1 ============================================
2 Dynamic DMA mapping using the generic device
3 ============================================
5 :Author: James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
7 This document describes the DMA API. For a more gentle introduction
8 of the API (and actual examples), see Documentation/DMA-API-HOWTO.txt.
10 This API is split into two pieces. Part I describes the basic API.
11 Part II describes extensions for supporting non-consistent memory
12 machines. Unless you know that your driver absolutely has to support
13 non-consistent platforms (this is usually only legacy platforms) you
14 should only use the API described in part I.
19 To get the dma_API, you must #include <linux/dma-mapping.h>. This
20 provides dma_addr_t and the interfaces described below.
22 A dma_addr_t can hold any valid DMA address for the platform. It can be
23 given to a device to use as a DMA source or target. A CPU cannot reference
24 a dma_addr_t directly because there may be translation between its physical
25 address space and the DMA address space.
27 Part Ia - Using large DMA-coherent buffers
28 ------------------------------------------
33 dma_alloc_coherent(struct device *dev, size_t size,
34 dma_addr_t *dma_handle, gfp_t flag)
36 Consistent memory is memory for which a write by either the device or
37 the processor can immediately be read by the processor or device
38 without having to worry about caching effects. (You may however need
39 to make sure to flush the processor's write buffers before telling
40 devices to read that memory.)
42 This routine allocates a region of <size> bytes of consistent memory.
44 It returns a pointer to the allocated region (in the processor's virtual
45 address space) or NULL if the allocation failed.
47 It also returns a <dma_handle> which may be cast to an unsigned integer the
48 same width as the bus and given to the device as the DMA address base of
51 Note: consistent memory can be expensive on some platforms, and the
52 minimum allocation length may be as big as a page, so you should
53 consolidate your requests for consistent memory as much as possible.
54 The simplest way to do that is to use the dma_pool calls (see below).
56 The flag parameter (dma_alloc_coherent() only) allows the caller to
57 specify the ``GFP_`` flags (see kmalloc()) for the allocation (the
58 implementation may choose to ignore flags that affect the location of
59 the returned memory, like GFP_DMA).
64 dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
65 dma_addr_t dma_handle)
67 Free a region of consistent memory you previously allocated. dev,
68 size and dma_handle must all be the same as those passed into
69 dma_alloc_coherent(). cpu_addr must be the virtual address returned by
70 the dma_alloc_coherent().
72 Note that unlike their sibling allocation calls, these routines
73 may only be called with IRQs enabled.
76 Part Ib - Using small DMA-coherent buffers
77 ------------------------------------------
79 To get this part of the dma_API, you must #include <linux/dmapool.h>
81 Many drivers need lots of small DMA-coherent memory regions for DMA
82 descriptors or I/O buffers. Rather than allocating in units of a page
83 or more using dma_alloc_coherent(), you can use DMA pools. These work
84 much like a struct kmem_cache, except that they use the DMA-coherent allocator,
85 not __get_free_pages(). Also, they understand common hardware constraints
86 for alignment, like queue heads needing to be aligned on N-byte boundaries.
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
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.
109 dma_pool_zalloc(struct dma_pool *pool, gfp_t mem_flags,
112 Wraps dma_pool_alloc() and also zeroes the returned memory if the
113 allocation attempt succeeded.
119 dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
120 dma_addr_t *dma_handle);
122 This allocates memory from the pool; the returned memory will meet the
123 size and alignment requirements specified at creation time. Pass
124 GFP_ATOMIC to prevent blocking, or if it's permitted (not
125 in_interrupt, not holding SMP locks), pass GFP_KERNEL to allow
126 blocking. Like dma_alloc_coherent(), this returns two values: an
127 address usable by the CPU, and the DMA address usable by the pool's
133 dma_pool_free(struct dma_pool *pool, void *vaddr,
136 This puts memory back into the pool. The pool is what was passed to
137 dma_pool_alloc(); the CPU (vaddr) and DMA addresses are what
138 were returned when that routine allocated the memory being freed.
