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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	============================================
2	Dynamic DMA mapping using the generic device
3	============================================
4	
5	:Author: James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
6	
7	This document describes the DMA API.  For a more gentle introduction
8	of the API (and actual examples), see Documentation/DMA-API-HOWTO.txt.
9	
10	This API is split into two pieces.  Part I describes the basic API.
11	Part II describes extensions for supporting non-consistent memory
12	machines.  Unless you know that your driver absolutely has to support
13	non-consistent platforms (this is usually only legacy platforms) you
14	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>.  This
20	provides dma_addr_t and the interfaces described below.
21	
22	A dma_addr_t can hold any valid DMA address for the platform.  It can be
23	given to a device to use as a DMA source or target.  A CPU cannot reference
24	a dma_addr_t directly because there may be translation between its physical
25	address space and the DMA address space.
26	
27	Part Ia - Using large DMA-coherent buffers
28	------------------------------------------
29	
30	::
31	
32		void *
33		dma_alloc_coherent(struct device *dev, size_t size,
34				   dma_addr_t *dma_handle, gfp_t flag)
35	
36	Consistent memory is memory for which a write by either the device or
37	the processor can immediately be read by the processor or device
38	without having to worry about caching effects.  (You may however need
39	to make sure to flush the processor's write buffers before telling
40	devices to read that memory.)
41	
42	This routine allocates a region of <size> bytes of consistent memory.
43	
44	It returns a pointer to the allocated region (in the processor's virtual
45	address space) or NULL if the allocation failed.
46	
47	It also returns a <dma_handle> which may be cast to an unsigned integer the
48	same width as the bus and given to the device as the DMA address base of
49	the region.
50	
51	Note: consistent memory can be expensive on some platforms, and the
52	minimum allocation length may be as big as a page, so you should
53	consolidate your requests for consistent memory as much as possible.
54	The simplest way to do that is to use the dma_pool calls (see below).
55	
56	The flag parameter (dma_alloc_coherent() only) allows the caller to
57	specify the ``GFP_`` flags (see kmalloc()) for the allocation (the
58	implementation may choose to ignore flags that affect the location of
59	the returned memory, like GFP_DMA).
60	
61	::
62	
63		void *
64		dma_zalloc_coherent(struct device *dev, size_t size,
65				    dma_addr_t *dma_handle, gfp_t flag)
66	
67	Wraps dma_alloc_coherent() and also zeroes the returned memory if the
68	allocation attempt succeeded.
69	
70	::
71	
72		void
73		dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
74				  dma_addr_t dma_handle)
75	
76	Free a region of consistent memory you previously allocated.  dev,
77	size and dma_handle must all be the same as those passed into
78	dma_alloc_coherent().  cpu_addr must be the virtual address returned by
79	the dma_alloc_coherent().
80	
81	Note that unlike their sibling allocation calls, these routines
82	may only be called with IRQs enabled.
83	
84	
85	Part Ib - Using small DMA-coherent buffers
86	------------------------------------------
87	
88	To get this part of the dma_API, you must #include <linux/dmapool.h>
89	
90	Many drivers need lots of small DMA-coherent memory regions for DMA
91	descriptors or I/O buffers.  Rather than allocating in units of a page
92	or more using dma_alloc_coherent(), you can use DMA pools.  These work
93	much like a struct kmem_cache, except that they use the DMA-coherent allocator,
94	not __get_free_pages().  Also, they understand common hardware constraints
95	for alignment, like queue heads needing to be aligned on N-byte boundaries.
96	
97	
98	::
99	
100		struct dma_pool *
101		dma_pool_create(const char *name, struct device *dev,
102				size_t size, size_t align, size_t alloc);
103	
104	dma_pool_create() initializes a pool of DMA-coherent buffers
105	for use with a given device.  It must be called in a context which
106	can sleep.
107	
108	The "name" is for diagnostics (like a struct kmem_cache name); dev and size
109	are like what you'd pass to dma_alloc_coherent().  The device's hardware
110	alignment requirement for this type of data is "align" (which is expressed
111	in bytes, and must be a power of two).  If your device has no boundary
112	crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated
113	from this pool must not cross 4KByte boundaries.
114	
115	::
116	
117		void *
118		dma_pool_zalloc(struct dma_pool *pool, gfp_t mem_flags,
119			        dma_addr_t *handle)
120	
121	Wraps dma_pool_alloc() and also zeroes the returned memory if the
122	allocation attempt succeeded.
