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Based on kernel version 3.16. Page generated on 2014-08-06 21:38 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 Documentation/DMA-API-HOWTO.txt.
8	
9	This API is split into two pieces.  Part I describes the basic API.
10	Part II describes extensions for supporting non-consistent memory
11	machines.  Unless you know that your driver absolutely has to support
12	non-consistent platforms (this is usually only legacy platforms) you
13	should only use the API described in part I.
14	
15	Part I - dma_ API
16	-------------------------------------
17	
18	To get the dma_ API, you must #include <linux/dma-mapping.h>.  This
19	provides dma_addr_t and the interfaces described below.
20	
21	A dma_addr_t can hold any valid DMA or bus address for the platform.  It
22	can be given to a device to use as a DMA source or target.  A CPU cannot
23	reference a dma_addr_t directly because there may be translation between
24	its physical address space and the bus address space.
25	
26	Part Ia - Using large DMA-coherent buffers
27	------------------------------------------
28	
29	void *
30	dma_alloc_coherent(struct device *dev, size_t size,
31				     dma_addr_t *dma_handle, gfp_t flag)
32	
33	Consistent memory is memory for which a write by either the device or
34	the processor can immediately be read by the processor or device
35	without having to worry about caching effects.  (You may however need
36	to make sure to flush the processor's write buffers before telling
37	devices to read that memory.)
38	
39	This routine allocates a region of <size> bytes of consistent memory.
40	
41	It returns a pointer to the allocated region (in the processor's virtual
42	address space) or NULL if the allocation failed.
43	
44	It also returns a <dma_handle> which may be cast to an unsigned integer the
45	same width as the bus and given to the device as the bus address base of
46	the region.
47	
48	Note: consistent memory can be expensive on some platforms, and the
49	minimum allocation length may be as big as a page, so you should
50	consolidate your requests for consistent memory as much as possible.
51	The simplest way to do that is to use the dma_pool calls (see below).
52	
53	The flag parameter (dma_alloc_coherent() only) allows the caller to
54	specify the GFP_ flags (see kmalloc()) for the allocation (the
55	implementation may choose to ignore flags that affect the location of
56	the returned memory, like GFP_DMA).
57	
58	void *
59	dma_zalloc_coherent(struct device *dev, size_t size,
60				     dma_addr_t *dma_handle, gfp_t flag)
61	
62	Wraps dma_alloc_coherent() and also zeroes the returned memory if the
63	allocation attempt succeeded.
64	
65	void
66	dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
67				   dma_addr_t dma_handle)
68	
69	Free a region of consistent memory you previously allocated.  dev,
70	size and dma_handle must all be the same as those passed into
71	dma_alloc_coherent().  cpu_addr must be the virtual address returned by
72	the dma_alloc_coherent().
73	
74	Note that unlike their sibling allocation calls, these routines
75	may only be called with IRQs enabled.
76	
77	
78	Part Ib - Using small DMA-coherent buffers
79	------------------------------------------
80	
81	To get this part of the dma_ API, you must #include <linux/dmapool.h>
82	
83	Many drivers need lots of small DMA-coherent memory regions for DMA
84	descriptors or I/O buffers.  Rather than allocating in units of a page
85	or more using dma_alloc_coherent(), you can use DMA pools.  These work
86	much like a struct kmem_cache, except that they use the DMA-coherent allocator,
87	not __get_free_pages().  Also, they understand common hardware constraints
88	for alignment, like queue heads needing to be aligned on N-byte boundaries.
89	
90	
91		struct dma_pool *
92		dma_pool_create(const char *name, struct device *dev,
93				size_t size, size_t align, size_t alloc);
94	
95	dma_pool_create() initializes a pool of DMA-coherent buffers
96	for use with a given device.  It must be called in a context which
97	can sleep.
