Documentation/core-api/dma-api.rst

============================================
Dynamic DMA mapping using the generic device
============================================

:Author: 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/core-api/dma-api-howto.rst.

This API is split into two pieces.  Part I describes the basic API.
Part II describes extensions for supporting non-coherent memory
machines.  Unless you know that your driver absolutely has to support
non-coherent 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>.  This
provides dma_addr_t and the interfaces described below.

A dma_addr_t can hold any valid DMA address for the platform.  It can be
given to a device to use as a DMA source or target.  A CPU cannot reference
a dma_addr_t directly because there may be translation between its physical
address space and the DMA address space.

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)

Coherent 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 coherent memory.

It returns a pointer to the allocated region (in the processor's virtual
address space) or NULL if the allocation failed.

It also returns a <dma_handle> which may be cast to an unsigned integer the
same width as the bus and given to the device as the DMA address base of
the region.

Note: coherent 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 coherent memory as much as possible.
The simplest way to do that is to use the dma_pool calls (see below).

The flag parameter allows the caller to specify the ``GFP_`` flags (see
kmalloc()) for the allocation (the implementation may ignore flags that affect
the location of the returned memory, like GFP_DMA).

::

	void
	dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
			  dma_addr_t dma_handle)

Free a previously allocated region of coherent memory.  dev, size and dma_handle
must all be the same as those passed into dma_alloc_coherent().  cpu_addr must
be the virtual address returned by dma_alloc_coherent().

Note that unlike the sibling allocation call, this routine 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.

.. kernel-doc:: mm/dmapool.c
   :export:

.. kernel-doc:: include/linux/dmapool.h


Part Ic - DMA addressing limitations
------------------------------------

DMA mask is a bit mask of the addressable region for the device. In other words,
if applying the DMA mask (a bitwise AND operation) to the DMA address of a
memory region does not clear any bits in the address, then the device can
perform DMA to that memory region.

All the below functions which set a DMA mask may fail if the requested mask
cannot be used with the device, or if the device is not capable of doing DMA.

::

	int
	dma_set_mask_and_coherent(struct device *dev, u64 mask)

Updates both streaming and coherent DMA masks.

Returns: 0 if successful and a negative error if not.

::

	int
	dma_set_mask(struct device *dev, u64 mask)

Updates only the streaming DMA mask.

Returns: 0 if successful and a negative error if not.

::

	int
	dma_set_coherent_mask(struct device *dev, u64 mask)

Updates only the coherent DMA mask.

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.

::

	size_t
	dma_max_mapping_size(struct device *dev);

Returns the maximum size of a mapping for the device. The size parameter
of the mapping functions like dma_map_single(), dma_map_page() and
others should not be larger than the returned value.

::

	size_t
	dma_opt_mapping_size(struct device *dev);

Returns the maximum optimal size of a mapping for the device.

Mapping larger buffers may take much longer in certain scenarios. In
addition, for high-rate short-lived streaming mappings, the upfront time
spent on the mapping may account for an appreciable part of the total
request lifetime. As such, if splitting larger requests incurs no
significant performance penalty, then device drivers are advised to
limit total DMA streaming mappings length to the returned value.

::

	bool
	dma_need_sync(struct device *dev, dma_addr_t dma_addr);

Returns %true if dma_sync_single_for_{device,cpu} calls are required to
transfer memory ownership.  Returns %false if those calls can be skipped.

::

	unsigned long
	dma_get_merge_boundary(struct device *dev);

Returns the DMA merge boundary. If the device cannot merge any DMA address
segments, the function returns 0.

Part Id - Streaming DMA mappings
--------------------------------

Streaming DMA allows to map an existing buffer for DMA transfers and then
unmap it when finished.  Map functions are not guaranteed to succeed, so the
return value must be checked.

