1               Dynamic DMA mapping using the generic device
   2               ============================================
   4        James E.J. Bottomley <>
   6This document describes the DMA API.  For a more gentle introduction
   7of the API (and actual examples) see
  10This API is split into two pieces.  Part I describes the API.  Part II
  11describes the extensions to the API for supporting non-consistent
  12memory machines.  Unless you know that your driver absolutely has to
  13support non-consistent platforms (this is usually only legacy
  14platforms) you should only use the API described in part I.
  16Part I - dma_ API
  19To get the dma_ API, you must #include <linux/dma-mapping.h>
  22Part Ia - Using large dma-coherent buffers
  25void *
  26dma_alloc_coherent(struct device *dev, size_t size,
  27                             dma_addr_t *dma_handle, gfp_t flag)
  29Consistent memory is memory for which a write by either the device or
  30the processor can immediately be read by the processor or device
  31without having to worry about caching effects.  (You may however need
  32to make sure to flush the processor's write buffers before telling
  33devices to read that memory.)
  35This routine allocates a region of <size> bytes of consistent memory.
  36It also returns a <dma_handle> which may be cast to an unsigned
  37integer the same width as the bus and used as the physical address
  38base of the region.
  40Returns: a pointer to the allocated region (in the processor's virtual
  41address space) or NULL if the allocation failed.
  43Note: consistent memory can be expensive on some platforms, and the
  44minimum allocation length may be as big as a page, so you should
  45consolidate your requests for consistent memory as much as possible.
  46The simplest way to do that is to use the dma_pool calls (see below).
  48The flag parameter (dma_alloc_coherent only) allows the caller to
  49specify the GFP_ flags (see kmalloc) for the allocation (the
  50implementation may choose to ignore flags that affect the location of
  51the returned memory, like GFP_DMA).
  54dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
  55                           dma_addr_t dma_handle)
  57Free the region of consistent memory you previously allocated.  dev,
  58size and dma_handle must all be the same as those passed into the
  59consistent allocate.  cpu_addr must be the virtual address returned by
  60the consistent allocate.
  62Note that unlike their sibling allocation calls, these routines
  63may only be called with IRQs enabled.
  66Part Ib - Using small dma-coherent buffers
  69To get this part of the dma_ API, you must #include <linux/dmapool.h>
  71Many drivers need lots of small dma-coherent memory regions for DMA
  72descriptors or I/O buffers.  Rather than allocating in units of a page
  73or more using dma_alloc_coherent(), you can use DMA pools.  These work
  74much like a struct kmem_cache, except that they use the dma-coherent allocator,
  75not __get_free_pages().  Also, they understand common hardware constraints
  76for alignment, like queue heads needing to be aligned on N-byte boundaries.
  79        struct dma_pool *
  80        dma_pool_create(const char *name, struct device *dev,
  81                        size_t size, size_t align, size_t alloc);
  83The pool create() routines initialize a pool of dma-coherent buffers
  84for use with a given device.  It must be called in a context which
  85can sleep.
  87The "name" is for diagnostics (like a struct kmem_cache name); dev and size
  88are like what you'd pass to dma_alloc_coherent().  The device's hardware
  89alignment requirement for this type of data is "align" (which is expressed
  90in bytes, and must be a power of two).  If your device has no boundary
  91crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated
  92from this pool must not cross 4KByte boundaries.
  95        void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
  96                        dma_addr_t *dma_handle);
  98This allocates memory from the pool; the returned memory will meet the size
  99and alignment requirements specified at creation time.  Pass GFP_ATOMIC to
 100prevent blocking, or if it's permitted (not in_interrupt, not holding SMP locks),
 101pass GFP_KERNEL to allow blocking.  Like dma_alloc_coherent(), this returns
 102two values:  an address usable by the cpu, and the dma address usable by the
 103pool's device.
 106        void dma_pool_free(struct dma_pool *pool, void *vaddr,
 107                        dma_addr_t addr);
 109This puts memory back into the pool.  The pool is what was passed to
 110the pool allocation routine; the cpu (vaddr) and dma addresses are what
 111were returned when that routine allocated the memory being freed.