143 dma_pool_destroy(struct dma_pool *pool);
145 dma_pool_destroy() frees the resources of the pool. It must be
146 called in a context which can sleep. Make sure you've freed all allocated
147 memory back to the pool before you destroy it.
150 Part Ic - DMA addressing limitations
151 ------------------------------------
156 dma_set_mask_and_coherent(struct device *dev, u64 mask)
158 Checks to see if the mask is possible and updates the device
159 streaming and coherent DMA mask parameters if it is.
161 Returns: 0 if successful and a negative error if not.
166 dma_set_mask(struct device *dev, u64 mask)
168 Checks to see if the mask is possible and updates the device
171 Returns: 0 if successful and a negative error if not.
176 dma_set_coherent_mask(struct device *dev, u64 mask)
178 Checks to see if the mask is possible and updates the device
181 Returns: 0 if successful and a negative error if not.
186 dma_get_required_mask(struct device *dev)
188 This API returns the mask that the platform requires to
189 operate efficiently. Usually this means the returned mask
190 is the minimum required to cover all of memory. Examining the
191 required mask gives drivers with variable descriptor sizes the
192 opportunity to use smaller descriptors as necessary.
194 Requesting the required mask does not alter the current mask. If you
195 wish to take advantage of it, you should issue a dma_set_mask()
196 call to set the mask to the value returned.
201 dma_direct_max_mapping_size(struct device *dev);
203 Returns the maximum size of a mapping for the device. The size parameter
204 of the mapping functions like dma_map_single(), dma_map_page() and
205 others should not be larger than the returned value.
207 Part Id - Streaming DMA mappings
208 --------------------------------
213 dma_map_single(struct device *dev, void *cpu_addr, size_t size,
214 enum dma_data_direction direction)
216 Maps a piece of processor virtual memory so it can be accessed by the
217 device and returns the DMA address of the memory.
219 The direction for both APIs may be converted freely by casting.
220 However the dma_API uses a strongly typed enumerator for its
223 ======================= =============================================
224 DMA_NONE no direction (used for debugging)
225 DMA_TO_DEVICE data is going from the memory to the device
226 DMA_FROM_DEVICE data is coming from the device to the memory
227 DMA_BIDIRECTIONAL direction isn't known
228 ======================= =============================================
232 Not all memory regions in a machine can be mapped by this API.
233 Further, contiguous kernel virtual space may not be contiguous as
234 physical memory. Since this API does not provide any scatter/gather
235 capability, it will fail if the user tries to map a non-physically
236 contiguous piece of memory. For this reason, memory to be mapped by
237 this API should be obtained from sources which guarantee it to be
238 physically contiguous (like kmalloc).
240 Further, the DMA address of the memory must be within the
241 dma_mask of the device (the dma_mask is a bit mask of the
242 addressable region for the device, i.e., if the DMA address of
243 the memory ANDed with the dma_mask is still equal to the DMA
244 address, then the device can perform DMA to the memory). To
245 ensure that the memory allocated by kmalloc is within the dma_mask,
246 the driver may specify various platform-dependent flags to restrict
247 the DMA address range of the allocation (e.g., on x86, GFP_DMA
248 guarantees to be within the first 16MB of available DMA addresses,
249 as required by ISA devices).
251 Note also that the above constraints on physical contiguity and
252 dma_mask may not apply if the platform has an IOMMU (a device which
253 maps an I/O DMA address to a physical memory address). However, to be
254 portable, device driver writers may *not* assume that such an IOMMU
259 Memory coherency operates at a granularity called the cache
260 line width. In order for memory mapped by this API to operate
261 correctly, the mapped region must begin exactly on a cache line
262 boundary and end exactly on one (to prevent two separately mapped
263 regions from sharing a single cache line). Since the cache line size
264 may not be known at compile time, the API will not enforce this
265 requirement. Therefore, it is recommended that driver writers who
266 don't take special care to determine the cache line size at run time
267 only map virtual regions that begin and end on page boundaries (which
268 are guaranteed also to be cache line boundaries).
