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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 Documentation/DMA-API-HOWTO.txt.
  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.
 15 Part I - dma_ API
 16 -------------------------------------
 18 To get the dma_ API, you must #include <linux/dma-mapping.h>.  This
 19 provides dma_addr_t and the interfaces described below.
 21 A dma_addr_t can hold any valid DMA address for the platform.  It can be
 22 given to a device to use as a DMA source or target.  A CPU cannot reference
 23 a dma_addr_t directly because there may be translation between its physical
 24 address space and the DMA address space.
 26 Part Ia - Using large DMA-coherent buffers
 27 ------------------------------------------
 29 void *
 30 dma_alloc_coherent(struct device *dev, size_t size,
 31                              dma_addr_t *dma_handle, gfp_t flag)
 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.)
 39 This routine allocates a region of <size> bytes of consistent memory.
 41 It returns a pointer to the allocated region (in the processor's virtual
 42 address space) or NULL if the allocation failed.
 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 DMA address base of
 46 the region.
 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).
 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).
 58 void *
 59 dma_zalloc_coherent(struct device *dev, size_t size,
 60                              dma_addr_t *dma_handle, gfp_t flag)
 62 Wraps dma_alloc_coherent() and also zeroes the returned memory if the
 63 allocation attempt succeeded.
 65 void
 66 dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
 67                            dma_addr_t dma_handle)
 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().
 74 Note that unlike their sibling allocation calls, these routines
 75 may only be called with IRQs enabled.
 78 Part Ib - Using small DMA-coherent buffers
 79 ------------------------------------------
 81 To get this part of the dma_ API, you must #include <linux/dmapool.h>
 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.
 91         struct dma_pool *
 92         dma_pool_create(const char *name, struct device *dev,
 93                         size_t size, size_t align, size_t alloc);
 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.
 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.
107         void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
108                         dma_addr_t *dma_handle);
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.
119         void dma_pool_free(struct dma_pool *pool, void *vaddr,
120                         dma_addr_t addr);
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.
127         void dma_pool_destroy(struct dma_pool *pool);
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.
134 Part Ic - DMA addressing limitations
135 ------------------------------------
137 int
138 dma_supported(struct device *dev, u64 mask)
140 Checks to see if the device can support DMA to the memory described by
141 mask.
143 Returns: 1 if it can and 0 if it can't.
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.
150 int
151 dma_set_mask_and_coherent(struct device *dev, u64 mask)
153 Checks to see if the mask is possible and updates the device
154 streaming and coherent DMA mask parameters if it is.
156 Returns: 0 if successful and a negative error if not.
158 int
159 dma_set_mask(struct device *dev, u64 mask)
161 Checks to see if the mask is possible and updates the device
162 parameters if it is.
164 Returns: 0 if successful and a negative error if not.
166 int
167 dma_set_coherent_mask(struct device *dev, u64 mask)
169 Checks to see if the mask is possible and updates the device
170 parameters if it is.
172 Returns: 0 if successful and a negative error if not.
174 u64
175 dma_get_required_mask(struct device *dev)
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.
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.
188 Part Id - Streaming DMA mappings
189 --------------------------------
191 dma_addr_t
192 dma_map_single(struct device *dev, void *cpu_addr, size_t size,
193                       enum dma_data_direction direction)
195 Maps a piece of processor virtual memory so it can be accessed by the
196 device and returns the DMA address of the memory.
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:
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
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).
215 Further, the DMA 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 DMA address of
218 the memory ANDed with the dma_mask is still equal to the DMA
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 DMA address range of the allocation (e.g., on x86, GFP_DMA
223 guarantees to be within the first 16MB of available DMA addresses,
224 as required by ISA devices).
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 DMA address to a physical memory address).  However, to be
229 portable, device driver writers may *not* assume that such an IOMMU
230 exists.
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).
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).
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).
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).
264 void
265 dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
266                  enum dma_data_direction direction)
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.
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)
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.
286 int
287 dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
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).
295         int
296         dma_map_sg(struct device *dev, struct scatterlist *sg,
297                 int nents, enum dma_data_direction direction)
299 Returns: the number of DMA 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).
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.
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.
313 With scatterlists, you use the resulting mapping like this:
315         int i, count = dma_map_sg(dev, sglist, nents, direction);
316         struct scatterlist *sg;
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         }
323 where nents is the number of entries in the sglist.
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.
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.
334         void
335         dma_unmap_sg(struct device *dev, struct scatterlist *sg,
336                 int nhwentries, enum dma_data_direction direction)
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.
342 Note: <nents> must be the number you passed in, *not* the number of
343 DMA address entries returned.
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)
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.
364 Notes:  You must do this:
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
373 See also dma_map_single().
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)
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)
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)
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)
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*.
399 struct dma_attrs encapsulates a set of "DMA attributes". For the
400 definition of struct dma_attrs see linux/dma-attrs.h.
402 The interpretation of DMA attributes is architecture-specific, and
403 each attribute should be documented in Documentation/DMA-attributes.txt.
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.
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:
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 ...
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         ....
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.:
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         ....
441 Part II - Advanced dma_ usage
442 -----------------------------
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.
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.
452 void *
453 dma_alloc_noncoherent(struct device *dev, size_t size,
454                                dma_addr_t *dma_handle, gfp_t flag)
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.
462 Note: where the platform can return consistent memory, it will
463 guarantee that the sync points become nops.
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.
470 void
471 dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
472                               dma_addr_t dma_handle)
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()).
478 int
479 dma_get_cache_alignment(void)
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.
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.
490 void
491 dma_cache_sync(struct device *dev, void *vaddr, size_t size,
492                enum dma_data_direction direction)
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.
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)
504 Declare region of memory to be handed out by dma_alloc_coherent() when
505 it's asked for coherent memory for this device.
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).
510 device_addr is the DMA 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()).
514 size is the size of the area (must be multiples of PAGE_SIZE).
516 flags can be ORed together and are:
518 DMA_MEMORY_MAP - request that the memory returned from
519 dma_alloc_coherent() be directly writable.
521 DMA_MEMORY_IO - request that the memory returned from
522 dma_alloc_coherent() be addressable using read()/write()/memcpy_toio() etc.
524 One or both of these flags must be present.
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).
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.
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.
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.
545 As a simplification for the platforms, only *one* such region of
546 memory may be declared per device.
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.
552 void
553 dma_release_declared_memory(struct device *dev)
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.
561 void *
562 dma_mark_declared_memory_occupied(struct device *dev,
563                                   dma_addr_t device_addr, size_t size)
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).
568 device_addr is the *device* address of the region requested.
570 size is the size (and should be a page-sized multiple).
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.
576 Part III - Debug drivers use of the DMA-API
577 -------------------------------------------
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.
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.
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:
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 ]---
627 The driver developer can find the driver and the device including a stacktrace
628 of the DMA-API call which caused this warning.
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.
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:
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.
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
650         dma-api/error_count     This file is read-only and shows the total
651                                 numbers of errors found.
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
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.
666         dma-api/num_free_entries
667                                 The current number of free dma_debug_entries
668                                 in the allocator.
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.
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.
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.
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.
694 void debug_dmap_mapping_error(struct device *dev, dma_addr_t dma_addr);
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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