1============================================
  2Dynamic DMA mapping using the generic device
  3============================================
  4
  5:Author: James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
  6
  7This document describes the DMA API.  For a more gentle introduction
  8of the API (and actual examples), see Documentation/DMA-API-HOWTO.txt.
  9
 10This API is split into two pieces.  Part I describes the basic API.
 11Part II describes extensions for supporting non-consistent memory
 12machines.  Unless you know that your driver absolutely has to support
 13non-consistent platforms (this is usually only legacy platforms) you
 14should only use the API described in part I.
 15
 16Part I - dma_API
 17----------------
 18
 19To get the dma_API, you must #include <linux/dma-mapping.h>.  This
 20provides dma_addr_t and the interfaces described below.
 21
 22A dma_addr_t can hold any valid DMA address for the platform.  It can be
 23given to a device to use as a DMA source or target.  A CPU cannot reference
 24a dma_addr_t directly because there may be translation between its physical
 25address space and the DMA address space.
 26
 27Part 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
 36Consistent memory is memory for which a write by either the device or
 37the processor can immediately be read by the processor or device
 38without having to worry about caching effects.  (You may however need
 39to make sure to flush the processor's write buffers before telling
 40devices to read that memory.)
 41
 42This routine allocates a region of <size> bytes of consistent memory.
 43
 44It returns a pointer to the allocated region (in the processor's virtual
 45address space) or NULL if the allocation failed.
 46
 47It also returns a <dma_handle> which may be cast to an unsigned integer the
 48same width as the bus and given to the device as the DMA address base of
 49the region.
 50
 51Note: consistent memory can be expensive on some platforms, and the
 52minimum allocation length may be as big as a page, so you should
 53consolidate your requests for consistent memory as much as possible.
 54The simplest way to do that is to use the dma_pool calls (see below).
 55
 56The flag parameter (dma_alloc_coherent() only) allows the caller to
 57specify the ``GFP_`` flags (see kmalloc()) for the allocation (the
 58implementation may choose to ignore flags that affect the location of
 59the returned memory, like GFP_DMA).
 60
 61::
 62
 63	void
 64	dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
 65			  dma_addr_t dma_handle)
 66
 67Free a region of consistent memory you previously allocated.  dev,
 68size and dma_handle must all be the same as those passed into
 69dma_alloc_coherent().  cpu_addr must be the virtual address returned by
 70the dma_alloc_coherent().
 71
 72Note that unlike their sibling allocation calls, these routines
 73may only be called with IRQs enabled.
 74
 75
 76Part Ib - Using small DMA-coherent buffers
 77------------------------------------------
 78
 79To get this part of the dma_API, you must #include <linux/dmapool.h>
 80
 81Many drivers need lots of small DMA-coherent memory regions for DMA
 82descriptors or I/O buffers.  Rather than allocating in units of a page
 83or more using dma_alloc_coherent(), you can use DMA pools.  These work
 84much like a struct kmem_cache, except that they use the DMA-coherent allocator,
 85not __get_free_pages().  Also, they understand common hardware constraints
 86for alignment, like queue heads needing to be aligned on N-byte boundaries.
 87
 88
 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
 95dma_pool_create() initializes a pool of DMA-coherent buffers
 96for use with a given device.  It must be called in a context which
 97can sleep.
 98
 99The "name" is for diagnostics (like a struct kmem_cache name); dev and size
100are like what you'd pass to dma_alloc_coherent().  The device's hardware
101alignment requirement for this type of data is "align" (which is expressed
102in bytes, and must be a power of two).  If your device has no boundary
103crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated
104from this pool must not cross 4KByte boundaries.
105
106::
107
108	void *
109	dma_pool_zalloc(struct dma_pool *pool, gfp_t mem_flags,
110		        dma_addr_t *handle)
111
112Wraps dma_pool_alloc() and also zeroes the returned memory if the
113allocation attempt succeeded.
114
115
116::
117
118	void *
119	dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
120		       dma_addr_t *dma_handle);
121
122This allocates memory from the pool; the returned memory will meet the
123size and alignment requirements specified at creation time.  Pass
124GFP_ATOMIC to prevent blocking, or if it's permitted (not
125in_interrupt, not holding SMP locks), pass GFP_KERNEL to allow
126blocking.  Like dma_alloc_coherent(), this returns two values:  an
127address usable by the CPU, and the DMA address usable by the pool's
128device.