.. note::

	In particular, mapping may fail for memory not addressable by the
	device, e.g. if it is not within the DMA mask of the device and/or a
	connecting bus bridge.  Streaming DMA functions try to overcome such
	addressing constraints, either by using an IOMMU (a device which maps
	I/O DMA addresses to physical memory addresses), or by copying the
	data to/from a bounce buffer if the kernel is configured with a
	:doc:`SWIOTLB <swiotlb>`.  However, these methods are not always
	available, and even if they are, they may still fail for a number of
	reasons.

	In short, a device driver may need to be wary of where buffers are
	located in physical memory, especially if the DMA mask is less than 32
	bits.

::

	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 DMA address of the memory.

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
======================= =============================================

.. note::

	Contiguous 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, memory to be mapped by
	this API should be obtained from sources which guarantee it to be
	physically contiguous (like kmalloc).

.. warning::

	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 device.  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 to (and returned by) dma_map_single().

::

	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.

::

	dma_addr_t
	dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size,
			 enum dma_data_direction dir, unsigned long attrs)

	void
	dma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size,
			   enum dma_data_direction dir, unsigned long attrs)

API for mapping and unmapping for MMIO resources. All the notes and
warnings for the other mapping APIs apply here. The API should only be
used to map device MMIO resources, mapping of RAM is not permitted.

::

	int
	dma_mapping_error(struct device *dev, dma_addr_t dma_addr)

In some circumstances dma_map_single(), dma_map_page() and dma_map_resource()
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)

Maps a scatter/gather list for DMA. Returns the number of DMA address segments
mapped, which may be smaller than <nents> passed in if several consecutive
sglist entries are merged (e.g. with an IOMMU, or if some adjacent segments
just happen to be physically contiguous).

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.  The returned number is 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 nents, 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
DMA address 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 nents,
			    enum dma_data_direction direction)

	void
	dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
			       int nents,
			       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 sg 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.


.. note::

   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,
			     unsigned long attrs)

	void
	dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
			       size_t size, enum dma_data_direction dir,
			       unsigned long attrs)

	int
	dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
			 int nents, enum dma_data_direction dir,
			 unsigned long attrs)

	void
	dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
			   int nents, enum dma_data_direction dir,
			   unsigned long attrs)

The four functions above are just like the counterpart functions
without the _attrs suffixes, except that they pass an optional
dma_attrs.

The interpretation of DMA attributes is architecture-specific, and
each attribute should be documented in
Documentation/core-api/dma-attributes.rst.

If dma_attrs are 0, 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-mapping.h>
	/* DMA_ATTR_FOO should be defined in linux/dma-mapping.h and
	* documented in Documentation/core-api/dma-attributes.rst */
	...

		unsigned long attr;
		attr |= DMA_ATTR_FOO;
		....
		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,
				     unsigned long attrs)
	{
		....
		if (attrs & DMA_ATTR_FOO)
			/* twizzle the frobnozzle */
		....
	}

Part Ie - IOVA-based DMA mappings
---------------------------------

These APIs allow a very efficient mapping when using an IOMMU.  They are an
optional path that requires extra code and are only recommended for drivers
where DMA mapping performance, or the space usage for storing the DMA addresses
matter.  All the considerations from the previous section apply here as well.

::

    bool dma_iova_try_alloc(struct device *dev, struct dma_iova_state *state,
		phys_addr_t phys, size_t size);

Is used to try to allocate IOVA space for mapping operation.  If it returns
false this API can't be used for the given device and the normal streaming
DMA mapping API should be used.  The ``struct dma_iova_state`` is allocated
by the driver and must be kept around until unmap time.

::

    static inline bool dma_use_iova(struct dma_iova_state *state)

Can be used by the driver to check if the IOVA-based API is used after a
call to dma_iova_try_alloc.  This can be useful in the unmap path.