270 DMA_TO_DEVICE synchronisation must be done after the last modification
271 of the memory region by the software and before it is handed off to
272 the device. Once this primitive is used, memory covered by this
273 primitive should be treated as read-only by the device. If the device
274 may write to it at any point, it should be DMA_BIDIRECTIONAL (see
277 DMA_FROM_DEVICE synchronisation must be done before the driver
278 accesses data that may be changed by the device. This memory should
279 be treated as read-only by the driver. If the driver needs to write
280 to it at any point, it should be DMA_BIDIRECTIONAL (see below).
282 DMA_BIDIRECTIONAL requires special handling: it means that the driver
283 isn't sure if the memory was modified before being handed off to the
284 device and also isn't sure if the device will also modify it. Thus,
285 you must always sync bidirectional memory twice: once before the
286 memory is handed off to the device (to make sure all memory changes
287 are flushed from the processor) and once before the data may be
288 accessed after being used by the device (to make sure any processor
289 cache lines are updated with data that the device may have changed).
294 dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
295 enum dma_data_direction direction)
297 Unmaps the region previously mapped. All the parameters passed in
298 must be identical to those passed in (and returned) by the mapping
304 dma_map_page(struct device *dev, struct page *page,
305 unsigned long offset, size_t size,
306 enum dma_data_direction direction)
309 dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
310 enum dma_data_direction direction)
312 API for mapping and unmapping for pages. All the notes and warnings
313 for the other mapping APIs apply here. Also, although the <offset>
314 and <size> parameters are provided to do partial page mapping, it is
315 recommended that you never use these unless you really know what the
321 dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size,
322 enum dma_data_direction dir, unsigned long attrs)
325 dma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size,
326 enum dma_data_direction dir, unsigned long attrs)
328 API for mapping and unmapping for MMIO resources. All the notes and
329 warnings for the other mapping APIs apply here. The API should only be
330 used to map device MMIO resources, mapping of RAM is not permitted.
335 dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
337 In some circumstances dma_map_single(), dma_map_page() and dma_map_resource()
338 will fail to create a mapping. A driver can check for these errors by testing
339 the returned DMA address with dma_mapping_error(). A non-zero return value
340 means the mapping could not be created and the driver should take appropriate
341 action (e.g. reduce current DMA mapping usage or delay and try again later).
346 dma_map_sg(struct device *dev, struct scatterlist *sg,
347 int nents, enum dma_data_direction direction)
349 Returns: the number of DMA address segments mapped (this may be shorter
350 than <nents> passed in if some elements of the scatter/gather list are
351 physically or virtually adjacent and an IOMMU maps them with a single
354 Please note that the sg cannot be mapped again if it has been mapped once.
355 The mapping process is allowed to destroy information in the sg.
357 As with the other mapping interfaces, dma_map_sg() can fail. When it
358 does, 0 is returned and a driver must take appropriate action. It is
359 critical that the driver do something, in the case of a block driver
360 aborting the request or even oopsing is better than doing nothing and
361 corrupting the filesystem.
363 With scatterlists, you use the resulting mapping like this::
365 int i, count = dma_map_sg(dev, sglist, nents, direction);
366 struct scatterlist *sg;
368 for_each_sg(sglist, sg, count, i) {
369 hw_address[i] = sg_dma_address(sg);
370 hw_len[i] = sg_dma_len(sg);
373 where nents is the number of entries in the sglist.
375 The implementation is free to merge several consecutive sglist entries
376 into one (e.g. with an IOMMU, or if several pages just happen to be
377 physically contiguous) and returns the actual number of sg entries it
378 mapped them to. On failure 0, is returned.
380 Then you should loop count times (note: this can be less than nents times)
381 and use sg_dma_address() and sg_dma_len() macros where you previously
382 accessed sg->address and sg->length as shown above.