 114        void dma_pool_destroy(struct dma_pool *pool);
 116The pool destroy() routines free the resources of the pool.  They must be
 117called in a context which can sleep.  Make sure you've freed all allocated
 118memory back to the pool before you destroy it.
 121Part Ic - DMA addressing limitations
 125dma_supported(struct device *dev, u64 mask)
 127Checks to see if the device can support DMA to the memory described by
 130Returns: 1 if it can and 0 if it can't.
 132Notes: This routine merely tests to see if the mask is possible.  It
 133won't change the current mask settings.  It is more intended as an
 134internal API for use by the platform than an external API for use by
 135driver writers.
 138dma_set_mask(struct device *dev, u64 mask)
 140Checks to see if the mask is possible and updates the device
 141parameters if it is.
 143Returns: 0 if successful and a negative error if not.
 146dma_set_coherent_mask(struct device *dev, u64 mask)
 148Checks to see if the mask is possible and updates the device
 149parameters if it is.
 151Returns: 0 if successful and a negative error if not.
 154dma_get_required_mask(struct device *dev)
 156This API returns the mask that the platform requires to
 157operate efficiently.  Usually this means the returned mask
 158is the minimum required to cover all of memory.  Examining the
 159required mask gives drivers with variable descriptor sizes the
 160opportunity to use smaller descriptors as necessary.
 162Requesting the required mask does not alter the current mask.  If you
 163wish to take advantage of it, you should issue a dma_set_mask()
 164call to set the mask to the value returned.
 167Part Id - Streaming DMA mappings
 171dma_map_single(struct device *dev, void *cpu_addr, size_t size,
 172                      enum dma_data_direction direction)
 174Maps a piece of processor virtual memory so it can be accessed by the
 175device and returns the physical handle of the memory.
 177The direction for both api's may be converted freely by casting.
 178However the dma_ API uses a strongly typed enumerator for its
 181DMA_NONE                no direction (used for debugging)
 182DMA_TO_DEVICE           data is going from the memory to the device
 183DMA_FROM_DEVICE         data is coming from the device to the memory
 184DMA_BIDIRECTIONAL       direction isn't known
 186Notes:  Not all memory regions in a machine can be mapped by this
 187API.  Further, regions that appear to be physically contiguous in
 188kernel virtual space may not be contiguous as physical memory.  Since
 189this API does not provide any scatter/gather capability, it will fail
 190if the user tries to map a non-physically contiguous piece of memory.
 191For this reason, it is recommended that memory mapped by this API be
 192obtained only from sources which guarantee it to be physically contiguous
 193(like kmalloc).
 195Further, the physical address of the memory must be within the
 196dma_mask of the device (the dma_mask represents a bit mask of the
 197addressable region for the device.  I.e., if the physical address of
 198the memory anded with the dma_mask is still equal to the physical
 199address, then the device can perform DMA to the memory).  In order to
 200ensure that the memory allocated by kmalloc is within the dma_mask,
 201the driver may specify various platform-dependent flags to restrict
 202the physical memory range of the allocation (e.g. on x86, GFP_DMA
 203guarantees to be within the first 16Mb of available physical memory,
 204as required by ISA devices).
 206Note also that the above constraints on physical contiguity and
 207dma_mask may not apply if the platform has an IOMMU (a device which
 208supplies a physical to virtual mapping between the I/O memory bus and
 209the device).  However, to be portable, device driver writers may *not*
 210assume that such an IOMMU exists.
 212Warnings:  Memory coherency operates at a granularity called the cache
 213line width.  In order for memory mapped by this API to operate
 214correctly, the mapped region must begin exactly on a cache line
 215boundary and end exactly on one (to prevent two separately mapped
 216regions from sharing a single cache line).  Since the cache line size
 217may not be known at compile time, the API will not enforce this
 218requirement.  Therefore, it is recommended that driver writers who
 219don't take special care to determine the cache line size at run time
 220only map virtual regions that begin and end on page boundaries (which
 221are guaranteed also to be cache line boundaries).