129
130::
131
132	void
133	dma_pool_free(struct dma_pool *pool, void *vaddr,
134		      dma_addr_t addr);
135
136This puts memory back into the pool.  The pool is what was passed to
137dma_pool_alloc(); the CPU (vaddr) and DMA addresses are what
138were returned when that routine allocated the memory being freed.
139
140::
141
142	void
143	dma_pool_destroy(struct dma_pool *pool);
144
145dma_pool_destroy() frees the resources of the pool.  It must be
146called in a context which can sleep.  Make sure you've freed all allocated
147memory back to the pool before you destroy it.
148
149
150Part Ic - DMA addressing limitations
151------------------------------------
152
153::
154
155	int
156	dma_set_mask_and_coherent(struct device *dev, u64 mask)
157
158Checks to see if the mask is possible and updates the device
159streaming and coherent DMA mask parameters if it is.
160
161Returns: 0 if successful and a negative error if not.
162
163::
164
165	int
166	dma_set_mask(struct device *dev, u64 mask)
167
168Checks to see if the mask is possible and updates the device
169parameters if it is.
170
171Returns: 0 if successful and a negative error if not.
172
173::
174
175	int
176	dma_set_coherent_mask(struct device *dev, u64 mask)
177
178Checks to see if the mask is possible and updates the device
179parameters if it is.
180
181Returns: 0 if successful and a negative error if not.
182
183::
184
185	u64
186	dma_get_required_mask(struct device *dev)
187
188This API returns the mask that the platform requires to
189operate efficiently.  Usually this means the returned mask
190is the minimum required to cover all of memory.  Examining the
191required mask gives drivers with variable descriptor sizes the
192opportunity to use smaller descriptors as necessary.
193
194Requesting the required mask does not alter the current mask.  If you
195wish to take advantage of it, you should issue a dma_set_mask()
196call to set the mask to the value returned.
197
198::
199
200	size_t
201	dma_max_mapping_size(struct device *dev);
202
203Returns the maximum size of a mapping for the device. The size parameter
204of the mapping functions like dma_map_single(), dma_map_page() and
205others should not be larger than the returned value.
206
207::
208
209	unsigned long
210	dma_get_merge_boundary(struct device *dev);
211
212Returns the DMA merge boundary. If the device cannot merge any the DMA address
213segments, the function returns 0.
214
215Part Id - Streaming DMA mappings
216--------------------------------
217
218::
219
220	dma_addr_t
221	dma_map_single(struct device *dev, void *cpu_addr, size_t size,
222		       enum dma_data_direction direction)
223
224Maps a piece of processor virtual memory so it can be accessed by the
225device and returns the DMA address of the memory.
226
227The direction for both APIs may be converted freely by casting.
228However the dma_API uses a strongly typed enumerator for its
229direction:
230
231======================= =============================================
232DMA_NONE		no direction (used for debugging)
233DMA_TO_DEVICE		data is going from the memory to the device
234DMA_FROM_DEVICE		data is coming from the device to the memory
235DMA_BIDIRECTIONAL	direction isn't known
236======================= =============================================
237
238.. note::
239
240	Not all memory regions in a machine can be mapped by this API.
241	Further, contiguous kernel virtual space may not be contiguous as
242	physical memory.  Since this API does not provide any scatter/gather
243	capability, it will fail if the user tries to map a non-physically
244	contiguous piece of memory.  For this reason, memory to be mapped by
245	this API should be obtained from sources which guarantee it to be
246	physically contiguous (like kmalloc).
247
248	Further, the DMA address of the memory must be within the
249	dma_mask of the device (the dma_mask is a bit mask of the
250	addressable region for the device, i.e., if the DMA address of
251	the memory ANDed with the dma_mask is still equal to the DMA
252	address, then the device can perform DMA to the memory).  To
253	ensure that the memory allocated by kmalloc is within the dma_mask,
254	the driver may specify various platform-dependent flags to restrict
255	the DMA address range of the allocation (e.g., on x86, GFP_DMA
256	guarantees to be within the first 16MB of available DMA addresses,
257	as required by ISA devices).
