle this contingency, it should not specify
 543DMA_MEMORY_IO in the input flags.
 544
 545As a simplification for the platforms, only *one* such region of
 546memory may be declared per device.
 547
 548For reasons of efficiency, most platforms choose to track the declared
 549region only at the granularity of a page.  For smaller allocations,
 550you should use the dma_pool() API.
 551
 552void
 553dma_release_declared_memory(struct device *dev)
 554
 555Remove the memory region previously declared from the system.  This
 556API performs *no* in-use checking for this region and will return
 557unconditionally having removed all the required structures.  It is the
 558driver's job to ensure that no parts of this memory region are
 559currently in use.
 560
 561void *
 562dma_mark_declared_memory_occupied(struct device *dev,
 563                                  dma_addr_t device_addr, size_t size)
 564
 565This is used to occupy specific regions of the declared space
 566(dma_alloc_coherent() will hand out the first free region it finds).
 567
 568device_addr is the *device* address of the region requested.
 569
 570size is the size (and should be a page-sized multiple).
 571
 572The return value will be either a pointer to the processor virtual
 573address of the memory, or an error (via PTR_ERR()) if any part of the
 574region is occupied.
 575
 576Part III - Debug drivers use of the DMA-API
 577-------------------------------------------
 578
 579The DMA-API as described above has some constraints. DMA addresses must be
 580released with the corresponding function with the same size for example. With
 581the advent of hardware IOMMUs it becomes more and more important that drivers
 582do not violate those constraints. In the worst case such a violation can
 583result in data corruption up to destroyed filesystems.
 584
 585To debug drivers and find bugs in the usage of the DMA-API checking code can
 586be compiled into the kernel which will tell the developer about those
 587violations. If your architecture supports it you can select the "Enable
 588debugging of DMA-API usage" option in your kernel configuration. Enabling this
 589option has a performance impact. Do not enable it in production kernels.
 590
 591If you boot the resulting kernel will contain code which does some bookkeeping
 592about what DMA memory was allocated for which device. If this code detects an
 593error it prints a warning message with some details into your kernel log. An
 594example warning message may look like this:
 595
 596------------[ cut here ]------------
 597WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
 598        check_unmap+0x203/0x490()
 599Hardware name:
 600forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
 601        function [device address=0x00000000640444be] [size=66 bytes] [mapped as
 602single] [unmapped as page]
 603Modules linked in: nfsd exportfs bridge stp llc r8169
 604Pid: 0, comm: swapper Tainted: G        W  2.6.28-dmatest-09289-g8bb99c0 #1
 605Call Trace:
 606 <IRQ>  [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
 607 [<ffffffff80647b70>] _spin_unlock+0x10/0x30
 608 [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
 609 [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
 610 [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
 611 [<ffffffff80252f96>] queue_work+0x56/0x60
 612 [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
 613 [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
 614 [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
 615 [<ffffffff80235177>] find_busiest_group+0x207/0x8a0
 616 [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
 617 [<ffffffff803c7ea3>] check_unmap+0x203/0x490
 618 [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
 619 [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
 620 [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
 621 [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
 622 [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
 623 [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
 624 [<ffffffff8020c093>] ret_from_intr+0x0/0xa
 625 <EOI> <4>---[ end trace f6435a98e2a38c0e ]---
 626
 627The driver developer can find the driver and the device including a stacktrace
 628of the DMA-API call which caused this warning.
 629
 630Per default only the first error will result in a warning message. All other
 631errors will only silently counted. This limitation exist to prevent the code
 632from flooding your kernel log. To support debugging a device driver this can
 633be disabled via debugfs. See the debugfs interface documentation below for
 634details.
 635
 636The debugfs directory for the DMA-API debugging code is called dma-api/. In
 637this directory the following files can currently be found:
 638
 639        dma-api/all_errors      This file contains a numeric value. If this
 640                                value is not equal to zero the debugging code
 641                                will print a warning for every error it finds
 642                                into the kernel log. Be careful with this
 643                                option, as it can easily flood your logs.
 644
 645        dma-api/disabled        This read-only file contains the character 'Y'
 646                                if the debugging code is disabled. This can
 647                                happen when it runs out of memory or if it was
 648                                disabled at boot time
 649
 650        dma-api/error_count     This file is read-only and shows the total
 651                                numbers of errors found.
 652
 653        dma-api/num_errors      The number in this file shows how many
 654                                warnings will be printed to the kernel log
 655                                before it stops. This number is initialized to
 656                                one at system boot and be set by writing into
 657                                this file
 658
 659        dma-api/min_free_entries
 660                                This read-only file can be read to get the
 661                                minimum number of free dma_debug_entries the
 662                                allocator has ever seen. If this value goes
 663                                down to zero the code will disable itself
 664                                because it is not longer reliable.
 665
 666        dma-api/num_free_entries
 667                                The current number of free dma_debug_entries
 668                                in the allocator.
 669
 670        dma-api/driver-filter
 671                                You can write a name of a driver into this file
 672                                to limit the debug output to requests from that
 673                                particular driver. Write an empty string to
 674                                that file to disable the filter and see
 675                                all errors again.
 676
 677If you have this code compiled into your kernel it will be enabled by default.
 678If you want to boot without the bookkeeping anyway you can provide
 679'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
 680Notice that you can not enable it again at runtime. You have to reboot to do
 681so.
 682
 683If you want to see debug messages only for a special device driver you can
 684specify the dma_debug_driver=<drivername> parameter. This will enable the
 685driver filter at boot time. The debug code will only print errors for that
 686driver afterwards. This filter can be disabled or changed later using debugfs.
 687
 688When the code disables itself at runtime this is most likely because it ran
 689out of dma_debug_entries. These entries are preallocated at boot. The number
 690of preallocated entries is defined per architecture. If it is too low for you
 691boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
 692architectural default.
 693
 694void debug_dmap_mapping_error(struct device *dev, dma_addr_t dma_addr);
 695
 696dma-debug interface debug_dma_mapping_error() to debug drivers that fail
 697to check DMA mapping errors on addresses returned by dma_map_single() and
 698dma_map_page() interfaces. This interface clears a flag set by
 699debug_dma_map_page() to indicate that dma_mapping_error() has been called by
 700the driver. When driver does unmap, debug_dma_unmap() checks the flag and if
 701this flag is still set, prints warning message that includes call trace that
 702leads up to the unmap. This interface can be called from dma_mapping_error()
 703routines to enable DMA mapping error check debugging.
 704
 705
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.