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Based on kernel version 3.13. Page generated on 2014-01-20 22:02 EST.

1	               Dynamic DMA mapping using the generic device
2	               ============================================
3	
4	        James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
5	
6	This document describes the DMA API.  For a more gentle introduction
7	of the API (and actual examples) see
8	Documentation/DMA-API-HOWTO.txt.
9	
10	This API is split into two pieces.  Part I describes the API.  Part II
11	describes the extensions to the API for supporting non-consistent
12	memory machines.  Unless you know that your driver absolutely has to
13	support non-consistent platforms (this is usually only legacy
14	platforms) you should only use the API described in part I.
15	
16	Part I - dma_ API
17	-------------------------------------
18	
19	To get the dma_ API, you must #include <linux/dma-mapping.h>
20	
21	
22	Part Ia - Using large dma-coherent buffers
23	------------------------------------------
24	
25	void *
26	dma_alloc_coherent(struct device *dev, size_t size,
27				     dma_addr_t *dma_handle, gfp_t flag)
28	
29	Consistent memory is memory for which a write by either the device or
30	the processor can immediately be read by the processor or device
31	without having to worry about caching effects.  (You may however need
32	to make sure to flush the processor's write buffers before telling
33	devices to read that memory.)
34	
35	This routine allocates a region of <size> bytes of consistent memory.
36	It also returns a <dma_handle> which may be cast to an unsigned
37	integer the same width as the bus and used as the physical address
38	base of the region.
39	
40	Returns: a pointer to the allocated region (in the processor's virtual
41	address space) or NULL if the allocation failed.
42	
43	Note: consistent memory can be expensive on some platforms, and the
44	minimum allocation length may be as big as a page, so you should
45	consolidate your requests for consistent memory as much as possible.
46	The simplest way to do that is to use the dma_pool calls (see below).
47	
48	The flag parameter (dma_alloc_coherent only) allows the caller to
49	specify the GFP_ flags (see kmalloc) for the allocation (the
50	implementation may choose to ignore flags that affect the location of
51	the returned memory, like GFP_DMA).
52	
53	void *
54	dma_zalloc_coherent(struct device *dev, size_t size,
55				     dma_addr_t *dma_handle, gfp_t flag)
56	
57	Wraps dma_alloc_coherent() and also zeroes the returned memory if the
58	allocation attempt succeeded.
59	
60	void
61	dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
62				   dma_addr_t dma_handle)
63	
64	Free the region of consistent memory you previously allocated.  dev,
65	size and dma_handle must all be the same as those passed into the
66	consistent allocate.  cpu_addr must be the virtual address returned by
67	the consistent allocate.
68	
69	Note that unlike their sibling allocation calls, these routines
70	may only be called with IRQs enabled.
71	
72	
73	Part Ib - Using small dma-coherent buffers
74	------------------------------------------
75	
76	To get this part of the dma_ API, you must #include <linux/dmapool.h>
77	
78	Many drivers need lots of small dma-coherent memory regions for DMA
79	descriptors or I/O buffers.  Rather than allocating in units of a page
80	or more using dma_alloc_coherent(), you can use DMA pools.  These work
81	much like a struct kmem_cache, except that they use the dma-coherent allocator,
82	not __get_free_pages().  Also, they understand common hardware constraints
83	for alignment, like queue heads needing to be aligned on N-byte boundaries.
84	
85	
86		struct dma_pool *
87		dma_pool_create(const char *name, struct device *dev,
88				size_t size, size_t align, size_t alloc);
89	
90	The pool create() routines initialize a pool of dma-coherent buffers
91	for use with a given device.  It must be called in a context which
92	can sleep.
93	
94	The "name" is for diagnostics (like a struct kmem_cache name); dev and size
95	are like what you'd pass to dma_alloc_coherent().  The device's hardware
96	alignment requirement for this type of data is "align" (which is expressed
97	in bytes, and must be a power of two).  If your device has no boundary
98	crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated
99	from this pool must not cross 4KByte boundaries.