123	
124	
125	::
126	
127		void *
128		dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
129			       dma_addr_t *dma_handle);
130	
131	This allocates memory from the pool; the returned memory will meet the
132	size and alignment requirements specified at creation time.  Pass
133	GFP_ATOMIC to prevent blocking, or if it's permitted (not
134	in_interrupt, not holding SMP locks), pass GFP_KERNEL to allow
135	blocking.  Like dma_alloc_coherent(), this returns two values:  an
136	address usable by the CPU, and the DMA address usable by the pool's
137	device.
138	
139	::
140	
141		void
142		dma_pool_free(struct dma_pool *pool, void *vaddr,
143			      dma_addr_t addr);
144	
145	This puts memory back into the pool.  The pool is what was passed to
146	dma_pool_alloc(); the CPU (vaddr) and DMA addresses are what
147	were returned when that routine allocated the memory being freed.
148	
149	::
150	
151		void
152		dma_pool_destroy(struct dma_pool *pool);
153	
154	dma_pool_destroy() frees the resources of the pool.  It must be
155	called in a context which can sleep.  Make sure you've freed all allocated
156	memory back to the pool before you destroy it.
157	
158	
159	Part Ic - DMA addressing limitations
160	------------------------------------
161	
162	::
163	
164		int
165		dma_set_mask_and_coherent(struct device *dev, u64 mask)
166	
167	Checks to see if the mask is possible and updates the device
168	streaming and coherent DMA mask parameters if it is.
169	
170	Returns: 0 if successful and a negative error if not.
171	
172	::
173	
174		int
175		dma_set_mask(struct device *dev, u64 mask)
176	
177	Checks to see if the mask is possible and updates the device
178	parameters if it is.
179	
180	Returns: 0 if successful and a negative error if not.
181	
182	::
183	
184		int
185		dma_set_coherent_mask(struct device *dev, u64 mask)
186	
187	Checks to see if the mask is possible and updates the device
188	parameters if it is.
189	
190	Returns: 0 if successful and a negative error if not.
191	
192	::
193	
194		u64
195		dma_get_required_mask(struct device *dev)
196	
197	This API returns the mask that the platform requires to
198	operate efficiently.  Usually this means the returned mask
199	is the minimum required to cover all of memory.  Examining the
200	required mask gives drivers with variable descriptor sizes the
201	opportunity to use smaller descriptors as necessary.
202	
203	Requesting the required mask does not alter the current mask.  If you
204	wish to take advantage of it, you should issue a dma_set_mask()
205	call to set the mask to the value returned.
206	
207	
208	Part Id - Streaming DMA mappings
209	--------------------------------
210	
211	::
212	
213		dma_addr_t
214		dma_map_single(struct device *dev, void *cpu_addr, size_t size,
215			       enum dma_data_direction direction)
216	
217	Maps a piece of processor virtual memory so it can be accessed by the
218	device and returns the DMA address of the memory.
219	
220	The direction for both APIs may be converted freely by casting.
221	However the dma_API uses a strongly typed enumerator for its
222	direction:
223	
224	======================= =============================================
225	DMA_NONE		no direction (used for debugging)
226	DMA_TO_DEVICE		data is going from the memory to the device
227	DMA_FROM_DEVICE		data is coming from the device to the memory
228	DMA_BIDIRECTIONAL	direction isn't known
229	======================= =============================================
230	
231	.. note::
232	
233		Not all memory regions in a machine can be mapped by this API.
234		Further, contiguous kernel virtual space may not be contiguous as
235		physical memory.  Since this API does not provide any scatter/gather
236		capability, it will fail if the user tries to map a non-physically
237		contiguous piece of memory.  For this reason, memory to be mapped by
238		this API should be obtained from sources which guarantee it to be
239		physically contiguous (like kmalloc).
240	
241		Further, the DMA address of the memory must be within the
242		dma_mask of the device (the dma_mask is a bit mask of the
243		addressable region for the device, i.e., if the DMA address of
244		the memory ANDed with the dma_mask is still equal to the DMA
245		address, then the device can perform DMA to the memory).  To
246		ensure that the memory allocated by kmalloc is within the dma_mask,
247		the driver may specify various platform-dependent flags to restrict
248		the DMA address range of the allocation (e.g., on x86, GFP_DMA
249		guarantees to be within the first 16MB of available DMA addresses,
250		as required by ISA devices).
251	
252		Note also that the above constraints on physical contiguity and
253		dma_mask may not apply if the platform has an IOMMU (a device which
254		maps an I/O DMA address to a physical memory address).  However, to be
255		portable, device driver writers may *not* assume that such an IOMMU
256		exists.