98	
99	The "name" is for diagnostics (like a struct kmem_cache name); dev and size
100	are like what you'd pass to dma_alloc_coherent().  The device's hardware
101	alignment requirement for this type of data is "align" (which is expressed
102	in bytes, and must be a power of two).  If your device has no boundary
103	crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated
104	from this pool must not cross 4KByte boundaries.
105	
106	
107		void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
108				dma_addr_t *dma_handle);
109	
110	This allocates memory from the pool; the returned memory will meet the
111	size and alignment requirements specified at creation time.  Pass
112	GFP_ATOMIC to prevent blocking, or if it's permitted (not
113	in_interrupt, not holding SMP locks), pass GFP_KERNEL to allow
114	blocking.  Like dma_alloc_coherent(), this returns two values:  an
115	address usable by the CPU, and the DMA address usable by the pool's
116	device.
117	
118	
119		void dma_pool_free(struct dma_pool *pool, void *vaddr,
120				dma_addr_t addr);
121	
122	This puts memory back into the pool.  The pool is what was passed to
123	dma_pool_alloc(); the CPU (vaddr) and DMA addresses are what
124	were returned when that routine allocated the memory being freed.
125	
126	
127		void dma_pool_destroy(struct dma_pool *pool);
128	
129	dma_pool_destroy() frees the resources of the pool.  It must be
130	called in a context which can sleep.  Make sure you've freed all allocated
131	memory back to the pool before you destroy it.
132	
133	
134	Part Ic - DMA addressing limitations
135	------------------------------------
136	
137	int
138	dma_supported(struct device *dev, u64 mask)
139	
140	Checks to see if the device can support DMA to the memory described by
141	mask.
142	
143	Returns: 1 if it can and 0 if it can't.
144	
145	Notes: This routine merely tests to see if the mask is possible.  It
146	won't change the current mask settings.  It is more intended as an
147	internal API for use by the platform than an external API for use by
148	driver writers.
149	
150	int
151	dma_set_mask_and_coherent(struct device *dev, u64 mask)
152	
153	Checks to see if the mask is possible and updates the device
154	streaming and coherent DMA mask parameters if it is.
155	
156	Returns: 0 if successful and a negative error if not.
157	
158	int
159	dma_set_mask(struct device *dev, u64 mask)
160	
161	Checks to see if the mask is possible and updates the device
162	parameters if it is.
163	
164	Returns: 0 if successful and a negative error if not.
165	
166	int
167	dma_set_coherent_mask(struct device *dev, u64 mask)
168	
169	Checks to see if the mask is possible and updates the device
170	parameters if it is.
171	
172	Returns: 0 if successful and a negative error if not.
173	
174	u64
175	dma_get_required_mask(struct device *dev)
176	
177	This API returns the mask that the platform requires to
178	operate efficiently.  Usually this means the returned mask
179	is the minimum required to cover all of memory.  Examining the
180	required mask gives drivers with variable descriptor sizes the
181	opportunity to use smaller descriptors as necessary.
182	
183	Requesting the required mask does not alter the current mask.  If you
184	wish to take advantage of it, you should issue a dma_set_mask()
185	call to set the mask to the value returned.
186	
187	
188	Part Id - Streaming DMA mappings
189	--------------------------------
190	
191	dma_addr_t
192	dma_map_single(struct device *dev, void *cpu_addr, size_t size,
193			      enum dma_data_direction direction)
194	
195	Maps a piece of processor virtual memory so it can be accessed by the
196	device and returns the bus address of the memory.
197	
198	The direction for both APIs may be converted freely by casting.
199	However the dma_ API uses a strongly typed enumerator for its
200	direction:
201	
202	DMA_NONE		no direction (used for debugging)
203	DMA_TO_DEVICE		data is going from the memory to the device
204	DMA_FROM_DEVICE		data is coming from the device to the memory
205	DMA_BIDIRECTIONAL	direction isn't known
206	
207	Notes:  Not all memory regions in a machine can be mapped by this API.
208	Further, contiguous kernel virtual space may not be contiguous as
209	physical memory.  Since this API does not provide any scatter/gather
210	capability, it will fail if the user tries to map a non-physically
211	contiguous piece of memory.  For this reason, memory to be mapped by
212	this API should be obtained from sources which guarantee it to be
213	physically contiguous (like kmalloc).