387 dma_unmap_sg(struct device *dev, struct scatterlist *sg,
388 int nents, enum dma_data_direction direction)
390 Unmap the previously mapped scatter/gather list. All the parameters
391 must be the same as those and passed in to the scatter/gather mapping
394 Note: <nents> must be the number you passed in, *not* the number of
395 DMA address entries returned.
400 dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle,
402 enum dma_data_direction direction)
405 dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle,
407 enum dma_data_direction direction)
410 dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
412 enum dma_data_direction direction)
415 dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
417 enum dma_data_direction direction)
419 Synchronise a single contiguous or scatter/gather mapping for the CPU
420 and device. With the sync_sg API, all the parameters must be the same
421 as those passed into the single mapping API. With the sync_single API,
422 you can use dma_handle and size parameters that aren't identical to
423 those passed into the single mapping API to do a partial sync.
430 - Before reading values that have been written by DMA from the device
431 (use the DMA_FROM_DEVICE direction)
432 - After writing values that will be written to the device using DMA
433 (use the DMA_TO_DEVICE) direction
434 - before *and* after handing memory to the device if the memory is
437 See also dma_map_single().
442 dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
443 enum dma_data_direction dir,
447 dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
448 size_t size, enum dma_data_direction dir,
452 dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
453 int nents, enum dma_data_direction dir,
457 dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
458 int nents, enum dma_data_direction dir,
461 The four functions above are just like the counterpart functions
462 without the _attrs suffixes, except that they pass an optional
465 The interpretation of DMA attributes is architecture-specific, and
466 each attribute should be documented in Documentation/DMA-attributes.txt.
468 If dma_attrs are 0, the semantics of each of these functions
469 is identical to those of the corresponding function
470 without the _attrs suffix. As a result dma_map_single_attrs()
471 can generally replace dma_map_single(), etc.
473 As an example of the use of the ``*_attrs`` functions, here's how
474 you could pass an attribute DMA_ATTR_FOO when mapping memory
477 #include <linux/dma-mapping.h>
478 /* DMA_ATTR_FOO should be defined in linux/dma-mapping.h and
479 * documented in Documentation/DMA-attributes.txt */
483 attr |= DMA_ATTR_FOO;
485 n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, attr);
488 Architectures that care about DMA_ATTR_FOO would check for its
489 presence in their implementations of the mapping and unmapping
492 void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
493 size_t size, enum dma_data_direction dir,
497 if (attrs & DMA_ATTR_FOO)
498 /* twizzle the frobnozzle */
503 Part II - Advanced dma usage
504 ----------------------------
506 Warning: These pieces of the DMA API should not be used in the
507 majority of cases, since they cater for unlikely corner cases that
508 don't belong in usual drivers.
510 If you don't understand how cache line coherency works between a
511 processor and an I/O device, you should not be using this part of the
517 dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
518 gfp_t flag, unsigned long attrs)
520 Identical to dma_alloc_coherent() except that when the
521 DMA_ATTR_NON_CONSISTENT flags is passed in the attrs argument, the
522 platform will choose to return either consistent or non-consistent memory
523 as it sees fit. By using this API, you are guaranteeing to the platform
524 that you have all the correct and necessary sync points for this memory
525 in the driver should it choose to return non-consistent memory.
527 Note: where the platform can return consistent memory, it will
528 guarantee that the sync points become nops.
530 Warning: Handling non-consistent memory is a real pain. You should
531 only use this API if you positively know your driver will be
532 required to work on one of the rare (usually non-PCI) architectures
533 that simply cannot make consistent memory.
538 dma_free_attrs(struct device *dev, size_t size, void *cpu_addr,
539 dma_addr_t dma_handle, unsigned long attrs)
541 Free memory allocated by the dma_alloc_attrs(). All parameters common
542 parameters must identical to those otherwise passed to dma_fre_coherent,
543 and the attrs argument must be identical to the attrs passed to
549 dma_get_cache_alignment(void)
551 Returns the processor cache alignment. This is the absolute minimum
552 alignment *and* width that you must observe when either mapping
553 memory or doing partial flushes.
557 This API may return a number *larger* than the actual cache
558 line, but it will guarantee that one or more cache lines fit exactly
559 into the width returned by this call. It will also always be a power
560 of two for easy alignment.