::

    int dma_iova_link(struct device *dev, struct dma_iova_state *state,
		phys_addr_t phys, size_t offset, size_t size,
		enum dma_data_direction dir, unsigned long attrs);

Is used to link ranges to the IOVA previously allocated.  The start of all
but the first call to dma_iova_link for a given state must be aligned
to the DMA merge boundary returned by ``dma_get_merge_boundary()``, and
the size of all but the last range must be aligned to the DMA merge boundary
as well.

::

    int dma_iova_sync(struct device *dev, struct dma_iova_state *state,
		size_t offset, size_t size);

Must be called to sync the IOMMU page tables for IOVA-range mapped by one or
more calls to ``dma_iova_link()``.

For drivers that use a one-shot mapping, all ranges can be unmapped and the
IOVA freed by calling:

::

   void dma_iova_destroy(struct device *dev, struct dma_iova_state *state,
		size_t mapped_len, enum dma_data_direction dir,
                unsigned long attrs);

Alternatively drivers can dynamically manage the IOVA space by unmapping
and mapping individual regions.  In that case

::

    void dma_iova_unlink(struct device *dev, struct dma_iova_state *state,
		size_t offset, size_t size, enum dma_data_direction dir,
		unsigned long attrs);

is used to unmap a range previously mapped, and

::

   void dma_iova_free(struct device *dev, struct dma_iova_state *state);

is used to free the IOVA space.  All regions must have been unmapped using
``dma_iova_unlink()`` before calling ``dma_iova_free()``.

Part II - Non-coherent DMA allocations
--------------------------------------

These APIs allow to allocate pages that are guaranteed to be DMA addressable
by the passed in device, but which need explicit management of memory ownership
for the kernel vs the device.

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.

::

	struct page *
	dma_alloc_pages(struct device *dev, size_t size, dma_addr_t *dma_handle,
			enum dma_data_direction dir, gfp_t gfp)

This routine allocates a region of <size> bytes of non-coherent memory.  It
returns a pointer to first struct page for the region, or NULL if the
allocation failed. The resulting struct page can be used for everything a
struct page is suitable for.

It also returns a <dma_handle> which may be cast to an unsigned integer the
same width as the bus and given to the device as the DMA address base of
the region.

The dir parameter specified if data is read and/or written by the device,
see dma_map_single() for details.

The gfp parameter allows the caller to specify the ``GFP_`` flags (see
kmalloc()) for the allocation, but rejects flags used to specify a memory
zone such as GFP_DMA or GFP_HIGHMEM.

Before giving the memory to the device, dma_sync_single_for_device() needs
to be called, and before reading memory written by the device,
dma_sync_single_for_cpu(), just like for streaming DMA mappings that are
reused.

::

	void
	dma_free_pages(struct device *dev, size_t size, struct page *page,
			dma_addr_t dma_handle, enum dma_data_direction dir)

Free a region of memory previously allocated using dma_alloc_pages().
dev, size, dma_handle and dir must all be the same as those passed into
dma_alloc_pages().  page must be the pointer returned by dma_alloc_pages().

::

	int
	dma_mmap_pages(struct device *dev, struct vm_area_struct *vma,
		       size_t size, struct page *page)

Map an allocation returned from dma_alloc_pages() into a user address space.
dev and size must be the same as those passed into dma_alloc_pages().
page must be the pointer returned by dma_alloc_pages().

::

	void *
	dma_alloc_noncoherent(struct device *dev, size_t size,
			dma_addr_t *dma_handle, enum dma_data_direction dir,
			gfp_t gfp)

This routine is a convenient wrapper around dma_alloc_pages that returns the
kernel virtual address for the allocated memory instead of the page structure.

::

	void
	dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
			dma_addr_t dma_handle, enum dma_data_direction dir)

Free a region of memory previously allocated using dma_alloc_noncoherent().
dev, size, dma_handle and dir must all be the same as those passed into
dma_alloc_noncoherent().  cpu_addr must be the virtual address returned by
dma_alloc_noncoherent().