 223DMA_TO_DEVICE synchronisation must be done after the last modification
 224of the memory region by the software and before it is handed off to
 225the driver.  Once this primitive is used, memory covered by this
 226primitive should be treated as read-only by the device.  If the device
 227may write to it at any point, it should be DMA_BIDIRECTIONAL (see
 230DMA_FROM_DEVICE synchronisation must be done before the driver
 231accesses data that may be changed by the device.  This memory should
 232be treated as read-only by the driver.  If the driver needs to write
 233to it at any point, it should be DMA_BIDIRECTIONAL (see below).
 235DMA_BIDIRECTIONAL requires special handling: it means that the driver
 236isn't sure if the memory was modified before being handed off to the
 237device and also isn't sure if the device will also modify it.  Thus,
 238you must always sync bidirectional memory twice: once before the
 239memory is handed off to the device (to make sure all memory changes
 240are flushed from the processor) and once before the data may be
 241accessed after being used by the device (to make sure any processor
 242cache lines are updated with data that the device may have changed).
 245dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
 246                 enum dma_data_direction direction)
 248Unmaps the region previously mapped.  All the parameters passed in
 249must be identical to those passed in (and returned) by the mapping
 253dma_map_page(struct device *dev, struct page *page,
 254                    unsigned long offset, size_t size,
 255                    enum dma_data_direction direction)
 257dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
 258               enum dma_data_direction direction)
 260API for mapping and unmapping for pages.  All the notes and warnings
 261for the other mapping APIs apply here.  Also, although the <offset>
 262and <size> parameters are provided to do partial page mapping, it is
 263recommended that you never use these unless you really know what the
 264cache width is.
 267dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
 269In some circumstances dma_map_single and dma_map_page will fail to create
 270a mapping. A driver can check for these errors by testing the returned
 271dma address with dma_mapping_error(). A non-zero return value means the mapping
 272could not be created and the driver should take appropriate action (e.g.
 273reduce current DMA mapping usage or delay and try again later).
 275        int
 276        dma_map_sg(struct device *dev, struct scatterlist *sg,
 277                int nents, enum dma_data_direction direction)
 279Returns: the number of physical segments mapped (this may be shorter
 280than <nents> passed in if some elements of the scatter/gather list are
 281physically or virtually adjacent and an IOMMU maps them with a single
 284Please note that the sg cannot be mapped again if it has been mapped once.
 285The mapping process is allowed to destroy information in the sg.
 287As with the other mapping interfaces, dma_map_sg can fail. When it
 288does, 0 is returned and a driver must take appropriate action. It is
 289critical that the driver do something, in the case of a block driver
 290aborting the request or even oopsing is better than doing nothing and
 291corrupting the filesystem.
 293With scatterlists, you use the resulting mapping like this:
 295        int i, count = dma_map_sg(dev, sglist, nents, direction);
 296        struct scatterlist *sg;
 298        for_each_sg(sglist, sg, count, i) {
 299                hw_address[i] = sg_dma_address(sg);
 300                hw_len[i] = sg_dma_len(sg);
 301        }
 303where nents is the number of entries in the sglist.
 305The implementation is free to merge several consecutive sglist entries
 306into one (e.g. with an IOMMU, or if several pages just happen to be
 307physically contiguous) and returns the actual number of sg entries it
 308mapped them to. On failure 0, is returned.
 310Then you should loop count times (note: this can be less than nents times)
 311and use sg_dma_address() and sg_dma_len() macros where you previously
 312accessed sg->address and sg->length as shown above.
 314        void
 315        dma_unmap_sg(struct device *dev, struct scatterlist *sg,
 316                int nhwentries, enum dma_data_direction direction)
 318Unmap the previously mapped scatter/gather list.  All the parameters
 319must be the same as those and passed in to the scatter/gather mapping
 322Note: <nents> must be the number you passed in, *not* the number of
 323physical entries returned.
