Dynamic DMA mapping using the generic device
                 ============================================
  
          James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
  
  This document describes the DMA API.  For a more gentle introduction

  of the API (and actual examples) see

  Documentation/DMA-API-HOWTO.txt.

  This API is split into two pieces.  Part I describes the API.  Part II
  describes the extensions to the API for supporting non-consistent
  memory machines.  Unless you know that your driver absolutely has to
  support non-consistent platforms (this is usually only legacy
  platforms) you should only use the API described in part I.

  Part I - dma_ API

  -------------------------------------

  To get the dma_ API, you must #include <linux/dma-mapping.h>
  
  
  Part Ia - Using large dma-coherent buffers
  ------------------------------------------
  
  void *
  dma_alloc_coherent(struct device *dev, size_t size,

  			     dma_addr_t *dma_handle, gfp_t flag)

  
  Consistent memory is memory for which a write by either the device or
  the processor can immediately be read by the processor or device

  without having to worry about caching effects.  (You may however need
  to make sure to flush the processor's write buffers before telling
  devices to read that memory.)

  
  This routine allocates a region of <size> bytes of consistent memory.

  It also returns a <dma_handle> which may be cast to an unsigned

  integer the same width as the bus and used as the physical address
  base of the region.
  
  Returns: a pointer to the allocated region (in the processor's virtual
  address space) or NULL if the allocation failed.
  
  Note: consistent memory can be expensive on some platforms, and the
  minimum allocation length may be as big as a page, so you should
  consolidate your requests for consistent memory as much as possible.
  The simplest way to do that is to use the dma_pool calls (see below).
  
  The flag parameter (dma_alloc_coherent only) allows the caller to
  specify the GFP_ flags (see kmalloc) for the allocation (the

  implementation may choose to ignore flags that affect the location of

  the returned memory, like GFP_DMA).

  void *
  dma_zalloc_coherent(struct device *dev, size_t size,
  			     dma_addr_t *dma_handle, gfp_t flag)
  
  Wraps dma_alloc_coherent() and also zeroes the returned memory if the
  allocation attempt succeeded.

  void

  dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,

  			   dma_addr_t dma_handle)

  
  Free the region of consistent memory you previously allocated.  dev,
  size and dma_handle must all be the same as those passed into the
  consistent allocate.  cpu_addr must be the virtual address returned by

  the consistent allocate.

  Note that unlike their sibling allocation calls, these routines
  may only be called with IRQs enabled.

  
  Part Ib - Using small dma-coherent buffers
  ------------------------------------------
  
  To get this part of the dma_ API, you must #include <linux/dmapool.h>
  
  Many drivers need lots of small dma-coherent memory regions for DMA
  descriptors or I/O buffers.  Rather than allocating in units of a page
  or more using dma_alloc_coherent(), you can use DMA pools.  These work

  much like a struct kmem_cache, except that they use the dma-coherent allocator,

  not __get_free_pages().  Also, they understand common hardware constraints

  for alignment, like queue heads needing to be aligned on N-byte boundaries.

  
  
  	struct dma_pool *
  	dma_pool_create(const char *name, struct device *dev,
  			size_t size, size_t align, size_t alloc);

  The pool create() routines initialize a pool of dma-coherent buffers
  for use with a given device.  It must be called in a context which
  can sleep.

  The "name" is for diagnostics (like a struct kmem_cache name); dev and size

  are like what you'd pass to dma_alloc_coherent().  The device's hardware
  alignment requirement for this type of data is "align" (which is expressed
  in bytes, and must be a power of two).  If your device has no boundary
  crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated
  from this pool must not cross 4KByte boundaries.

  	void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,

  			dma_addr_t *dma_handle);

  This allocates memory from the pool; the returned memory will meet the size
  and alignment requirements specified at creation time.  Pass GFP_ATOMIC to

  prevent blocking, or if it's permitted (not in_interrupt, not holding SMP locks),

  pass GFP_KERNEL to allow blocking.  Like dma_alloc_coherent(), this returns
  two values:  an address usable by the cpu, and the dma address usable by the
  pool's device.
  
  
  	void dma_pool_free(struct dma_pool *pool, void *vaddr,
  			dma_addr_t addr);

  This puts memory back into the pool.  The pool is what was passed to

  the pool allocation routine; the cpu (vaddr) and dma addresses are what

  were returned when that routine allocated the memory being freed.
  
  
  	void dma_pool_destroy(struct dma_pool *pool);

  The pool destroy() routines free the resources of the pool.  They must be
  called in a context which can sleep.  Make sure you've freed all allocated
  memory back to the pool before you destroy it.
  