258
259	Note also that the above constraints on physical contiguity and
260	dma_mask may not apply if the platform has an IOMMU (a device which
261	maps an I/O DMA address to a physical memory address).  However, to be
262	portable, device driver writers may *not* assume that such an IOMMU
263	exists.
264
265.. warning::
266
267	Memory coherency operates at a granularity called the cache
268	line width.  In order for memory mapped by this API to operate
269	correctly, the mapped region must begin exactly on a cache line
270	boundary and end exactly on one (to prevent two separately mapped
271	regions from sharing a single cache line).  Since the cache line size
272	may not be known at compile time, the API will not enforce this
273	requirement.  Therefore, it is recommended that driver writers who
274	don't take special care to determine the cache line size at run time
275	only map virtual regions that begin and end on page boundaries (which
276	are guaranteed also to be cache line boundaries).
277
278	DMA_TO_DEVICE synchronisation must be done after the last modification
279	of the memory region by the software and before it is handed off to
280	the device.  Once this primitive is used, memory covered by this
281	primitive should be treated as read-only by the device.  If the device
282	may write to it at any point, it should be DMA_BIDIRECTIONAL (see
283	below).
284
285	DMA_FROM_DEVICE synchronisation must be done before the driver
286	accesses data that may be changed by the device.  This memory should
287	be treated as read-only by the driver.  If the driver needs to write
288	to it at any point, it should be DMA_BIDIRECTIONAL (see below).
289
290	DMA_BIDIRECTIONAL requires special handling: it means that the driver
291	isn't sure if the memory was modified before being handed off to the
292	device and also isn't sure if the device will also modify it.  Thus,
293	you must always sync bidirectional memory twice: once before the
294	memory is handed off to the device (to make sure all memory changes
295	are flushed from the processor) and once before the data may be
296	accessed after being used by the device (to make sure any processor
297	cache lines are updated with data that the device may have changed).
298
299::
300
301	void
302	dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
303			 enum dma_data_direction direction)
304
305Unmaps the region previously mapped.  All the parameters passed in
306must be identical to those passed in (and returned) by the mapping
307API.
308
309::
310
311	dma_addr_t
312	dma_map_page(struct device *dev, struct page *page,
313		     unsigned long offset, size_t size,
314		     enum dma_data_direction direction)
315
316	void
317	dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
318		       enum dma_data_direction direction)
319
320API for mapping and unmapping for pages.  All the notes and warnings
321for the other mapping APIs apply here.  Also, although the <offset>
322and <size> parameters are provided to do partial page mapping, it is
323recommended that you never use these unless you really know what the
324cache width is.
325
326::
327
328	dma_addr_t
329	dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size,
330			 enum dma_data_direction dir, unsigned long attrs)
331
332	void
333	dma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size,
334			   enum dma_data_direction dir, unsigned long attrs)
335
336API for mapping and unmapping for MMIO resources. All the notes and
337warnings for the other mapping APIs apply here. The API should only be
338used to map device MMIO resources, mapping of RAM is not permitted.
339
340::
341
342	int
343	dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
344
345In some circumstances dma_map_single(), dma_map_page() and dma_map_resource()
346will fail to create a mapping. A driver can check for these errors by testing
347the returned DMA address with dma_mapping_error(). A non-zero return value
348means the mapping could not be created and the driver should take appropriate
349action (e.g. reduce current DMA mapping usage or delay and try again later).
350
351::
352
353	int
354	dma_map_sg(struct device *dev, struct scatterlist *sg,
355		   int nents, enum dma_data_direction direction)
356
357Returns: the number of DMA address segments mapped (this may be shorter
358than <nents> passed in if some elements of the scatter/gather list are
359physically or virtually adjacent and an IOMMU maps them with a single
360entry).
361
362Please note that the sg cannot be mapped again if it has been mapped once.
363The mapping process is allowed to destroy information in the sg.
364
365As with the other mapping interfaces, dma_map_sg() can fail. When it
366does, 0 is returned and a driver must take appropriate action. It is
367critical that the driver do something, in the case of a block driver
368aborting the request or even oopsing is better than doing nothing and
369corrupting the filesystem.