 326dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size,
 327                        enum dma_data_direction direction)
 329dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size,
 330                           enum dma_data_direction direction)
 332dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nelems,
 333                    enum dma_data_direction direction)
 335dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nelems,
 336                       enum dma_data_direction direction)
 338Synchronise a single contiguous or scatter/gather mapping for the cpu
 339and device. With the sync_sg API, all the parameters must be the same
 340as those passed into the single mapping API. With the sync_single API,
 341you can use dma_handle and size parameters that aren't identical to
 342those passed into the single mapping API to do a partial sync.
 344Notes:  You must do this:
 346- Before reading values that have been written by DMA from the device
 347  (use the DMA_FROM_DEVICE direction)
 348- After writing values that will be written to the device using DMA
 349  (use the DMA_TO_DEVICE) direction
 350- before *and* after handing memory to the device if the memory is
 353See also dma_map_single().
 356dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
 357                     enum dma_data_direction dir,
 358                     struct dma_attrs *attrs)
 361dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
 362                       size_t size, enum dma_data_direction dir,
 363                       struct dma_attrs *attrs)
 366dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
 367                 int nents, enum dma_data_direction dir,
 368                 struct dma_attrs *attrs)
 371dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
 372                   int nents, enum dma_data_direction dir,
 373                   struct dma_attrs *attrs)
 375The four functions above are just like the counterpart functions
 376without the _attrs suffixes, except that they pass an optional
 377struct dma_attrs*.
 379struct dma_attrs encapsulates a set of "dma attributes". For the
 380definition of struct dma_attrs see linux/dma-attrs.h.
 382The interpretation of dma attributes is architecture-specific, and
 383each attribute should be documented in Documentation/DMA-attributes.txt.
 385If struct dma_attrs* is NULL, the semantics of each of these
 386functions is identical to those of the corresponding function
 387without the _attrs suffix. As a result dma_map_single_attrs()
 388can generally replace dma_map_single(), etc.
 390As an example of the use of the *_attrs functions, here's how
 391you could pass an attribute DMA_ATTR_FOO when mapping memory
 392for DMA:
 394#include <linux/dma-attrs.h>
 395/* DMA_ATTR_FOO should be defined in linux/dma-attrs.h and
 396 * documented in Documentation/DMA-attributes.txt */
 399        DEFINE_DMA_ATTRS(attrs);
 400        dma_set_attr(DMA_ATTR_FOO, &attrs);
 401        ....
 402        n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, &attr);
 403        ....
 405Architectures that care about DMA_ATTR_FOO would check for its
 406presence in their implementations of the mapping and unmapping
 407routines, e.g.:
 409void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
 410                             size_t size, enum dma_data_direction dir,
 411                             struct dma_attrs *attrs)
 413        ....
 414        int foo =  dma_get_attr(DMA_ATTR_FOO, attrs);
 415        ....
 416        if (foo)
 417                /* twizzle the frobnozzle */
 418        ....
 421Part II - Advanced dma_ usage
 424Warning: These pieces of the DMA API should not be used in the
 425majority of cases, since they cater for unlikely corner cases that
 426don't belong in usual drivers.
 428If you don't understand how cache line coherency works between a
 429processor and an I/O device, you should not be using this part of the
 430API at all.
 432void *
 433dma_alloc_noncoherent(struct device *dev, size_t size,
 434                               dma_addr_t *dma_handle, gfp_t flag)
 436Identical to dma_alloc_coherent() except that the platform will
 437choose to return either consistent or non-consistent memory as it sees
 438fit.  By using this API, you are guaranteeing to the platform that you
 439have all the correct and necessary sync points for this memory in the
 440driver should it choose to return non-consistent memory.
 442Note: where the platform can return consistent memory, it will
 443guarantee that the sync points become nops.
 445Warning:  Handling non-consistent memory is a real pain.  You should
 446only ever use this API if you positively know your driver will be
 447required to work on one of the rare (usually non-PCI) architectures
 448that simply cannot make consistent memory.