  
  Part Ic - DMA addressing limitations
  ------------------------------------
  
  int
  dma_supported(struct device *dev, u64 mask)

  
  Checks to see if the device can support DMA to the memory described by
  mask.
  
  Returns: 1 if it can and 0 if it can't.
  
  Notes: This routine merely tests to see if the mask is possible.  It
  won't change the current mask settings.  It is more intended as an
  internal API for use by the platform than an external API for use by
  driver writers.
  
  int
  dma_set_mask(struct device *dev, u64 mask)

  
  Checks to see if the mask is possible and updates the device
  parameters if it is.
  
  Returns: 0 if successful and a negative error if not.

  int
  dma_set_coherent_mask(struct device *dev, u64 mask)

  
  Checks to see if the mask is possible and updates the device
  parameters if it is.
  
  Returns: 0 if successful and a negative error if not.

  u64
  dma_get_required_mask(struct device *dev)

  This API returns the mask that the platform requires to
  operate efficiently.  Usually this means the returned mask

  is the minimum required to cover all of memory.  Examining the
  required mask gives drivers with variable descriptor sizes the
  opportunity to use smaller descriptors as necessary.
  
  Requesting the required mask does not alter the current mask.  If you

  wish to take advantage of it, you should issue a dma_set_mask()
  call to set the mask to the value returned.

  
  
  Part Id - Streaming DMA mappings
  --------------------------------
  
  dma_addr_t
  dma_map_single(struct device *dev, void *cpu_addr, size_t size,
  		      enum dma_data_direction direction)

  
  Maps a piece of processor virtual memory so it can be accessed by the
  device and returns the physical handle of the memory.
  
  The direction for both api's may be converted freely by casting.
  However the dma_ API uses a strongly typed enumerator for its
  direction:

  DMA_NONE		no direction (used for debugging)
  DMA_TO_DEVICE		data is going from the memory to the device
  DMA_FROM_DEVICE		data is coming from the device to the memory
  DMA_BIDIRECTIONAL	direction isn't known

  
  Notes:  Not all memory regions in a machine can be mapped by this
  API.  Further, regions that appear to be physically contiguous in
  kernel virtual space may not be contiguous as physical memory.  Since
  this API does not provide any scatter/gather capability, it will fail

  if the user tries to map a non-physically contiguous piece of memory.

  For this reason, it is recommended that memory mapped by this API be

  obtained only from sources which guarantee it to be physically contiguous

  (like kmalloc).
  
  Further, the physical address of the memory must be within the
  dma_mask of the device (the dma_mask represents a bit mask of the

  addressable region for the device.  I.e., if the physical address of

  the memory anded with the dma_mask is still equal to the physical
  address, then the device can perform DMA to the memory).  In order to
  ensure that the memory allocated by kmalloc is within the dma_mask,

  the driver may specify various platform-dependent flags to restrict

  the physical memory range of the allocation (e.g. on x86, GFP_DMA
  guarantees to be within the first 16Mb of available physical memory,
  as required by ISA devices).
  
  Note also that the above constraints on physical contiguity and
  dma_mask may not apply if the platform has an IOMMU (a device which
  supplies a physical to virtual mapping between the I/O memory bus and
  the device).  However, to be portable, device driver writers may *not*
  assume that such an IOMMU exists.
  
