path: root/Documentation/DMA-API.txt

============================================
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/DMA-API-HOWTO.txt.

This API is split into two pieces.  Part I describes the basic API.
Part II describes extensions 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>.  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)

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

Free a region of consistent memory you previously allocated.  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
the dma_alloc_coherent().

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);

dma_pool_create() initializes 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_zalloc(struct dma_pool *pool, gfp_t mem_flags,
		        dma_addr_t *handle)

Wraps dma_pool_alloc() and also zeroes the returned memory if the
allocation attempt succeeded.


::

	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
dma_pool_alloc(); 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);

dma_pool_destroy() frees the resources of the pool.  It 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_set_mask_and_coherent(struct device *dev, u64 mask)

Checks to see if the mask is possible and updates the device
streaming and coherent DMA mask parameters if it is.

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

::

	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 DMA address of the memory.

The direction for both APIs 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
======================= =============================================

.. note::

	Not all memory regions in a machine can be mapped by this API.
	Further, 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).

	Further, the DMA address of the memory must be within the
	dma_mask of the device (the dma_mask is a bit mask of the
	addressable region for the device, i.e., if the DMA address of
	the memory ANDed with the dma_mask is still equal to the DMA
	address, then the device can perform DMA to the memory).  To
	ensure that the memory allocated by kmalloc is within the dma_mask,
	the driver may specify various platform-dependent flags to restrict
	the DMA address range of the allocation (e.g., on x86, GFP_DMA
	guarantees to be within the first 16MB of available DMA addresses,
	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
	maps an I/O DMA address to a physical memory address).  However, to be
	portable, device driver writers may *not* assume that such an IOMMU
	exists.

.. 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 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.

::

	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)

Returns: the number of DMA address 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 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 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.


.. 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/DMA-attributes.txt.

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/DMA-attributes.txt */
	...

		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 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_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
			gfp_t flag, unsigned long attrs)

Identical to dma_alloc_coherent() except that when the
DMA_ATTR_NON_CONSISTENT flags is passed in the attrs argument, 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 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_attrs(struct device *dev, size_t size, void *cpu_addr,
		       dma_addr_t dma_handle, unsigned long attrs)

Free memory allocated by the dma_alloc_attrs().  All common
parameters must be identical to those otherwise passed to dma_free_coherent,
and the attrs argument must be identical to the attrs passed to
dma_alloc_attrs().

::

	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.

::

	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_attrs() with
the DMA_ATTR_NON_CONSISTENT flag 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, phys_addr_t phys_addr,
				    dma_addr_t device_addr, size_t size);

Declare region of memory to be handed out by dma_alloc_coherent() when
it's asked for coherent memory for this device.

phys_addr is the CPU physical address to which the memory is currently
assigned (this will be ioremapped so the CPU can access the region).

device_addr is the DMA address the device needs to be programmed
with to actually 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).

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.

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_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/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_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.