370
371With scatterlists, you use the resulting mapping like this::
372
373	int i, count = dma_map_sg(dev, sglist, nents, direction);
374	struct scatterlist *sg;
375
376	for_each_sg(sglist, sg, count, i) {
377		hw_address[i] = sg_dma_address(sg);
378		hw_len[i] = sg_dma_len(sg);
379	}
380
381where nents is the number of entries in the sglist.
382
383The implementation is free to merge several consecutive sglist entries
384into one (e.g. with an IOMMU, or if several pages just happen to be
385physically contiguous) and returns the actual number of sg entries it
386mapped them to. On failure 0, is returned.
387
388Then you should loop count times (note: this can be less than nents times)
389and use sg_dma_address() and sg_dma_len() macros where you previously
390accessed sg->address and sg->length as shown above.
391
392::
393
394	void
395	dma_unmap_sg(struct device *dev, struct scatterlist *sg,
396		     int nents, enum dma_data_direction direction)
397
398Unmap the previously mapped scatter/gather list.  All the parameters
399must be the same as those and passed in to the scatter/gather mapping
400API.
401
402Note: <nents> must be the number you passed in, *not* the number of
403DMA address entries returned.
404
405::
406
407	void
408	dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle,
409				size_t size,
410				enum dma_data_direction direction)
411
412	void
413	dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle,
414				   size_t size,
415				   enum dma_data_direction direction)
416
417	void
418	dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
419			    int nents,
420			    enum dma_data_direction direction)
421
422	void
423	dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
424			       int nents,
425			       enum dma_data_direction direction)
426
427Synchronise a single contiguous or scatter/gather mapping for the CPU
428and device. With the sync_sg API, all the parameters must be the same
429as those passed into the single mapping API. With the sync_single API,
430you can use dma_handle and size parameters that aren't identical to
431those passed into the single mapping API to do a partial sync.
432
433
434.. note::
435
436   You must do this:
437
438   - Before reading values that have been written by DMA from the device
439     (use the DMA_FROM_DEVICE direction)
440   - After writing values that will be written to the device using DMA
441     (use the DMA_TO_DEVICE) direction
442   - before *and* after handing memory to the device if the memory is
443     DMA_BIDIRECTIONAL
444
445See also dma_map_single().
446
447::
448
449	dma_addr_t
450	dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
451			     enum dma_data_direction dir,
452			     unsigned long attrs)
453
454	void
455	dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
456			       size_t size, enum dma_data_direction dir,
457			       unsigned long attrs)
458
459	int
460	dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
461			 int nents, enum dma_data_direction dir,
462			 unsigned long attrs)
463
464	void
465	dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
466			   int nents, enum dma_data_direction dir,
467			   unsigned long attrs)
468
469The four functions above are just like the counterpart functions
470without the _attrs suffixes, except that they pass an optional
471dma_attrs.
472
473The interpretation of DMA attributes is architecture-specific, and
474each attribute should be documented in Documentation/DMA-attributes.txt.
475
476If dma_attrs are 0, the semantics of each of these functions
477is identical to those of the corresponding function
478without the _attrs suffix. As a result dma_map_single_attrs()
479can generally replace dma_map_single(), etc.
480
481As an example of the use of the ``*_attrs`` functions, here's how
482you could pass an attribute DMA_ATTR_FOO when mapping memory
483for DMA::
484
485	#include <linux/dma-mapping.h>
486	/* DMA_ATTR_FOO should be defined in linux/dma-mapping.h and
487	* documented in Documentation/DMA-attributes.txt */
488	...
489
490		unsigned long attr;
491		attr |= DMA_ATTR_FOO;
492		....
493		n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, attr);
494		....
495
496Architectures that care about DMA_ATTR_FOO would check for its
497presence in their implementations of the mapping and unmapping
498routines, e.g.:::
499
500	void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
501				     size_t size, enum dma_data_direction dir,
502				     unsigned long attrs)
503	{
504		....
505		if (attrs & DMA_ATTR_FOO)
506			/* twizzle the frobnozzle */
507		....
508	}
509
510
511Part II - Advanced dma usage
512----------------------------
513
514Warning: These pieces of the DMA API should not be used in the
515majority of cases, since they cater for unlikely corner cases that
516don't belong in usual drivers.