257	
258	.. warning::
259	
260		Memory coherency operates at a granularity called the cache
261		line width.  In order for memory mapped by this API to operate
262		correctly, the mapped region must begin exactly on a cache line
263		boundary and end exactly on one (to prevent two separately mapped
264		regions from sharing a single cache line).  Since the cache line size
265		may not be known at compile time, the API will not enforce this
266		requirement.  Therefore, it is recommended that driver writers who
267		don't take special care to determine the cache line size at run time
268		only map virtual regions that begin and end on page boundaries (which
269		are guaranteed also to be cache line boundaries).
270	
271		DMA_TO_DEVICE synchronisation must be done after the last modification
272		of the memory region by the software and before it is handed off to
273		the device.  Once this primitive is used, memory covered by this
274		primitive should be treated as read-only by the device.  If the device
275		may write to it at any point, it should be DMA_BIDIRECTIONAL (see
276		below).
277	
278		DMA_FROM_DEVICE synchronisation must be done before the driver
279		accesses data that may be changed by the device.  This memory should
280		be treated as read-only by the driver.  If the driver needs to write
281		to it at any point, it should be DMA_BIDIRECTIONAL (see below).
282	
283		DMA_BIDIRECTIONAL requires special handling: it means that the driver
284		isn't sure if the memory was modified before being handed off to the
285		device and also isn't sure if the device will also modify it.  Thus,
286		you must always sync bidirectional memory twice: once before the
287		memory is handed off to the device (to make sure all memory changes
288		are flushed from the processor) and once before the data may be
289		accessed after being used by the device (to make sure any processor
290		cache lines are updated with data that the device may have changed).
291	
292	::
293	
294		void
295		dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
296				 enum dma_data_direction direction)
297	
298	Unmaps the region previously mapped.  All the parameters passed in
299	must be identical to those passed in (and returned) by the mapping
300	API.
301	
302	::
303	
304		dma_addr_t
305		dma_map_page(struct device *dev, struct page *page,
306			     unsigned long offset, size_t size,
307			     enum dma_data_direction direction)
308	
309		void
310		dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
311			       enum dma_data_direction direction)
312	
313	API for mapping and unmapping for pages.  All the notes and warnings
314	for the other mapping APIs apply here.  Also, although the <offset>
315	and <size> parameters are provided to do partial page mapping, it is
316	recommended that you never use these unless you really know what the
317	cache width is.
318	
319	::
320	
321		dma_addr_t
322		dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size,
323				 enum dma_data_direction dir, unsigned long attrs)
324	
325		void
326		dma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size,
327				   enum dma_data_direction dir, unsigned long attrs)
328	
329	API for mapping and unmapping for MMIO resources. All the notes and
330	warnings for the other mapping APIs apply here. The API should only be
331	used to map device MMIO resources, mapping of RAM is not permitted.
332	
333	::
334	
335		int
336		dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
337	
338	In some circumstances dma_map_single(), dma_map_page() and dma_map_resource()
339	will fail to create a mapping. A driver can check for these errors by testing
340	the returned DMA address with dma_mapping_error(). A non-zero return value
341	means the mapping could not be created and the driver should take appropriate
342	action (e.g. reduce current DMA mapping usage or delay and try again later).
343	
344	::
345	
346		int
347		dma_map_sg(struct device *dev, struct scatterlist *sg,
348			   int nents, enum dma_data_direction direction)
349	
350	Returns: the number of DMA address segments mapped (this may be shorter
351	than <nents> passed in if some elements of the scatter/gather list are
352	physically or virtually adjacent and an IOMMU maps them with a single
353	entry).
354	
355	Please note that the sg cannot be mapped again if it has been mapped once.
356	The mapping process is allowed to destroy information in the sg.
357	
358	As with the other mapping interfaces, dma_map_sg() can fail. When it
359	does, 0 is returned and a driver must take appropriate action. It is
360	critical that the driver do something, in the case of a block driver
361	aborting the request or even oopsing is better than doing nothing and
362	corrupting the filesystem.
363	
364	With scatterlists, you use the resulting mapping like this::
365	
366		int i, count = dma_map_sg(dev, sglist, nents, direction);
367		struct scatterlist *sg;
368	
369		for_each_sg(sglist, sg, count, i) {
370			hw_address[i] = sg_dma_address(sg);
371			hw_len[i] = sg_dma_len(sg);
372		}
373	
374	where nents is the number of entries in the sglist.