  Warnings:  Memory coherency operates at a granularity called the cache
  line width.  In order for memory mapped by this API to operate
  correctly, the mapped region must begin exactly on a cache line
  boundary and end exactly on one (to prevent two separately mapped
  regions from sharing a single cache line).  Since the cache line size
  may not be known at compile time, the API will not enforce this
  requirement.  Therefore, it is recommended that driver writers who
  don't take special care to determine the cache line size at run time
  only map virtual regions that begin and end on page boundaries (which
  are guaranteed also to be cache line boundaries).
  
  DMA_TO_DEVICE synchronisation must be done after the last modification
  of the memory region by the software and before it is handed off to

  the driver.  Once this primitive is used, memory covered by this
  primitive should be treated as read-only by the device.  If the device

  may write to it at any point, it should be DMA_BIDIRECTIONAL (see
  below).
  
  DMA_FROM_DEVICE synchronisation must be done before the driver
  accesses data that may be changed by the device.  This memory should

  be treated as read-only by the driver.  If the driver needs to write

  to it at any point, it should be DMA_BIDIRECTIONAL (see below).
  
  DMA_BIDIRECTIONAL requires special handling: it means that the driver
  isn't sure if the memory was modified before being handed off to the
  device and also isn't sure if the device will also modify it.  Thus,
  you must always sync bidirectional memory twice: once before the
  memory is handed off to the device (to make sure all memory changes
  are flushed from the processor) and once before the data may be
  accessed after being used by the device (to make sure any processor

  cache lines are updated with data that the device may have changed).

  
  void
  dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
  		 enum dma_data_direction direction)

  
  Unmaps the region previously mapped.  All the parameters passed in
  must be identical to those passed in (and returned) by the mapping
  API.
  
  dma_addr_t
  dma_map_page(struct device *dev, struct page *page,
  		    unsigned long offset, size_t size,
  		    enum dma_data_direction direction)

  void
  dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
  	       enum dma_data_direction direction)

  
  API for mapping and unmapping for pages.  All the notes and warnings
  for the other mapping APIs apply here.  Also, although the <offset>
  and <size> parameters are provided to do partial page mapping, it is
  recommended that you never use these unless you really know what the
  cache width is.
  
  int

  dma_mapping_error(struct device *dev, dma_addr_t dma_addr)

  In some circumstances dma_map_single and dma_map_page will fail to create
  a mapping. A driver can check for these errors by testing the returned

  dma address with dma_mapping_error(). A non-zero return value means the mapping
  could not be created and the driver should take appropriate action (e.g.

  reduce current DMA mapping usage or delay and try again later).

  	int
  	dma_map_sg(struct device *dev, struct scatterlist *sg,
  		int nents, enum dma_data_direction direction)

  Returns: the number of physical segments mapped (this may be shorter

  than <nents> passed in if some elements of the scatter/gather list are
  physically or virtually adjacent and an IOMMU maps them with a single
  entry).

  
  Please note that the sg cannot be mapped again if it has been mapped once.
  The mapping process is allowed to destroy information in the sg.
  
  As with the other mapping interfaces, dma_map_sg can fail. When it
  does, 0 is returned and a driver must take appropriate action. It is
  critical that the driver do something, in the case of a block driver
  aborting the request or even oopsing is better than doing nothing and
  corrupting the filesystem.

  With scatterlists, you use the resulting mapping like this:
  
  	int i, count = dma_map_sg(dev, sglist, nents, direction);
  	struct scatterlist *sg;

  	for_each_sg(sglist, sg, count, i) {

  		hw_address[i] = sg_dma_address(sg);
  		hw_len[i] = sg_dma_len(sg);
  	}
  
  where nents is the number of entries in the sglist.
  
  The implementation is free to merge several consecutive sglist entries
  into one (e.g. with an IOMMU, or if several pages just happen to be
  physically contiguous) and returns the actual number of sg entries it
  mapped them to. On failure 0, is returned.
  
  Then you should loop count times (note: this can be less than nents times)
  and use sg_dma_address() and sg_dma_len() macros where you previously
  accessed sg->address and sg->length as shown above.
  
  	void
  	dma_unmap_sg(struct device *dev, struct scatterlist *sg,
  		int nhwentries, enum dma_data_direction direction)

  Unmap the previously mapped scatter/gather list.  All the parameters

  must be the same as those and passed in to the scatter/gather mapping
  API.
  
