FreeBSD/Linux Kernel Cross Reference
sys/Documentation/DMA-API.txt

                    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.
   
   void debug_dmap_mapping_error(struct device *dev, dma_addr_t dma_addr);
   
   dma-debug interface debug_dma_mapping_error() to debug drivers that fail
   to check dma mapping errors on addresses returned by dma_map_single() and
   dma_map_page() interfaces. This interface clears a flag set by
   debug_dma_map_page() to indicate that dma_mapping_error() has been called by
   the driver. When driver does unmap, debug_dma_unmap() checks the flag and if
   this flag is still set, prints warning message that includes call trace that
   leads up to the unmap. This interface can be called from dma_mapping_error()
   routines to enable dma mapping error check debugging.
   

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