375	
376	The implementation is free to merge several consecutive sglist entries
377	into one (e.g. with an IOMMU, or if several pages just happen to be
378	physically contiguous) and returns the actual number of sg entries it
379	mapped them to. On failure 0, is returned.
380	
381	Then you should loop count times (note: this can be less than nents times)
382	and use sg_dma_address() and sg_dma_len() macros where you previously
383	accessed sg->address and sg->length as shown above.
384	
385	::
386	
387		void
388		dma_unmap_sg(struct device *dev, struct scatterlist *sg,
389			     int nents, enum dma_data_direction direction)
390	
391	Unmap the previously mapped scatter/gather list.  All the parameters
392	must be the same as those and passed in to the scatter/gather mapping
393	API.
394	
395	Note: <nents> must be the number you passed in, *not* the number of
396	DMA address entries returned.
397	
398	::
399	
400		void
401		dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle,
402					size_t size,
403					enum dma_data_direction direction)
404	
405		void
406		dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle,
407					   size_t size,
408					   enum dma_data_direction direction)
409	
410		void
411		dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
412				    int nents,
413				    enum dma_data_direction direction)
414	
415		void
416		dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
417				       int nents,
418				       enum dma_data_direction direction)
419	
420	Synchronise a single contiguous or scatter/gather mapping for the CPU
421	and device. With the sync_sg API, all the parameters must be the same
422	as those passed into the single mapping API. With the sync_single API,
423	you can use dma_handle and size parameters that aren't identical to
424	those passed into the single mapping API to do a partial sync.
425	
426	
427	.. note::
428	
429	   You must do this:
430	
431	   - Before reading values that have been written by DMA from the device
432	     (use the DMA_FROM_DEVICE direction)
433	   - After writing values that will be written to the device using DMA
434	     (use the DMA_TO_DEVICE) direction
435	   - before *and* after handing memory to the device if the memory is
436	     DMA_BIDIRECTIONAL
437	
438	See also dma_map_single().
439	
440	::
441	
442		dma_addr_t
443		dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
444				     enum dma_data_direction dir,
445				     unsigned long attrs)
446	
447		void
448		dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
449				       size_t size, enum dma_data_direction dir,
450				       unsigned long attrs)
451	
452		int
453		dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
454				 int nents, enum dma_data_direction dir,
455				 unsigned long attrs)
456	
457		void
458		dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
459				   int nents, enum dma_data_direction dir,
460				   unsigned long attrs)
461	
462	The four functions above are just like the counterpart functions
463	without the _attrs suffixes, except that they pass an optional
464	dma_attrs.
465	
466	The interpretation of DMA attributes is architecture-specific, and
467	each attribute should be documented in Documentation/DMA-attributes.txt.
468	
469	If dma_attrs are 0, the semantics of each of these functions
470	is identical to those of the corresponding function
471	without the _attrs suffix. As a result dma_map_single_attrs()
472	can generally replace dma_map_single(), etc.
473	
474	As an example of the use of the ``*_attrs`` functions, here's how
475	you could pass an attribute DMA_ATTR_FOO when mapping memory
476	for DMA::
477	
478		#include <linux/dma-mapping.h>
479		/* DMA_ATTR_FOO should be defined in linux/dma-mapping.h and
480		* documented in Documentation/DMA-attributes.txt */
481		...
482	
483			unsigned long attr;
484			attr |= DMA_ATTR_FOO;
485			....
486			n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, attr);
487			....
488	
489	Architectures that care about DMA_ATTR_FOO would check for its
490	presence in their implementations of the mapping and unmapping
491	routines, e.g.:::
492	
493		void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
494					     size_t size, enum dma_data_direction dir,
495					     unsigned long attrs)
496		{
497			....
498			if (attrs & DMA_ATTR_FOO)
499				/* twizzle the frobnozzle */
500			....
501		}
502	
503	
504	Part II - Advanced dma usage
505	----------------------------
506	
507	Warning: These pieces of the DMA API should not be used in the
508	majority of cases, since they cater for unlikely corner cases that
509	don't belong in usual drivers.
510	
511	If you don't understand how cache line coherency works between a
512	processor and an I/O device, you should not be using this part of the
513	API at all.
514	
515	::
516	
517		void *
518		dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
519				gfp_t flag, unsigned long attrs)
520	
521	Identical to dma_alloc_coherent() except that when the
522	DMA_ATTR_NON_CONSISTENT flags is passed in the attrs argument, the
523	platform will choose to return either consistent or non-consistent memory
524	as it sees fit.  By using this API, you are guaranteeing to the platform
525	that you have all the correct and necessary sync points for this memory
526	in the driver should it choose to return non-consistent memory.