  Note: <nents> must be the number you passed in, *not* the number of
  physical entries returned.

  void
  dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size,
  			enum dma_data_direction direction)
  void

  dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size,
  			   enum dma_data_direction direction)
  void

  dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nelems,
  		    enum dma_data_direction direction)
  void

  dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nelems,
  		       enum dma_data_direction direction)

  
  Synchronise a single contiguous or scatter/gather mapping for the cpu
  and device. With the sync_sg API, all the parameters must be the same
  as those passed into the single mapping API. With the sync_single API,
  you can use dma_handle and size parameters that aren't identical to
  those passed into the single mapping API to do a partial sync.
  
  Notes:  You must do this:
  
  - Before reading values that have been written by DMA from the device
    (use the DMA_FROM_DEVICE direction)
  - After writing values that will be written to the device using DMA
    (use the DMA_TO_DEVICE) direction
  - before *and* after handing memory to the device if the memory is
    DMA_BIDIRECTIONAL
  
  See also dma_map_single().

  dma_addr_t
  dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
  		     enum dma_data_direction dir,
  		     struct dma_attrs *attrs)
  
  void
  dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
  		       size_t size, enum dma_data_direction dir,
  		       struct dma_attrs *attrs)
  
  int
  dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
  		 int nents, enum dma_data_direction dir,
  		 struct dma_attrs *attrs)
  
  void
  dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
  		   int nents, enum dma_data_direction dir,
  		   struct dma_attrs *attrs)
  
  The four functions above are just like the counterpart functions
  without the _attrs suffixes, except that they pass an optional
  struct dma_attrs*.
  
  struct dma_attrs encapsulates a set of "dma attributes". For the
  definition of struct dma_attrs see linux/dma-attrs.h.
  
  The interpretation of dma attributes is architecture-specific, and
  each attribute should be documented in Documentation/DMA-attributes.txt.
  
  If struct dma_attrs* is NULL, the semantics of each of these
  functions is identical to those of the corresponding function
  without the _attrs suffix. As a result dma_map_single_attrs()
  can generally replace dma_map_single(), etc.
  
  As an example of the use of the *_attrs functions, here's how
  you could pass an attribute DMA_ATTR_FOO when mapping memory
  for DMA:
  
  #include <linux/dma-attrs.h>
  /* DMA_ATTR_FOO should be defined in linux/dma-attrs.h and
   * documented in Documentation/DMA-attributes.txt */
  ...
  
  	DEFINE_DMA_ATTRS(attrs);
  	dma_set_attr(DMA_ATTR_FOO, &attrs);
  	....
  	n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, &attr);
  	....
  
  Architectures that care about DMA_ATTR_FOO would check for its
  presence in their implementations of the mapping and unmapping
  routines, e.g.:
  
  void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
  			     size_t size, enum dma_data_direction dir,
  			     struct dma_attrs *attrs)
  {
  	....
  	int foo =  dma_get_attr(DMA_ATTR_FOO, attrs);
  	....
  	if (foo)
  		/* twizzle the frobnozzle */
  	....

  
  Part II - Advanced dma_ usage
  -----------------------------

  Warning: These pieces of the DMA API should not be used in the
  majority of cases, since they cater for unlikely corner cases that
  don't belong in usual drivers.

  
  If you don't understand how cache line coherency works between a
  processor and an I/O device, you should not be using this part of the
  API at all.
  
  void *
  dma_alloc_noncoherent(struct device *dev, size_t size,

  			       dma_addr_t *dma_handle, gfp_t flag)

  
  Identical to dma_alloc_coherent() except that the platform will
  choose to return either consistent or non-consistent memory as it sees
  fit.  By using this API, you are guaranteeing to the platform that you
  have all the correct and necessary sync points for this memory in the
  driver should it choose to return non-consistent memory.
  
  Note: where the platform can return consistent memory, it will
  guarantee that the sync points become nops.
  