214	
215	Further, the bus address of the memory must be within the
216	dma_mask of the device (the dma_mask is a bit mask of the
217	addressable region for the device, i.e., if the bus address of
218	the memory ANDed with the dma_mask is still equal to the bus
219	address, then the device can perform DMA to the memory).  To
220	ensure that the memory allocated by kmalloc is within the dma_mask,
221	the driver may specify various platform-dependent flags to restrict
222	the bus address range of the allocation (e.g., on x86, GFP_DMA
223	guarantees to be within the first 16MB of available bus addresses,
224	as required by ISA devices).
225	
226	Note also that the above constraints on physical contiguity and
227	dma_mask may not apply if the platform has an IOMMU (a device which
228	maps an I/O bus address to a physical memory address).  However, to be
229	portable, device driver writers may *not* assume that such an IOMMU
230	exists.
231	
232	Warnings:  Memory coherency operates at a granularity called the cache
233	line width.  In order for memory mapped by this API to operate
234	correctly, the mapped region must begin exactly on a cache line
235	boundary and end exactly on one (to prevent two separately mapped
236	regions from sharing a single cache line).  Since the cache line size
237	may not be known at compile time, the API will not enforce this
238	requirement.  Therefore, it is recommended that driver writers who
239	don't take special care to determine the cache line size at run time
240	only map virtual regions that begin and end on page boundaries (which
241	are guaranteed also to be cache line boundaries).
242	
243	DMA_TO_DEVICE synchronisation must be done after the last modification
244	of the memory region by the software and before it is handed off to
245	the driver.  Once this primitive is used, memory covered by this
246	primitive should be treated as read-only by the device.  If the device
247	may write to it at any point, it should be DMA_BIDIRECTIONAL (see
248	below).
249	
250	DMA_FROM_DEVICE synchronisation must be done before the driver
251	accesses data that may be changed by the device.  This memory should
252	be treated as read-only by the driver.  If the driver needs to write
253	to it at any point, it should be DMA_BIDIRECTIONAL (see below).
254	
255	DMA_BIDIRECTIONAL requires special handling: it means that the driver
256	isn't sure if the memory was modified before being handed off to the
257	device and also isn't sure if the device will also modify it.  Thus,
258	you must always sync bidirectional memory twice: once before the
259	memory is handed off to the device (to make sure all memory changes
260	are flushed from the processor) and once before the data may be
261	accessed after being used by the device (to make sure any processor
262	cache lines are updated with data that the device may have changed).
263	
264	void
265	dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
266			 enum dma_data_direction direction)
267	
268	Unmaps the region previously mapped.  All the parameters passed in
269	must be identical to those passed in (and returned) by the mapping
270	API.
271	
272	dma_addr_t
273	dma_map_page(struct device *dev, struct page *page,
274			    unsigned long offset, size_t size,
275			    enum dma_data_direction direction)
276	void
277	dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
278		       enum dma_data_direction direction)
279	
280	API for mapping and unmapping for pages.  All the notes and warnings
281	for the other mapping APIs apply here.  Also, although the <offset>
282	and <size> parameters are provided to do partial page mapping, it is
283	recommended that you never use these unless you really know what the
284	cache width is.
285	
286	int
287	dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
288	
289	In some circumstances dma_map_single() and dma_map_page() will fail to create
290	a mapping. A driver can check for these errors by testing the returned
291	DMA address with dma_mapping_error(). A non-zero return value means the mapping
292	could not be created and the driver should take appropriate action (e.g.
293	reduce current DMA mapping usage or delay and try again later).