100	
101	
102		void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
103				dma_addr_t *dma_handle);
104	
105	This allocates memory from the pool; the returned memory will meet the size
106	and alignment requirements specified at creation time.  Pass GFP_ATOMIC to
107	prevent blocking, or if it's permitted (not in_interrupt, not holding SMP locks),
108	pass GFP_KERNEL to allow blocking.  Like dma_alloc_coherent(), this returns
109	two values:  an address usable by the cpu, and the dma address usable by the
110	pool's device.
111	
112	
113		void dma_pool_free(struct dma_pool *pool, void *vaddr,
114				dma_addr_t addr);
115	
116	This puts memory back into the pool.  The pool is what was passed to
117	the pool allocation routine; the cpu (vaddr) and dma addresses are what
118	were returned when that routine allocated the memory being freed.
119	
120	
121		void dma_pool_destroy(struct dma_pool *pool);
122	
123	The pool destroy() routines free the resources of the pool.  They must be
124	called in a context which can sleep.  Make sure you've freed all allocated
125	memory back to the pool before you destroy it.
126	
127	
128	Part Ic - DMA addressing limitations
129	------------------------------------
130	
131	int
132	dma_supported(struct device *dev, u64 mask)
133	
134	Checks to see if the device can support DMA to the memory described by
135	mask.
136	
137	Returns: 1 if it can and 0 if it can't.
138	
139	Notes: This routine merely tests to see if the mask is possible.  It
140	won't change the current mask settings.  It is more intended as an
141	internal API for use by the platform than an external API for use by
142	driver writers.
143	
144	int
145	dma_set_mask_and_coherent(struct device *dev, u64 mask)
146	
147	Checks to see if the mask is possible and updates the device
148	streaming and coherent DMA mask parameters if it is.
149	
150	Returns: 0 if successful and a negative error if not.
151	
152	int
153	dma_set_mask(struct device *dev, u64 mask)
154	
155	Checks to see if the mask is possible and updates the device
156	parameters if it is.
157	
158	Returns: 0 if successful and a negative error if not.
159	
160	int
161	dma_set_coherent_mask(struct device *dev, u64 mask)
162	
163	Checks to see if the mask is possible and updates the device
164	parameters if it is.
165	
166	Returns: 0 if successful and a negative error if not.
167	
168	u64
169	dma_get_required_mask(struct device *dev)
170	
171	This API returns the mask that the platform requires to
172	operate efficiently.  Usually this means the returned mask
173	is the minimum required to cover all of memory.  Examining the
174	required mask gives drivers with variable descriptor sizes the
175	opportunity to use smaller descriptors as necessary.
176	
177	Requesting the required mask does not alter the current mask.  If you
178	wish to take advantage of it, you should issue a dma_set_mask()
179	call to set the mask to the value returned.
180	
181	
182	Part Id - Streaming DMA mappings
183	--------------------------------
184	
185	dma_addr_t
186	dma_map_single(struct device *dev, void *cpu_addr, size_t size,
187			      enum dma_data_direction direction)
188	
189	Maps a piece of processor virtual memory so it can be accessed by the
190	device and returns the physical handle of the memory.
191	
192	The direction for both api's may be converted freely by casting.
193	However the dma_ API uses a strongly typed enumerator for its
194	direction:
195	
196	DMA_NONE		no direction (used for debugging)
197	DMA_TO_DEVICE		data is going from the memory to the device
198	DMA_FROM_DEVICE		data is coming from the device to the memory
199	DMA_BIDIRECTIONAL	direction isn't known
200	
201	Notes:  Not all memory regions in a machine can be mapped by this
202	API.  Further, regions that appear to be physically contiguous in
203	kernel virtual space may not be contiguous as physical memory.  Since
204	this API does not provide any scatter/gather capability, it will fail
205	if the user tries to map a non-physically contiguous piece of memory.