  Warning:  Handling non-consistent memory is a real pain.  You should
  only ever use this API if you positively know your driver will be
  required to work on one of the rare (usually non-PCI) architectures
  that simply cannot make consistent memory.
  
  void
  dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
  			      dma_addr_t dma_handle)

  Free memory allocated by the nonconsistent API.  All parameters must

  be identical to those passed in (and returned by
  dma_alloc_noncoherent()).
  
  int

  dma_get_cache_alignment(void)

  Returns the processor cache alignment.  This is the absolute minimum

  alignment *and* width that you must observe when either mapping
  memory or doing partial flushes.
  
  Notes: This API may return a number *larger* than the actual cache
  line, but it will guarantee that one or more cache lines fit exactly
  into the width returned by this call.  It will also always be a power

  of two for easy alignment.

  
  void

  dma_cache_sync(struct device *dev, void *vaddr, size_t size,

  	       enum dma_data_direction direction)
  
  Do a partial sync of memory that was allocated by
  dma_alloc_noncoherent(), starting at virtual address vaddr and
  continuing on for size.  Again, you *must* observe the cache line
  boundaries when doing this.
  
  int
  dma_declare_coherent_memory(struct device *dev, dma_addr_t bus_addr,
  			    dma_addr_t device_addr, size_t size, int
  			    flags)

  Declare region of memory to be handed out by dma_alloc_coherent when
  it's asked for coherent memory for this device.
  
  bus_addr is the physical address to which the memory is currently
  assigned in the bus responding region (this will be used by the

  platform to perform the mapping).

  
  device_addr is the physical address the device needs to be programmed
  with actually to address this memory (this will be handed out as the

  dma_addr_t in dma_alloc_coherent()).

  
  size is the size of the area (must be multiples of PAGE_SIZE).

  flags can be or'd together and are:

  
  DMA_MEMORY_MAP - request that the memory returned from

  dma_alloc_coherent() be directly writable.

  
  DMA_MEMORY_IO - request that the memory returned from
  dma_alloc_coherent() be addressable using read/write/memcpy_toio etc.

  One or both of these flags must be present.

  
  DMA_MEMORY_INCLUDES_CHILDREN - make the declared memory be allocated by
  dma_alloc_coherent of any child devices of this one (for memory residing
  on a bridge).
  
  DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions. 
  Do not allow dma_alloc_coherent() to fall back to system memory when
  it's out of memory in the declared region.
  
  The return value will be either DMA_MEMORY_MAP or DMA_MEMORY_IO and
  must correspond to a passed in flag (i.e. no returning DMA_MEMORY_IO
  if only DMA_MEMORY_MAP were passed in) for success or zero for
  failure.
  
  Note, for DMA_MEMORY_IO returns, all subsequent memory returned by
  dma_alloc_coherent() may no longer be accessed directly, but instead
  must be accessed using the correct bus functions.  If your driver
  isn't prepared to handle this contingency, it should not specify
  DMA_MEMORY_IO in the input flags.
  
  As a simplification for the platforms, only *one* such region of
  memory may be declared per device.
  
  For reasons of efficiency, most platforms choose to track the declared
  region only at the granularity of a page.  For smaller allocations,
  you should use the dma_pool() API.
  
  void
  dma_release_declared_memory(struct device *dev)
  
  Remove the memory region previously declared from the system.  This
  API performs *no* in-use checking for this region and will return
  unconditionally having removed all the required structures.  It is the

  driver's job to ensure that no parts of this memory region are

  currently in use.
  
  void *
  dma_mark_declared_memory_occupied(struct device *dev,
  				  dma_addr_t device_addr, size_t size)
  
  This is used to occupy specific regions of the declared space
  (dma_alloc_coherent() will hand out the first free region it finds).

  device_addr is the *device* address of the region requested.

  size is the size (and should be a page-sized multiple).

  
  The return value will be either a pointer to the processor virtual
  address of the memory, or an error (via PTR_ERR()) if any part of the
  region is occupied.

  
  Part III - Debug drivers use of the DMA-API
  -------------------------------------------
  
  The DMA-API as described above as some constraints. DMA addresses must be
  released with the corresponding function with the same size for example. With
  the advent of hardware IOMMUs it becomes more and more important that drivers
  do not violate those constraints. In the worst case such a violation can
  result in data corruption up to destroyed filesystems.
  