527	
528	Note: where the platform can return consistent memory, it will
529	guarantee that the sync points become nops.
530	
531	Warning:  Handling non-consistent memory is a real pain.  You should
532	only use this API if you positively know your driver will be
533	required to work on one of the rare (usually non-PCI) architectures
534	that simply cannot make consistent memory.
535	
536	::
537	
538		void
539		dma_free_attrs(struct device *dev, size_t size, void *cpu_addr,
540			       dma_addr_t dma_handle, unsigned long attrs)
541	
542	Free memory allocated by the dma_alloc_attrs().  All parameters common
543	parameters must identical to those otherwise passed to dma_fre_coherent,
544	and the attrs argument must be identical to the attrs passed to
545	dma_alloc_attrs().
546	
547	::
548	
549		int
550		dma_get_cache_alignment(void)
551	
552	Returns the processor cache alignment.  This is the absolute minimum
553	alignment *and* width that you must observe when either mapping
554	memory or doing partial flushes.
555	
556	.. note::
557	
558		This API may return a number *larger* than the actual cache
559		line, but it will guarantee that one or more cache lines fit exactly
560		into the width returned by this call.  It will also always be a power
561		of two for easy alignment.
562	
563	::
564	
565		void
566		dma_cache_sync(struct device *dev, void *vaddr, size_t size,
567			       enum dma_data_direction direction)
568	
569	Do a partial sync of memory that was allocated by dma_alloc_attrs() with
570	the DMA_ATTR_NON_CONSISTENT flag starting at virtual address vaddr and
571	continuing on for size.  Again, you *must* observe the cache line
572	boundaries when doing this.
573	
574	::
575	
576		int
577		dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
578					    dma_addr_t device_addr, size_t size, int
579					    flags)
580	
581	Declare region of memory to be handed out by dma_alloc_coherent() when
582	it's asked for coherent memory for this device.
583	
584	phys_addr is the CPU physical address to which the memory is currently
585	assigned (this will be ioremapped so the CPU can access the region).
586	
587	device_addr is the DMA address the device needs to be programmed
588	with to actually address this memory (this will be handed out as the
589	dma_addr_t in dma_alloc_coherent()).
590	
591	size is the size of the area (must be multiples of PAGE_SIZE).
592	
593	flags can be ORed together and are:
594	
595	- DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions.
596	  Do not allow dma_alloc_coherent() to fall back to system memory when
597	  it's out of memory in the declared region.
598	
599	As a simplification for the platforms, only *one* such region of
600	memory may be declared per device.
601	
602	For reasons of efficiency, most platforms choose to track the declared
603	region only at the granularity of a page.  For smaller allocations,
604	you should use the dma_pool() API.
605	
606	::
607	
608		void
609		dma_release_declared_memory(struct device *dev)
610	
611	Remove the memory region previously declared from the system.  This
612	API performs *no* in-use checking for this region and will return
613	unconditionally having removed all the required structures.  It is the
614	driver's job to ensure that no parts of this memory region are
615	currently in use.
616	
617	::
618	
619		void *
620		dma_mark_declared_memory_occupied(struct device *dev,
621						  dma_addr_t device_addr, size_t size)
622	
623	This is used to occupy specific regions of the declared space
624	(dma_alloc_coherent() will hand out the first free region it finds).
625	
626	device_addr is the *device* address of the region requested.
627	
628	size is the size (and should be a page-sized multiple).
629	
630	The return value will be either a pointer to the processor virtual
631	address of the memory, or an error (via PTR_ERR()) if any part of the
632	region is occupied.
633	
634	Part III - Debug drivers use of the DMA-API
635	-------------------------------------------
636	
637	The DMA-API as described above has some constraints. DMA addresses must be
638	released with the corresponding function with the same size for example. With
639	the advent of hardware IOMMUs it becomes more and more important that drivers
640	do not violate those constraints. In the worst case such a violation can
641	result in data corruption up to destroyed filesystems.
642	
643	To debug drivers and find bugs in the usage of the DMA-API checking code can
644	be compiled into the kernel which will tell the developer about those
645	violations. If your architecture supports it you can select the "Enable
646	debugging of DMA-API usage" option in your kernel configuration. Enabling this
647	option has a performance impact. Do not enable it in production kernels.