 451dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
 452                              dma_addr_t dma_handle)
 454Free memory allocated by the nonconsistent API.  All parameters must
 455be identical to those passed in (and returned by
 461Returns the processor cache alignment.  This is the absolute minimum
 462alignment *and* width that you must observe when either mapping
 463memory or doing partial flushes.
 465Notes: This API may return a number *larger* than the actual cache
 466line, but it will guarantee that one or more cache lines fit exactly
 467into the width returned by this call.  It will also always be a power
 468of two for easy alignment.
 471dma_cache_sync(struct device *dev, void *vaddr, size_t size,
 472               enum dma_data_direction direction)
 474Do a partial sync of memory that was allocated by
 475dma_alloc_noncoherent(), starting at virtual address vaddr and
 476continuing on for size.  Again, you *must* observe the cache line
 477boundaries when doing this.
 480dma_declare_coherent_memory(struct device *dev, dma_addr_t bus_addr,
 481                            dma_addr_t device_addr, size_t size, int
 482                            flags)
 484Declare region of memory to be handed out by dma_alloc_coherent when
 485it's asked for coherent memory for this device.
 487bus_addr is the physical address to which the memory is currently
 488assigned in the bus responding region (this will be used by the
 489platform to perform the mapping).
 491device_addr is the physical address the device needs to be programmed
 492with actually to address this memory (this will be handed out as the
 493dma_addr_t in dma_alloc_coherent()).
 495size is the size of the area (must be multiples of PAGE_SIZE).
 497flags can be or'd together and are:
 499DMA_MEMORY_MAP - request that the memory returned from
 500dma_alloc_coherent() be directly writable.
 502DMA_MEMORY_IO - request that the memory returned from
 503dma_alloc_coherent() be addressable using read/write/memcpy_toio etc.
 505One or both of these flags must be present.
 507DMA_MEMORY_INCLUDES_CHILDREN - make the declared memory be allocated by
 508dma_alloc_coherent of any child devices of this one (for memory residing
 509on a bridge).
 511DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions. 
 512Do not allow dma_alloc_coherent() to fall back to system memory when
 513it's out of memory in the declared region.
 515The return value will be either DMA_MEMORY_MAP or DMA_MEMORY_IO and
 516must correspond to a passed in flag (i.e. no returning DMA_MEMORY_IO
 517if only DMA_MEMORY_MAP were passed in) for success or zero for
 520Note, for DMA_MEMORY_IO returns, all subsequent memory returned by
 521dma_alloc_coherent() may no longer be accessed directly, but instead
 522must be accessed using the correct bus functions.  If your driver
 523isn't prepared to handle this contingency, it should not specify
 524DMA_MEMORY_IO in the input flags.
 526As a simplification for the platforms, only *one* such region of
 527memory may be declared per device.
 529For reasons of efficiency, most platforms choose to track the declared
 530region only at the granularity of a page.  For smaller allocations,
 531you should use the dma_pool() API.
 534dma_release_declared_memory(struct device *dev)
 536Remove the memory region previously declared from the system.  This
 537API performs *no* in-use checking for this region and will return
 538unconditionally having removed all the required structures.  It is the
 539driver's job to ensure that no parts of this memory region are
 540currently in use.
 542void *
 543dma_mark_declared_memory_occupied(struct device *dev,
 544                                  dma_addr_t device_addr, size_t size)
 546This is used to occupy specific regions of the declared space
 547(dma_alloc_coherent() will hand out the first free region it finds).
 549device_addr is the *device* address of the region requested.
 551size is the size (and should be a page-sized multiple).
 553The return value will be either a pointer to the processor virtual
 554address of the memory, or an error (via PTR_ERR()) if any part of the
 555region is occupied.
 557Part III - Debug drivers use of the DMA-API
 560The DMA-API as described above as some constraints. DMA addresses must be
 561released with the corresponding function with the same size for example. With
 562the advent of hardware IOMMUs it becomes more and more important that drivers
 563do not violate those constraints. In the worst case such a violation can
 564result in data corruption up to destroyed filesystems.
