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

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