648	
649	If you boot the resulting kernel will contain code which does some bookkeeping
650	about what DMA memory was allocated for which device. If this code detects an
651	error it prints a warning message with some details into your kernel log. An
652	example warning message may look like this::
653	
654		WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
655			check_unmap+0x203/0x490()
656		Hardware name:
657		forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
658			function [device address=0x00000000640444be] [size=66 bytes] [mapped as
659		single] [unmapped as page]
660		Modules linked in: nfsd exportfs bridge stp llc r8169
661		Pid: 0, comm: swapper Tainted: G        W  2.6.28-dmatest-09289-g8bb99c0 #1
662		Call Trace:
663		<IRQ>  [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
664		[<ffffffff80647b70>] _spin_unlock+0x10/0x30
665		[<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
666		[<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
667		[<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
668		[<ffffffff80252f96>] queue_work+0x56/0x60
669		[<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
670		[<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
671		[<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
672		[<ffffffff80235177>] find_busiest_group+0x207/0x8a0
673		[<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
674		[<ffffffff803c7ea3>] check_unmap+0x203/0x490
675		[<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
676		[<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
677		[<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
678		[<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
679		[<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
680		[<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
681		[<ffffffff8020c093>] ret_from_intr+0x0/0xa
682		<EOI> <4>---[ end trace f6435a98e2a38c0e ]---
683	
684	The driver developer can find the driver and the device including a stacktrace
685	of the DMA-API call which caused this warning.
686	
687	Per default only the first error will result in a warning message. All other
688	errors will only silently counted. This limitation exist to prevent the code
689	from flooding your kernel log. To support debugging a device driver this can
690	be disabled via debugfs. See the debugfs interface documentation below for
691	details.
692	
693	The debugfs directory for the DMA-API debugging code is called dma-api/. In
694	this directory the following files can currently be found:
695	
696	=============================== ===============================================
697	dma-api/all_errors		This file contains a numeric value. If this
698					value is not equal to zero the debugging code
699					will print a warning for every error it finds
700					into the kernel log. Be careful with this
701					option, as it can easily flood your logs.
702	
703	dma-api/disabled		This read-only file contains the character 'Y'
704					if the debugging code is disabled. This can
705					happen when it runs out of memory or if it was
706					disabled at boot time
707	
708	dma-api/error_count		This file is read-only and shows the total
709					numbers of errors found.
710	
711	dma-api/num_errors		The number in this file shows how many
712					warnings will be printed to the kernel log
713					before it stops. This number is initialized to
714					one at system boot and be set by writing into
715					this file
716	
717	dma-api/min_free_entries	This read-only file can be read to get the
718					minimum number of free dma_debug_entries the
719					allocator has ever seen. If this value goes
720					down to zero the code will disable itself
721					because it is not longer reliable.
722	
723	dma-api/num_free_entries	The current number of free dma_debug_entries
724					in the allocator.
725	
726	dma-api/driver-filter		You can write a name of a driver into this file
727					to limit the debug output to requests from that
728					particular driver. Write an empty string to
729					that file to disable the filter and see
730					all errors again.
731	=============================== ===============================================
732	
733	If you have this code compiled into your kernel it will be enabled by default.
734	If you want to boot without the bookkeeping anyway you can provide
735	'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
736	Notice that you can not enable it again at runtime. You have to reboot to do
737	so.
738	
739	If you want to see debug messages only for a special device driver you can
740	specify the dma_debug_driver=<drivername> parameter. This will enable the
741	driver filter at boot time. The debug code will only print errors for that
742	driver afterwards. This filter can be disabled or changed later using debugfs.
743	
744	When the code disables itself at runtime this is most likely because it ran
745	out of dma_debug_entries. These entries are preallocated at boot. The number
746	of preallocated entries is defined per architecture. If it is too low for you
747	boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
748	architectural default.
749	
750	::
751	
752		void
753		debug_dma_mapping_error(struct device *dev, dma_addr_t dma_addr);
754	
755	dma-debug interface debug_dma_mapping_error() to debug drivers that fail
756	to check DMA mapping errors on addresses returned by dma_map_single() and
757	dma_map_page() interfaces. This interface clears a flag set by
758	debug_dma_map_page() to indicate that dma_mapping_error() has been called by
759	the driver. When driver does unmap, debug_dma_unmap() checks the flag and if
760	this flag is still set, prints warning message that includes call trace that
761	leads up to the unmap. This interface can be called from dma_mapping_error()
762	routines to enable DMA mapping error check debugging.
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