 566To debug drivers and find bugs in the usage of the DMA-API checking code can
 567be compiled into the kernel which will tell the developer about those
 568violations. If your architecture supports it you can select the "Enable
 569debugging of DMA-API usage" option in your kernel configuration. Enabling this
 570option has a performance impact. Do not enable it in production kernels.
 572If you boot the resulting kernel will contain code which does some bookkeeping
 573about what DMA memory was allocated for which device. If this code detects an
 574error it prints a warning message with some details into your kernel log. An
 575example warning message may look like this:
 577------------[ cut here ]------------
 578WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
 579        check_unmap+0x203/0x490()
 580Hardware name:
 581forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
 582        function [device address=0x00000000640444be] [size=66 bytes] [mapped as
 583single] [unmapped as page]
 584Modules linked in: nfsd exportfs bridge stp llc r8169
 585Pid: 0, comm: swapper Tainted: G        W  2.6.28-dmatest-09289-g8bb99c0 #1
 586Call Trace:
 587 <IRQ>  [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
 588 [<ffffffff80647b70>] _spin_unlock+0x10/0x30
 589 [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
 590 [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
 591 [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
 592 [<ffffffff80252f96>] queue_work+0x56/0x60
 593 [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
 594 [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
 595 [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
 596 [<ffffffff80235177>] find_busiest_group+0x207/0x8a0
 597 [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
 598 [<ffffffff803c7ea3>] check_unmap+0x203/0x490
 599 [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
 600 [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
 601 [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
 602 [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
 603 [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
 604 [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
 605 [<ffffffff8020c093>] ret_from_intr+0x0/0xa
 606 <EOI> <4>---[ end trace f6435a98e2a38c0e ]---
 608The driver developer can find the driver and the device including a stacktrace
 609of the DMA-API call which caused this warning.
 611Per default only the first error will result in a warning message. All other
 612errors will only silently counted. This limitation exist to prevent the code
 613from flooding your kernel log. To support debugging a device driver this can
 614be disabled via debugfs. See the debugfs interface documentation below for
 617The debugfs directory for the DMA-API debugging code is called dma-api/. In
 618this directory the following files can currently be found:
 620        dma-api/all_errors      This file contains a numeric value. If this
 621                                value is not equal to zero the debugging code
 622                                will print a warning for every error it finds
 623                                into the kernel log. Be careful with this
 624                                option, as it can easily flood your logs.
 626        dma-api/disabled        This read-only file contains the character 'Y'
 627                                if the debugging code is disabled. This can
 628                                happen when it runs out of memory or if it was
 629                                disabled at boot time
 631        dma-api/error_count     This file is read-only and shows the total
 632                                numbers of errors found.
 634        dma-api/num_errors      The number in this file shows how many
 635                                warnings will be printed to the kernel log
 636                                before it stops. This number is initialized to
 637                                one at system boot and be set by writing into
 638                                this file
 640        dma-api/min_free_entries
 641                                This read-only file can be read to get the
 642                                minimum number of free dma_debug_entries the
 643                                allocator has ever seen. If this value goes
 644                                down to zero the code will disable itself
 645                                because it is not longer reliable.
 647        dma-api/num_free_entries
 648                                The current number of free dma_debug_entries
 649                                in the allocator.
 651        dma-api/driver-filter
 652                                You can write a name of a driver into this file
 653                                to limit the debug output to requests from that
 654                                particular driver. Write an empty string to
 655                                that file to disable the filter and see
 656                                all errors again.
 658If you have this code compiled into your kernel it will be enabled by default.
 659If you want to boot without the bookkeeping anyway you can provide
 660'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
 661Notice that you can not enable it again at runtime. You have to reboot to do
 664If you want to see debug messages only for a special device driver you can
 665specify the dma_debug_driver=<drivername> parameter. This will enable the
 666driver filter at boot time. The debug code will only print errors for that
 667driver afterwards. This filter can be disabled or changed later using debugfs.
 669When the code disables itself at runtime this is most likely because it ran
 670out of dma_debug_entries. These entries are preallocated at boot. The number
 671of preallocated entries is defined per architecture. If it is too low for you
 672boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
 673architectural default.
 674 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.