294	
295		int
296		dma_map_sg(struct device *dev, struct scatterlist *sg,
297			int nents, enum dma_data_direction direction)
298	
299	Returns: the number of bus address segments mapped (this may be shorter
300	than <nents> passed in if some elements of the scatter/gather list are
301	physically or virtually adjacent and an IOMMU maps them with a single
302	entry).
303	
304	Please note that the sg cannot be mapped again if it has been mapped once.
305	The mapping process is allowed to destroy information in the sg.
306	
307	As with the other mapping interfaces, dma_map_sg() can fail. When it
308	does, 0 is returned and a driver must take appropriate action. It is
309	critical that the driver do something, in the case of a block driver
310	aborting the request or even oopsing is better than doing nothing and
311	corrupting the filesystem.
312	
313	With scatterlists, you use the resulting mapping like this:
314	
315		int i, count = dma_map_sg(dev, sglist, nents, direction);
316		struct scatterlist *sg;
317	
318		for_each_sg(sglist, sg, count, i) {
319			hw_address[i] = sg_dma_address(sg);
320			hw_len[i] = sg_dma_len(sg);
321		}
322	
323	where nents is the number of entries in the sglist.
324	
325	The implementation is free to merge several consecutive sglist entries
326	into one (e.g. with an IOMMU, or if several pages just happen to be
327	physically contiguous) and returns the actual number of sg entries it
328	mapped them to. On failure 0, is returned.
329	
330	Then you should loop count times (note: this can be less than nents times)
331	and use sg_dma_address() and sg_dma_len() macros where you previously
332	accessed sg->address and sg->length as shown above.
333	
334		void
335		dma_unmap_sg(struct device *dev, struct scatterlist *sg,
336			int nhwentries, enum dma_data_direction direction)
337	
338	Unmap the previously mapped scatter/gather list.  All the parameters
339	must be the same as those and passed in to the scatter/gather mapping
340	API.
341	
342	Note: <nents> must be the number you passed in, *not* the number of
343	bus address entries returned.
344	
345	void
346	dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size,
347				enum dma_data_direction direction)
348	void
349	dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size,
350				   enum dma_data_direction direction)
351	void
352	dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nelems,
353			    enum dma_data_direction direction)
354	void
355	dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nelems,
356			       enum dma_data_direction direction)
357	
358	Synchronise a single contiguous or scatter/gather mapping for the CPU
359	and device. With the sync_sg API, all the parameters must be the same
360	as those passed into the single mapping API. With the sync_single API,
361	you can use dma_handle and size parameters that aren't identical to
362	those passed into the single mapping API to do a partial sync.
363	
364	Notes:  You must do this:
365	
366	- Before reading values that have been written by DMA from the device
367	  (use the DMA_FROM_DEVICE direction)
368	- After writing values that will be written to the device using DMA
369	  (use the DMA_TO_DEVICE) direction
370	- before *and* after handing memory to the device if the memory is
371	  DMA_BIDIRECTIONAL
372	
373	See also dma_map_single().
374	
375	dma_addr_t
376	dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
377			     enum dma_data_direction dir,
378			     struct dma_attrs *attrs)
379	
380	void
381	dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
382			       size_t size, enum dma_data_direction dir,
383			       struct dma_attrs *attrs)
384	
385	int
386	dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
387			 int nents, enum dma_data_direction dir,
388			 struct dma_attrs *attrs)
389	
390	void
391	dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
392			   int nents, enum dma_data_direction dir,
393			   struct dma_attrs *attrs)
394	
395	The four functions above are just like the counterpart functions
396	without the _attrs suffixes, except that they pass an optional
397	struct dma_attrs*.
398	
399	struct dma_attrs encapsulates a set of "DMA attributes". For the
400	definition of struct dma_attrs see linux/dma-attrs.h.