206	For this reason, it is recommended that memory mapped by this API be
207	obtained only from sources which guarantee it to be physically contiguous
208	(like kmalloc).
209	
210	Further, the physical address of the memory must be within the
211	dma_mask of the device (the dma_mask represents a bit mask of the
212	addressable region for the device.  I.e., if the physical address of
213	the memory anded with the dma_mask is still equal to the physical
214	address, then the device can perform DMA to the memory).  In order to
215	ensure that the memory allocated by kmalloc is within the dma_mask,
216	the driver may specify various platform-dependent flags to restrict
217	the physical memory range of the allocation (e.g. on x86, GFP_DMA
218	guarantees to be within the first 16Mb of available physical memory,
219	as required by ISA devices).
220	
221	Note also that the above constraints on physical contiguity and
222	dma_mask may not apply if the platform has an IOMMU (a device which
223	supplies a physical to virtual mapping between the I/O memory bus and
224	the device).  However, to be portable, device driver writers may *not*
225	assume that such an IOMMU exists.
226	
227	Warnings:  Memory coherency operates at a granularity called the cache
228	line width.  In order for memory mapped by this API to operate
229	correctly, the mapped region must begin exactly on a cache line
230	boundary and end exactly on one (to prevent two separately mapped
231	regions from sharing a single cache line).  Since the cache line size
232	may not be known at compile time, the API will not enforce this
233	requirement.  Therefore, it is recommended that driver writers who
234	don't take special care to determine the cache line size at run time
235	only map virtual regions that begin and end on page boundaries (which
236	are guaranteed also to be cache line boundaries).
237	
238	DMA_TO_DEVICE synchronisation must be done after the last modification
239	of the memory region by the software and before it is handed off to
240	the driver.  Once this primitive is used, memory covered by this
241	primitive should be treated as read-only by the device.  If the device
242	may write to it at any point, it should be DMA_BIDIRECTIONAL (see
243	below).
244	
245	DMA_FROM_DEVICE synchronisation must be done before the driver
246	accesses data that may be changed by the device.  This memory should
247	be treated as read-only by the driver.  If the driver needs to write
248	to it at any point, it should be DMA_BIDIRECTIONAL (see below).
249	
250	DMA_BIDIRECTIONAL requires special handling: it means that the driver
251	isn't sure if the memory was modified before being handed off to the
252	device and also isn't sure if the device will also modify it.  Thus,
253	you must always sync bidirectional memory twice: once before the
254	memory is handed off to the device (to make sure all memory changes
255	are flushed from the processor) and once before the data may be
256	accessed after being used by the device (to make sure any processor
257	cache lines are updated with data that the device may have changed).
258	
259	void
260	dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
261			 enum dma_data_direction direction)
262	
263	Unmaps the region previously mapped.  All the parameters passed in
264	must be identical to those passed in (and returned) by the mapping
265	API.
266	
267	dma_addr_t
268	dma_map_page(struct device *dev, struct page *page,
269			    unsigned long offset, size_t size,
270			    enum dma_data_direction direction)
271	void
272	dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
273		       enum dma_data_direction direction)
274	
275	API for mapping and unmapping for pages.  All the notes and warnings
276	for the other mapping APIs apply here.  Also, although the <offset>
277	and <size> parameters are provided to do partial page mapping, it is
278	recommended that you never use these unless you really know what the
279	cache width is.
280	
281	int
282	dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
283	
284	In some circumstances dma_map_single and dma_map_page will fail to create
285	a mapping. A driver can check for these errors by testing the returned
286	dma address with dma_mapping_error(). A non-zero return value means the mapping
287	could not be created and the driver should take appropriate action (e.g.
288	reduce current DMA mapping usage or delay and try again later).