::

	struct sg_table *
	dma_alloc_noncontiguous(struct device *dev, size_t size,
				enum dma_data_direction dir, gfp_t gfp,
				unsigned long attrs);

This routine allocates  <size> bytes of non-coherent and possibly non-contiguous
memory.  It returns a pointer to struct sg_table that describes the allocated
and DMA mapped memory, or NULL if the allocation failed. The resulting memory
can be used for struct page mapped into a scatterlist are suitable for.

The return sg_table is guaranteed to have 1 single DMA mapped segment as
indicated by sgt->nents, but it might have multiple CPU side segments as
indicated by sgt->orig_nents.

The dir parameter specified if data is read and/or written by the device,
see dma_map_single() for details.

The gfp parameter allows the caller to specify the ``GFP_`` flags (see
kmalloc()) for the allocation, but rejects flags used to specify a memory
zone such as GFP_DMA or GFP_HIGHMEM.

The attrs argument must be either 0 or DMA_ATTR_ALLOC_SINGLE_PAGES.

Before giving the memory to the device, dma_sync_sgtable_for_device() needs
to be called, and before reading memory written by the device,
dma_sync_sgtable_for_cpu(), just like for streaming DMA mappings that are
reused.

::

	void
	dma_free_noncontiguous(struct device *dev, size_t size,
			       struct sg_table *sgt,
			       enum dma_data_direction dir)

Free memory previously allocated using dma_alloc_noncontiguous().  dev, size,
and dir must all be the same as those passed into dma_alloc_noncontiguous().
sgt must be the pointer returned by dma_alloc_noncontiguous().

::

	void *
	dma_vmap_noncontiguous(struct device *dev, size_t size,
		struct sg_table *sgt)

Return a contiguous kernel mapping for an allocation returned from
dma_alloc_noncontiguous().  dev and size must be the same as those passed into
dma_alloc_noncontiguous().  sgt must be the pointer returned by
dma_alloc_noncontiguous().

Once a non-contiguous allocation is mapped using this function, the
flush_kernel_vmap_range() and invalidate_kernel_vmap_range() APIs must be used
to manage the coherency between the kernel mapping, the device and user space
mappings (if any).

::

	void
	dma_vunmap_noncontiguous(struct device *dev, void *vaddr)

Unmap a kernel mapping returned by dma_vmap_noncontiguous().  dev must be the
same the one passed into dma_alloc_noncontiguous().  vaddr must be the pointer
returned by dma_vmap_noncontiguous().


::

	int
	dma_mmap_noncontiguous(struct device *dev, struct vm_area_struct *vma,
			       size_t size, struct sg_table *sgt)

Map an allocation returned from dma_alloc_noncontiguous() into a user address
space.  dev and size must be the same as those passed into
dma_alloc_noncontiguous().  sgt must be the pointer returned by
dma_alloc_noncontiguous().

::

	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.

.. note::

	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.


Part III - Debug drivers use of the DMA API
-------------------------------------------

The DMA API as described above has 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::

	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_phys+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/dump			This read-only file contains current DMA
				mappings.

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 attempt to increase
				nr_total_entries to compensate.

dma-api/num_free_entries	The current number of free dma_debug_entries
				in the allocator.

dma-api/nr_total_entries	The total number of dma_debug_entries in the
				allocator, both free and used.

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 and was unable to allocate more on-demand. 65536
entries are preallocated at boot - if this is too low for you boot with
'dma_debug_entries=<your_desired_number>' to overwrite the default. Note
that the code allocates entries in batches, so the exact number of
preallocated entries may be greater than the actual number requested. The
code will print to the kernel log each time it has dynamically allocated
as many entries as were initially preallocated. This is to indicate that a
larger preallocation size may be appropriate, or if it happens continually
that a driver may be leaking mappings.

::

	void
	debug_dma_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_phys() 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.

Functions and structures
========================

.. kernel-doc:: include/linux/scatterlist.h
.. kernel-doc:: lib/scatterlist.c