401	
402	The interpretation of DMA attributes is architecture-specific, and
403	each attribute should be documented in Documentation/DMA-attributes.txt.
404	
405	If struct dma_attrs* is NULL, the semantics of each of these
406	functions is identical to those of the corresponding function
407	without the _attrs suffix. As a result dma_map_single_attrs()
408	can generally replace dma_map_single(), etc.
409	
410	As an example of the use of the *_attrs functions, here's how
411	you could pass an attribute DMA_ATTR_FOO when mapping memory
412	for DMA:
413	
414	#include <linux/dma-attrs.h>
415	/* DMA_ATTR_FOO should be defined in linux/dma-attrs.h and
416	 * documented in Documentation/DMA-attributes.txt */
417	...
418	
419		DEFINE_DMA_ATTRS(attrs);
420		dma_set_attr(DMA_ATTR_FOO, &attrs);
421		....
422		n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, &attr);
423		....
424	
425	Architectures that care about DMA_ATTR_FOO would check for its
426	presence in their implementations of the mapping and unmapping
427	routines, e.g.:
428	
429	void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
430				     size_t size, enum dma_data_direction dir,
431				     struct dma_attrs *attrs)
432	{
433		....
434		int foo =  dma_get_attr(DMA_ATTR_FOO, attrs);
435		....
436		if (foo)
437			/* twizzle the frobnozzle */
438		....
439	
440	
441	Part II - Advanced dma_ usage
442	-----------------------------
443	
444	Warning: These pieces of the DMA API should not be used in the
445	majority of cases, since they cater for unlikely corner cases that
446	don't belong in usual drivers.
447	
448	If you don't understand how cache line coherency works between a
449	processor and an I/O device, you should not be using this part of the
450	API at all.
451	
452	void *
453	dma_alloc_noncoherent(struct device *dev, size_t size,
454				       dma_addr_t *dma_handle, gfp_t flag)
455	
456	Identical to dma_alloc_coherent() except that the platform will
457	choose to return either consistent or non-consistent memory as it sees
458	fit.  By using this API, you are guaranteeing to the platform that you
459	have all the correct and necessary sync points for this memory in the
460	driver should it choose to return non-consistent memory.
461	
462	Note: where the platform can return consistent memory, it will
463	guarantee that the sync points become nops.
464	
465	Warning:  Handling non-consistent memory is a real pain.  You should
466	only use this API if you positively know your driver will be
467	required to work on one of the rare (usually non-PCI) architectures
468	that simply cannot make consistent memory.
469	
470	void
471	dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
472				      dma_addr_t dma_handle)
473	
474	Free memory allocated by the nonconsistent API.  All parameters must
475	be identical to those passed in (and returned by
476	dma_alloc_noncoherent()).
477	
478	int
479	dma_get_cache_alignment(void)
480	
481	Returns the processor cache alignment.  This is the absolute minimum
482	alignment *and* width that you must observe when either mapping
483	memory or doing partial flushes.
484	
485	Notes: This API may return a number *larger* than the actual cache
486	line, but it will guarantee that one or more cache lines fit exactly
487	into the width returned by this call.  It will also always be a power
488	of two for easy alignment.
489	
490	void
491	dma_cache_sync(struct device *dev, void *vaddr, size_t size,
492		       enum dma_data_direction direction)
493	
494	Do a partial sync of memory that was allocated by
495	dma_alloc_noncoherent(), starting at virtual address vaddr and
496	continuing on for size.  Again, you *must* observe the cache line
497	boundaries when doing this.
498	
499	int
500	dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
501				    dma_addr_t device_addr, size_t size, int
502				    flags)
503	
504	Declare region of memory to be handed out by dma_alloc_coherent() when
505	it's asked for coherent memory for this device.
506	
507	phys_addr is the CPU physical address to which the memory is currently
508	assigned (this will be ioremapped so the CPU can access the region).