289	
290		int
291		dma_map_sg(struct device *dev, struct scatterlist *sg,
292			int nents, enum dma_data_direction direction)
293	
294	Returns: the number of physical segments mapped (this may be shorter
295	than <nents> passed in if some elements of the scatter/gather list are
296	physically or virtually adjacent and an IOMMU maps them with a single
297	entry).
298	
299	Please note that the sg cannot be mapped again if it has been mapped once.
300	The mapping process is allowed to destroy information in the sg.
301	
302	As with the other mapping interfaces, dma_map_sg can fail. When it
303	does, 0 is returned and a driver must take appropriate action. It is
304	critical that the driver do something, in the case of a block driver
305	aborting the request or even oopsing is better than doing nothing and
306	corrupting the filesystem.
307	
308	With scatterlists, you use the resulting mapping like this:
309	
310		int i, count = dma_map_sg(dev, sglist, nents, direction);
311		struct scatterlist *sg;
312	
313		for_each_sg(sglist, sg, count, i) {
314			hw_address[i] = sg_dma_address(sg);
315			hw_len[i] = sg_dma_len(sg);
316		}
317	
318	where nents is the number of entries in the sglist.
319	
320	The implementation is free to merge several consecutive sglist entries
321	into one (e.g. with an IOMMU, or if several pages just happen to be
322	physically contiguous) and returns the actual number of sg entries it
323	mapped them to. On failure 0, is returned.
324	
325	Then you should loop count times (note: this can be less than nents times)
326	and use sg_dma_address() and sg_dma_len() macros where you previously
327	accessed sg->address and sg->length as shown above.
328	
329		void
330		dma_unmap_sg(struct device *dev, struct scatterlist *sg,
331			int nhwentries, enum dma_data_direction direction)
332	
333	Unmap the previously mapped scatter/gather list.  All the parameters
334	must be the same as those and passed in to the scatter/gather mapping
335	API.
336	
337	Note: <nents> must be the number you passed in, *not* the number of
338	physical entries returned.
339	
340	void
341	dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size,
342				enum dma_data_direction direction)
343	void
344	dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size,
345				   enum dma_data_direction direction)
346	void
347	dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nelems,
348			    enum dma_data_direction direction)
349	void
350	dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nelems,
351			       enum dma_data_direction direction)
352	
353	Synchronise a single contiguous or scatter/gather mapping for the cpu
354	and device. With the sync_sg API, all the parameters must be the same
355	as those passed into the single mapping API. With the sync_single API,
356	you can use dma_handle and size parameters that aren't identical to
357	those passed into the single mapping API to do a partial sync.
358	
359	Notes:  You must do this:
360	
361	- Before reading values that have been written by DMA from the device
362	  (use the DMA_FROM_DEVICE direction)
363	- After writing values that will be written to the device using DMA
364	  (use the DMA_TO_DEVICE) direction
365	- before *and* after handing memory to the device if the memory is
366	  DMA_BIDIRECTIONAL
367	
368	See also dma_map_single().
369	
370	dma_addr_t
371	dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
372			     enum dma_data_direction dir,
373			     struct dma_attrs *attrs)
374	
375	void
376	dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
377			       size_t size, enum dma_data_direction dir,
378			       struct dma_attrs *attrs)
379	
380	int
381	dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
382			 int nents, enum dma_data_direction dir,
383			 struct dma_attrs *attrs)
384	
385	void
386	dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
387			   int nents, enum dma_data_direction dir,
388			   struct dma_attrs *attrs)
389	
390	The four functions above are just like the counterpart functions
391	without the _attrs suffixes, except that they pass an optional
392	struct dma_attrs*.
393	
394	struct dma_attrs encapsulates a set of "dma attributes". For the
395	definition of struct dma_attrs see linux/dma-attrs.h.
396	
397	The interpretation of dma attributes is architecture-specific, and
398	each attribute should be documented in Documentation/DMA-attributes.txt.