  To debug drivers and find bugs in the usage of the DMA-API checking code can
  be compiled into the kernel which will tell the developer about those
  violations. If your architecture supports it you can select the "Enable
  debugging of DMA-API usage" option in your kernel configuration. Enabling this
  option has a performance impact. Do not enable it in production kernels.
  
  If you boot the resulting kernel will contain code which does some bookkeeping
  about what DMA memory was allocated for which device. If this code detects an
  error it prints a warning message with some details into your kernel log. An
  example warning message may look like this:
  
  ------------[ cut here ]------------
  WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
  	check_unmap+0x203/0x490()
  Hardware name:
  forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
  	function [device address=0x00000000640444be] [size=66 bytes] [mapped as
  single] [unmapped as page]
  Modules linked in: nfsd exportfs bridge stp llc r8169
  Pid: 0, comm: swapper Tainted: G        W  2.6.28-dmatest-09289-g8bb99c0 #1
  Call Trace:
   <IRQ>  [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
   [<ffffffff80647b70>] _spin_unlock+0x10/0x30
   [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
   [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
   [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
   [<ffffffff80252f96>] queue_work+0x56/0x60
   [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
   [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
   [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
   [<ffffffff80235177>] find_busiest_group+0x207/0x8a0
   [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
   [<ffffffff803c7ea3>] check_unmap+0x203/0x490
   [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
   [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
   [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
   [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
   [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
   [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
   [<ffffffff8020c093>] ret_from_intr+0x0/0xa
   <EOI> <4>---[ end trace f6435a98e2a38c0e ]---
  
  The driver developer can find the driver and the device including a stacktrace
  of the DMA-API call which caused this warning.
  
  Per default only the first error will result in a warning message. All other
  errors will only silently counted. This limitation exist to prevent the code
  from flooding your kernel log. To support debugging a device driver this can
  be disabled via debugfs. See the debugfs interface documentation below for
  details.
  
  The debugfs directory for the DMA-API debugging code is called dma-api/. In
  this directory the following files can currently be found:
  
  	dma-api/all_errors	This file contains a numeric value. If this
  				value is not equal to zero the debugging code
  				will print a warning for every error it finds

  				into the kernel log. Be careful with this
  				option, as it can easily flood your logs.

  
  	dma-api/disabled	This read-only file contains the character 'Y'
  				if the debugging code is disabled. This can
  				happen when it runs out of memory or if it was
  				disabled at boot time
  
  	dma-api/error_count	This file is read-only and shows the total
  				numbers of errors found.
  
  	dma-api/num_errors	The number in this file shows how many
  				warnings will be printed to the kernel log
  				before it stops. This number is initialized to
  				one at system boot and be set by writing into
  				this file
  
  	dma-api/min_free_entries
  				This read-only file can be read to get the
  				minimum number of free dma_debug_entries the
  				allocator has ever seen. If this value goes
  				down to zero the code will disable itself
  				because it is not longer reliable.
  
  	dma-api/num_free_entries
  				The current number of free dma_debug_entries
  				in the allocator.

  	dma-api/driver-filter
  				You can write a name of a driver into this file
  				to limit the debug output to requests from that
  				particular driver. Write an empty string to
  				that file to disable the filter and see
  				all errors again.

  If you have this code compiled into your kernel it will be enabled by default.
  If you want to boot without the bookkeeping anyway you can provide
  'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
  Notice that you can not enable it again at runtime. You have to reboot to do
  so.

  If you want to see debug messages only for a special device driver you can
  specify the dma_debug_driver=<drivername> parameter. This will enable the
  driver filter at boot time. The debug code will only print errors for that
  driver afterwards. This filter can be disabled or changed later using debugfs.

  When the code disables itself at runtime this is most likely because it ran
  out of dma_debug_entries. These entries are preallocated at boot. The number
  of preallocated entries is defined per architecture. If it is too low for you
  boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
  architectural default.