509	
510	device_addr is the bus address the device needs to be programmed
511	with to actually address this memory (this will be handed out as the
512	dma_addr_t in dma_alloc_coherent()).
513	
514	size is the size of the area (must be multiples of PAGE_SIZE).
515	
516	flags can be ORed together and are:
517	
518	DMA_MEMORY_MAP - request that the memory returned from
519	dma_alloc_coherent() be directly writable.
520	
521	DMA_MEMORY_IO - request that the memory returned from
522	dma_alloc_coherent() be addressable using read()/write()/memcpy_toio() etc.
523	
524	One or both of these flags must be present.
525	
526	DMA_MEMORY_INCLUDES_CHILDREN - make the declared memory be allocated by
527	dma_alloc_coherent of any child devices of this one (for memory residing
528	on a bridge).
529	
530	DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions. 
531	Do not allow dma_alloc_coherent() to fall back to system memory when
532	it's out of memory in the declared region.
533	
534	The return value will be either DMA_MEMORY_MAP or DMA_MEMORY_IO and
535	must correspond to a passed in flag (i.e. no returning DMA_MEMORY_IO
536	if only DMA_MEMORY_MAP were passed in) for success or zero for
537	failure.
538	
539	Note, for DMA_MEMORY_IO returns, all subsequent memory returned by
540	dma_alloc_coherent() may no longer be accessed directly, but instead
541	must be accessed using the correct bus functions.  If your driver
542	isn't prepared to handle this contingency, it should not specify
543	DMA_MEMORY_IO in the input flags.
544	
545	As a simplification for the platforms, only *one* such region of
546	memory may be declared per device.
547	
548	For reasons of efficiency, most platforms choose to track the declared
549	region only at the granularity of a page.  For smaller allocations,
550	you should use the dma_pool() API.
551	
552	void
553	dma_release_declared_memory(struct device *dev)
554	
555	Remove the memory region previously declared from the system.  This
556	API performs *no* in-use checking for this region and will return
557	unconditionally having removed all the required structures.  It is the
558	driver's job to ensure that no parts of this memory region are
559	currently in use.
560	
561	void *
562	dma_mark_declared_memory_occupied(struct device *dev,
563					  dma_addr_t device_addr, size_t size)
564	
565	This is used to occupy specific regions of the declared space
566	(dma_alloc_coherent() will hand out the first free region it finds).
567	
568	device_addr is the *device* address of the region requested.
569	
570	size is the size (and should be a page-sized multiple).
571	
572	The return value will be either a pointer to the processor virtual
573	address of the memory, or an error (via PTR_ERR()) if any part of the
574	region is occupied.
575	
576	Part III - Debug drivers use of the DMA-API
577	-------------------------------------------
578	
579	The DMA-API as described above has some constraints. DMA addresses must be
580	released with the corresponding function with the same size for example. With
581	the advent of hardware IOMMUs it becomes more and more important that drivers
582	do not violate those constraints. In the worst case such a violation can
583	result in data corruption up to destroyed filesystems.
584	
585	To debug drivers and find bugs in the usage of the DMA-API checking code can
586	be compiled into the kernel which will tell the developer about those
587	violations. If your architecture supports it you can select the "Enable
588	debugging of DMA-API usage" option in your kernel configuration. Enabling this
589	option has a performance impact. Do not enable it in production kernels.