399	
400	If struct dma_attrs* is NULL, the semantics of each of these
401	functions is identical to those of the corresponding function
402	without the _attrs suffix. As a result dma_map_single_attrs()
403	can generally replace dma_map_single(), etc.
404	
405	As an example of the use of the *_attrs functions, here's how
406	you could pass an attribute DMA_ATTR_FOO when mapping memory
407	for DMA:
408	
409	#include <linux/dma-attrs.h>
410	/* DMA_ATTR_FOO should be defined in linux/dma-attrs.h and
411	 * documented in Documentation/DMA-attributes.txt */
412	...
413	
414		DEFINE_DMA_ATTRS(attrs);
415		dma_set_attr(DMA_ATTR_FOO, &attrs);
416		....
417		n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, &attr);
418		....
419	
420	Architectures that care about DMA_ATTR_FOO would check for its
421	presence in their implementations of the mapping and unmapping
422	routines, e.g.:
423	
424	void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
425				     size_t size, enum dma_data_direction dir,
426				     struct dma_attrs *attrs)
427	{
428		....
429		int foo =  dma_get_attr(DMA_ATTR_FOO, attrs);
430		....
431		if (foo)
432			/* twizzle the frobnozzle */
433		....
434	
435	
436	Part II - Advanced dma_ usage
437	-----------------------------
438	
439	Warning: These pieces of the DMA API should not be used in the
440	majority of cases, since they cater for unlikely corner cases that
441	don't belong in usual drivers.
442	
443	If you don't understand how cache line coherency works between a
444	processor and an I/O device, you should not be using this part of the
445	API at all.
446	
447	void *
448	dma_alloc_noncoherent(struct device *dev, size_t size,
449				       dma_addr_t *dma_handle, gfp_t flag)
450	
451	Identical to dma_alloc_coherent() except that the platform will
452	choose to return either consistent or non-consistent memory as it sees
453	fit.  By using this API, you are guaranteeing to the platform that you
454	have all the correct and necessary sync points for this memory in the
455	driver should it choose to return non-consistent memory.
456	
457	Note: where the platform can return consistent memory, it will
458	guarantee that the sync points become nops.
459	
460	Warning:  Handling non-consistent memory is a real pain.  You should
461	only ever use this API if you positively know your driver will be
462	required to work on one of the rare (usually non-PCI) architectures
463	that simply cannot make consistent memory.
464	
465	void
466	dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
467				      dma_addr_t dma_handle)
468	
469	Free memory allocated by the nonconsistent API.  All parameters must
470	be identical to those passed in (and returned by
471	dma_alloc_noncoherent()).
472	
473	int
474	dma_get_cache_alignment(void)
475	
476	Returns the processor cache alignment.  This is the absolute minimum
477	alignment *and* width that you must observe when either mapping
478	memory or doing partial flushes.
479	
480	Notes: This API may return a number *larger* than the actual cache
481	line, but it will guarantee that one or more cache lines fit exactly
482	into the width returned by this call.  It will also always be a power
483	of two for easy alignment.
484	
485	void
486	dma_cache_sync(struct device *dev, void *vaddr, size_t size,
487		       enum dma_data_direction direction)
488	
489	Do a partial sync of memory that was allocated by
490	dma_alloc_noncoherent(), starting at virtual address vaddr and
491	continuing on for size.  Again, you *must* observe the cache line
492	boundaries when doing this.
493	
494	int
495	dma_declare_coherent_memory(struct device *dev, dma_addr_t bus_addr,
496				    dma_addr_t device_addr, size_t size, int
497				    flags)
498	
499	Declare region of memory to be handed out by dma_alloc_coherent when
500	it's asked for coherent memory for this device.
501	
502	bus_addr is the physical address to which the memory is currently
503	assigned in the bus responding region (this will be used by the
504	platform to perform the mapping).