517
518If you don't understand how cache line coherency works between a
519processor and an I/O device, you should not be using this part of the
520API at all.
521
522::
523
524	void *
525	dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
526			gfp_t flag, unsigned long attrs)
527
528Identical to dma_alloc_coherent() except that when the
529DMA_ATTR_NON_CONSISTENT flags is passed in the attrs argument, the
530platform will choose to return either consistent or non-consistent memory
531as it sees fit.  By using this API, you are guaranteeing to the platform
532that you have all the correct and necessary sync points for this memory
533in the driver should it choose to return non-consistent memory.
534
535Note: where the platform can return consistent memory, it will
536guarantee that the sync points become nops.
537
538Warning:  Handling non-consistent memory is a real pain.  You should
539only use this API if you positively know your driver will be
540required to work on one of the rare (usually non-PCI) architectures
541that simply cannot make consistent memory.
542
543::
544
545	void
546	dma_free_attrs(struct device *dev, size_t size, void *cpu_addr,
547		       dma_addr_t dma_handle, unsigned long attrs)
548
549Free memory allocated by the dma_alloc_attrs().  All common
550parameters must be identical to those otherwise passed to dma_free_coherent,
551and the attrs argument must be identical to the attrs passed to
552dma_alloc_attrs().
553
554::
555
556	int
557	dma_get_cache_alignment(void)
558
559Returns the processor cache alignment.  This is the absolute minimum
560alignment *and* width that you must observe when either mapping
561memory or doing partial flushes.
562
563.. note::
564
565	This API may return a number *larger* than the actual cache
566	line, but it will guarantee that one or more cache lines fit exactly
567	into the width returned by this call.  It will also always be a power
568	of two for easy alignment.
569
570::
571
572	void
573	dma_cache_sync(struct device *dev, void *vaddr, size_t size,
574		       enum dma_data_direction direction)
575
576Do a partial sync of memory that was allocated by dma_alloc_attrs() with
577the DMA_ATTR_NON_CONSISTENT flag starting at virtual address vaddr and
578continuing on for size.  Again, you *must* observe the cache line
579boundaries when doing this.
580
581::
582
583	int
584	dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
585				    dma_addr_t device_addr, size_t size);
586
587Declare region of memory to be handed out by dma_alloc_coherent() when
588it's asked for coherent memory for this device.
589
590phys_addr is the CPU physical address to which the memory is currently
591assigned (this will be ioremapped so the CPU can access the region).
592
593device_addr is the DMA address the device needs to be programmed
594with to actually address this memory (this will be handed out as the
595dma_addr_t in dma_alloc_coherent()).
596
597size is the size of the area (must be multiples of PAGE_SIZE).
598
599As a simplification for the platforms, only *one* such region of
600memory may be declared per device.
601
602For reasons of efficiency, most platforms choose to track the declared
603region only at the granularity of a page.  For smaller allocations,
604you should use the dma_pool() API.
605
606Part III - Debug drivers use of the DMA-API
607-------------------------------------------
608
609The DMA-API as described above has some constraints. DMA addresses must be
610released with the corresponding function with the same size for example. With
611the advent of hardware IOMMUs it becomes more and more important that drivers
612do not violate those constraints. In the worst case such a violation can
613result in data corruption up to destroyed filesystems.
614
615To debug drivers and find bugs in the usage of the DMA-API checking code can
616be compiled into the kernel which will tell the developer about those
617violations. If your architecture supports it you can select the "Enable
618debugging of DMA-API usage" option in your kernel configuration. Enabling this
619option has a performance impact. Do not enable it in production kernels.