590	
591	If you boot the resulting kernel will contain code which does some bookkeeping
592	about what DMA memory was allocated for which device. If this code detects an
593	error it prints a warning message with some details into your kernel log. An
594	example warning message may look like this:
595	
596	------------[ cut here ]------------
597	WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
598		check_unmap+0x203/0x490()
599	Hardware name:
600	forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
601		function [device address=0x00000000640444be] [size=66 bytes] [mapped as
602	single] [unmapped as page]
603	Modules linked in: nfsd exportfs bridge stp llc r8169
604	Pid: 0, comm: swapper Tainted: G        W  2.6.28-dmatest-09289-g8bb99c0 #1
605	Call Trace:
606	 <IRQ>  [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
607	 [<ffffffff80647b70>] _spin_unlock+0x10/0x30
608	 [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
609	 [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
610	 [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
611	 [<ffffffff80252f96>] queue_work+0x56/0x60
612	 [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
613	 [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
614	 [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
615	 [<ffffffff80235177>] find_busiest_group+0x207/0x8a0
616	 [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
617	 [<ffffffff803c7ea3>] check_unmap+0x203/0x490
618	 [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
619	 [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
620	 [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
621	 [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
622	 [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
623	 [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
624	 [<ffffffff8020c093>] ret_from_intr+0x0/0xa
625	 <EOI> <4>---[ end trace f6435a98e2a38c0e ]---
626	
627	The driver developer can find the driver and the device including a stacktrace
628	of the DMA-API call which caused this warning.
629	
630	Per default only the first error will result in a warning message. All other
631	errors will only silently counted. This limitation exist to prevent the code
632	from flooding your kernel log. To support debugging a device driver this can
633	be disabled via debugfs. See the debugfs interface documentation below for
634	details.
635	
636	The debugfs directory for the DMA-API debugging code is called dma-api/. In
637	this directory the following files can currently be found:
638	
639		dma-api/all_errors	This file contains a numeric value. If this
640					value is not equal to zero the debugging code
641					will print a warning for every error it finds
642					into the kernel log. Be careful with this
643					option, as it can easily flood your logs.
644	
645		dma-api/disabled	This read-only file contains the character 'Y'
646					if the debugging code is disabled. This can
647					happen when it runs out of memory or if it was
648					disabled at boot time
649	
650		dma-api/error_count	This file is read-only and shows the total
651					numbers of errors found.
652	
653		dma-api/num_errors	The number in this file shows how many
654					warnings will be printed to the kernel log
655					before it stops. This number is initialized to
656					one at system boot and be set by writing into
657					this file
658	
659		dma-api/min_free_entries
660					This read-only file can be read to get the
661					minimum number of free dma_debug_entries the
662					allocator has ever seen. If this value goes
663					down to zero the code will disable itself
664					because it is not longer reliable.
665	
666		dma-api/num_free_entries
667					The current number of free dma_debug_entries
668					in the allocator.
669	
670		dma-api/driver-filter
671					You can write a name of a driver into this file
672					to limit the debug output to requests from that
673					particular driver. Write an empty string to
674					that file to disable the filter and see
675					all errors again.
676	
677	If you have this code compiled into your kernel it will be enabled by default.
678	If you want to boot without the bookkeeping anyway you can provide
679	'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
680	Notice that you can not enable it again at runtime. You have to reboot to do
681	so.
682	
683	If you want to see debug messages only for a special device driver you can
684	specify the dma_debug_driver=<drivername> parameter. This will enable the
685	driver filter at boot time. The debug code will only print errors for that
686	driver afterwards. This filter can be disabled or changed later using debugfs.
687	
688	When the code disables itself at runtime this is most likely because it ran
689	out of dma_debug_entries. These entries are preallocated at boot. The number
690	of preallocated entries is defined per architecture. If it is too low for you
691	boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
692	architectural default.
693	
694	void debug_dmap_mapping_error(struct device *dev, dma_addr_t dma_addr);
695	
696	dma-debug interface debug_dma_mapping_error() to debug drivers that fail
697	to check DMA mapping errors on addresses returned by dma_map_single() and
698	dma_map_page() interfaces. This interface clears a flag set by
699	debug_dma_map_page() to indicate that dma_mapping_error() has been called by
700	the driver. When driver does unmap, debug_dma_unmap() checks the flag and if
701	this flag is still set, prints warning message that includes call trace that
702	leads up to the unmap. This interface can be called from dma_mapping_error()
703	routines to enable DMA mapping error check debugging.
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