505	
506	device_addr is the physical address the device needs to be programmed
507	with actually to address this memory (this will be handed out as the
508	dma_addr_t in dma_alloc_coherent()).
509	
510	size is the size of the area (must be multiples of PAGE_SIZE).
511	
512	flags can be or'd together and are:
513	
514	DMA_MEMORY_MAP - request that the memory returned from
515	dma_alloc_coherent() be directly writable.
516	
517	DMA_MEMORY_IO - request that the memory returned from
518	dma_alloc_coherent() be addressable using read/write/memcpy_toio etc.
519	
520	One or both of these flags must be present.
521	
522	DMA_MEMORY_INCLUDES_CHILDREN - make the declared memory be allocated by
523	dma_alloc_coherent of any child devices of this one (for memory residing
524	on a bridge).
525	
526	DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions. 
527	Do not allow dma_alloc_coherent() to fall back to system memory when
528	it's out of memory in the declared region.
529	
530	The return value will be either DMA_MEMORY_MAP or DMA_MEMORY_IO and
531	must correspond to a passed in flag (i.e. no returning DMA_MEMORY_IO
532	if only DMA_MEMORY_MAP were passed in) for success or zero for
533	failure.
534	
535	Note, for DMA_MEMORY_IO returns, all subsequent memory returned by
536	dma_alloc_coherent() may no longer be accessed directly, but instead
537	must be accessed using the correct bus functions.  If your driver
538	isn't prepared to handle this contingency, it should not specify
539	DMA_MEMORY_IO in the input flags.
540	
541	As a simplification for the platforms, only *one* such region of
542	memory may be declared per device.
543	
544	For reasons of efficiency, most platforms choose to track the declared
545	region only at the granularity of a page.  For smaller allocations,
546	you should use the dma_pool() API.
547	
548	void
549	dma_release_declared_memory(struct device *dev)
550	
551	Remove the memory region previously declared from the system.  This
552	API performs *no* in-use checking for this region and will return
553	unconditionally having removed all the required structures.  It is the
554	driver's job to ensure that no parts of this memory region are
555	currently in use.
556	
557	void *
558	dma_mark_declared_memory_occupied(struct device *dev,
559					  dma_addr_t device_addr, size_t size)
560	
561	This is used to occupy specific regions of the declared space
562	(dma_alloc_coherent() will hand out the first free region it finds).
563	
564	device_addr is the *device* address of the region requested.
565	
566	size is the size (and should be a page-sized multiple).
567	
568	The return value will be either a pointer to the processor virtual
569	address of the memory, or an error (via PTR_ERR()) if any part of the
570	region is occupied.
571	
572	Part III - Debug drivers use of the DMA-API
573	-------------------------------------------
574	
575	The DMA-API as described above as some constraints. DMA addresses must be
576	released with the corresponding function with the same size for example. With
577	the advent of hardware IOMMUs it becomes more and more important that drivers
578	do not violate those constraints. In the worst case such a violation can
579	result in data corruption up to destroyed filesystems.
580	
581	To debug drivers and find bugs in the usage of the DMA-API checking code can
582	be compiled into the kernel which will tell the developer about those
583	violations. If your architecture supports it you can select the "Enable
584	debugging of DMA-API usage" option in your kernel configuration. Enabling this
585	option has a performance impact. Do not enable it in production kernels.