620
621If you boot the resulting kernel will contain code which does some bookkeeping
622about what DMA memory was allocated for which device. If this code detects an
623error it prints a warning message with some details into your kernel log. An
624example warning message may look like this::
625
626	WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
627		check_unmap+0x203/0x490()
628	Hardware name:
629	forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
630		function [device address=0x00000000640444be] [size=66 bytes] [mapped as
631	single] [unmapped as page]
632	Modules linked in: nfsd exportfs bridge stp llc r8169
633	Pid: 0, comm: swapper Tainted: G        W  2.6.28-dmatest-09289-g8bb99c0 #1
634	Call Trace:
635	<IRQ>  [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
636	[<ffffffff80647b70>] _spin_unlock+0x10/0x30
637	[<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
638	[<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
639	[<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
640	[<ffffffff80252f96>] queue_work+0x56/0x60
641	[<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
642	[<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
643	[<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
644	[<ffffffff80235177>] find_busiest_group+0x207/0x8a0
645	[<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
646	[<ffffffff803c7ea3>] check_unmap+0x203/0x490
647	[<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
648	[<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
649	[<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
650	[<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
651	[<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
652	[<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
653	[<ffffffff8020c093>] ret_from_intr+0x0/0xa
654	<EOI> <4>---[ end trace f6435a98e2a38c0e ]---
655
656The driver developer can find the driver and the device including a stacktrace
657of the DMA-API call which caused this warning.
658
659Per default only the first error will result in a warning message. All other
660errors will only silently counted. This limitation exist to prevent the code
661from flooding your kernel log. To support debugging a device driver this can
662be disabled via debugfs. See the debugfs interface documentation below for
663details.
664
665The debugfs directory for the DMA-API debugging code is called dma-api/. In
666this directory the following files can currently be found:
667
668=============================== ===============================================
669dma-api/all_errors		This file contains a numeric value. If this
670				value is not equal to zero the debugging code
671				will print a warning for every error it finds
672				into the kernel log. Be careful with this
673				option, as it can easily flood your logs.
674
675dma-api/disabled		This read-only file contains the character 'Y'
676				if the debugging code is disabled. This can
677				happen when it runs out of memory or if it was
678				disabled at boot time
679
680dma-api/dump			This read-only file contains current DMA
681				mappings.
682
683dma-api/error_count		This file is read-only and shows the total
684				numbers of errors found.
685
686dma-api/num_errors		The number in this file shows how many
687				warnings will be printed to the kernel log
688				before it stops. This number is initialized to
689				one at system boot and be set by writing into
690				this file
691
692dma-api/min_free_entries	This read-only file can be read to get the
693				minimum number of free dma_debug_entries the
694				allocator has ever seen. If this value goes
695				down to zero the code will attempt to increase
696				nr_total_entries to compensate.
697
698dma-api/num_free_entries	The current number of free dma_debug_entries
699				in the allocator.
700
701dma-api/nr_total_entries	The total number of dma_debug_entries in the
702				allocator, both free and used.
703
704dma-api/driver_filter		You can write a name of a driver into this file
705				to limit the debug output to requests from that
706				particular driver. Write an empty string to
707				that file to disable the filter and see
708				all errors again.
709=============================== ===============================================
710
711If you have this code compiled into your kernel it will be enabled by default.
712If you want to boot without the bookkeeping anyway you can provide
713'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
714Notice that you can not enable it again at runtime. You have to reboot to do
715so.
716
717If you want to see debug messages only for a special device driver you can
718specify the dma_debug_driver=<drivername> parameter. This will enable the
719driver filter at boot time. The debug code will only print errors for that
720driver afterwards. This filter can be disabled or changed later using debugfs.
721
722When the code disables itself at runtime this is most likely because it ran
723out of dma_debug_entries and was unable to allocate more on-demand. 65536
724entries are preallocated at boot - if this is too low for you boot with
725'dma_debug_entries=<your_desired_number>' to overwrite the default. Note
726that the code allocates entries in batches, so the exact number of
727preallocated entries may be greater than the actual number requested. The
728code will print to the kernel log each time it has dynamically allocated
729as many entries as were initially preallocated. This is to indicate that a
730larger preallocation size may be appropriate, or if it happens continually
731that a driver may be leaking mappings.
732
733::
734
735	void
736	debug_dma_mapping_error(struct device *dev, dma_addr_t dma_addr);
737
738dma-debug interface debug_dma_mapping_error() to debug drivers that fail
739to check DMA mapping errors on addresses returned by dma_map_single() and
740dma_map_page() interfaces. This interface clears a flag set by
741debug_dma_map_page() to indicate that dma_mapping_error() has been called by
742the driver. When driver does unmap, debug_dma_unmap() checks the flag and if
743this flag is still set, prints warning message that includes call trace that
744leads up to the unmap. This interface can be called from dma_mapping_error()
745routines to enable DMA mapping error check debugging.