586	
587	If you boot the resulting kernel will contain code which does some bookkeeping
588	about what DMA memory was allocated for which device. If this code detects an
589	error it prints a warning message with some details into your kernel log. An
590	example warning message may look like this:
591	
592	------------[ cut here ]------------
593	WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
594		check_unmap+0x203/0x490()
595	Hardware name:
596	forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
597		function [device address=0x00000000640444be] [size=66 bytes] [mapped as
598	single] [unmapped as page]
599	Modules linked in: nfsd exportfs bridge stp llc r8169
600	Pid: 0, comm: swapper Tainted: G        W  2.6.28-dmatest-09289-g8bb99c0 #1
601	Call Trace:
602	 <IRQ>  [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
603	 [<ffffffff80647b70>] _spin_unlock+0x10/0x30
604	 [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
605	 [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
606	 [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
607	 [<ffffffff80252f96>] queue_work+0x56/0x60
608	 [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
609	 [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
610	 [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
611	 [<ffffffff80235177>] find_busiest_group+0x207/0x8a0
612	 [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
613	 [<ffffffff803c7ea3>] check_unmap+0x203/0x490
614	 [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
615	 [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
616	 [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
617	 [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
618	 [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
619	 [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
620	 [<ffffffff8020c093>] ret_from_intr+0x0/0xa
621	 <EOI> <4>---[ end trace f6435a98e2a38c0e ]---
622	
623	The driver developer can find the driver and the device including a stacktrace
624	of the DMA-API call which caused this warning.
625	
626	Per default only the first error will result in a warning message. All other
627	errors will only silently counted. This limitation exist to prevent the code
628	from flooding your kernel log. To support debugging a device driver this can
629	be disabled via debugfs. See the debugfs interface documentation below for
630	details.
631	
632	The debugfs directory for the DMA-API debugging code is called dma-api/. In
633	this directory the following files can currently be found:
634	
635		dma-api/all_errors	This file contains a numeric value. If this
636					value is not equal to zero the debugging code
637					will print a warning for every error it finds
638					into the kernel log. Be careful with this
639					option, as it can easily flood your logs.
640	
641		dma-api/disabled	This read-only file contains the character 'Y'
642					if the debugging code is disabled. This can
643					happen when it runs out of memory or if it was
644					disabled at boot time
645	
646		dma-api/error_count	This file is read-only and shows the total
647					numbers of errors found.
648	
649		dma-api/num_errors	The number in this file shows how many
650					warnings will be printed to the kernel log
651					before it stops. This number is initialized to
652					one at system boot and be set by writing into
653					this file
654	
655		dma-api/min_free_entries
656					This read-only file can be read to get the
657					minimum number of free dma_debug_entries the
658					allocator has ever seen. If this value goes
659					down to zero the code will disable itself
660					because it is not longer reliable.
661	
662		dma-api/num_free_entries
663					The current number of free dma_debug_entries
664					in the allocator.
665	
666		dma-api/driver-filter
667					You can write a name of a driver into this file
668					to limit the debug output to requests from that
669					particular driver. Write an empty string to
670					that file to disable the filter and see
671					all errors again.
672	
673	If you have this code compiled into your kernel it will be enabled by default.
674	If you want to boot without the bookkeeping anyway you can provide
675	'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
676	Notice that you can not enable it again at runtime. You have to reboot to do
677	so.
678	
679	If you want to see debug messages only for a special device driver you can
680	specify the dma_debug_driver=<drivername> parameter. This will enable the
681	driver filter at boot time. The debug code will only print errors for that
682	driver afterwards. This filter can be disabled or changed later using debugfs.
683	
684	When the code disables itself at runtime this is most likely because it ran
685	out of dma_debug_entries. These entries are preallocated at boot. The number
686	of preallocated entries is defined per architecture. If it is too low for you
687	boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
688	architectural default.
689	
690	void debug_dmap_mapping_error(struct device *dev, dma_addr_t dma_addr);
691	
692	dma-debug interface debug_dma_mapping_error() to debug drivers that fail
693	to check dma mapping errors on addresses returned by dma_map_single() and
694	dma_map_page() interfaces. This interface clears a flag set by
695	debug_dma_map_page() to indicate that dma_mapping_error() has been called by
696	the driver. When driver does unmap, debug_dma_unmap() checks the flag and if
697	this flag is still set, prints warning message that includes call trace that
698	leads up to the unmap. This interface can be called from dma_mapping_error()
699	routines to enable dma mapping error check debugging.
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