565 dma_cache_sync(struct device *dev, void *vaddr, size_t size,
566 enum dma_data_direction direction)
568 Do a partial sync of memory that was allocated by dma_alloc_attrs() with
569 the DMA_ATTR_NON_CONSISTENT flag starting at virtual address vaddr and
570 continuing on for size. Again, you *must* observe the cache line
571 boundaries when doing this.
576 dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
577 dma_addr_t device_addr, size_t size, int
580 Declare region of memory to be handed out by dma_alloc_coherent() when
581 it's asked for coherent memory for this device.
583 phys_addr is the CPU physical address to which the memory is currently
584 assigned (this will be ioremapped so the CPU can access the region).
586 device_addr is the DMA address the device needs to be programmed
587 with to actually address this memory (this will be handed out as the
588 dma_addr_t in dma_alloc_coherent()).
590 size is the size of the area (must be multiples of PAGE_SIZE).
592 flags can be ORed together and are:
594 - DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions.
595 Do not allow dma_alloc_coherent() to fall back to system memory when
596 it's out of memory in the declared region.
598 As a simplification for the platforms, only *one* such region of
599 memory may be declared per device.
601 For reasons of efficiency, most platforms choose to track the declared
602 region only at the granularity of a page. For smaller allocations,
603 you should use the dma_pool() API.
608 dma_release_declared_memory(struct device *dev)
610 Remove the memory region previously declared from the system. This
611 API performs *no* in-use checking for this region and will return
612 unconditionally having removed all the required structures. It is the
613 driver's job to ensure that no parts of this memory region are
619 dma_mark_declared_memory_occupied(struct device *dev,
620 dma_addr_t device_addr, size_t size)
622 This is used to occupy specific regions of the declared space
623 (dma_alloc_coherent() will hand out the first free region it finds).
625 device_addr is the *device* address of the region requested.
627 size is the size (and should be a page-sized multiple).
629 The return value will be either a pointer to the processor virtual
630 address of the memory, or an error (via PTR_ERR()) if any part of the
633 Part III - Debug drivers use of the DMA-API
634 -------------------------------------------
636 The DMA-API as described above has some constraints. DMA addresses must be
637 released with the corresponding function with the same size for example. With
638 the advent of hardware IOMMUs it becomes more and more important that drivers
639 do not violate those constraints. In the worst case such a violation can
640 result in data corruption up to destroyed filesystems.
642 To debug drivers and find bugs in the usage of the DMA-API checking code can
643 be compiled into the kernel which will tell the developer about those
644 violations. If your architecture supports it you can select the "Enable
645 debugging of DMA-API usage" option in your kernel configuration. Enabling this
646 option has a performance impact. Do not enable it in production kernels.
648 If you boot the resulting kernel will contain code which does some bookkeeping
649 about what DMA memory was allocated for which device. If this code detects an
650 error it prints a warning message with some details into your kernel log. An
651 example warning message may look like this::
653 WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
654 check_unmap+0x203/0x490()
656 forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
657 function [device address=0x00000000640444be] [size=66 bytes] [mapped as
658 single] [unmapped as page]
659 Modules linked in: nfsd exportfs bridge stp llc r8169
660 Pid: 0, comm: swapper Tainted: G W 2.6.28-dmatest-09289-g8bb99c0 #1
662 <IRQ> [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
663 [<ffffffff80647b70>] _spin_unlock+0x10/0x30
664 [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
665 [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
666 [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
667 [<ffffffff80252f96>] queue_work+0x56/0x60
668 [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
669 [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
670 [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
671 [<ffffffff80235177>] find_busiest_group+0x207/0x8a0
672 [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
673 [<ffffffff803c7ea3>] check_unmap+0x203/0x490
674 [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
675 [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
676 [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
677 [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
678 [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
679 [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
680 [<ffffffff8020c093>] ret_from_intr+0x0/0xa
681 <EOI> <4>---[ end trace f6435a98e2a38c0e ]---
683 The driver developer can find the driver and the device including a stacktrace
684 of the DMA-API call which caused this warning.
686 Per default only the first error will result in a warning message. All other
687 errors will only silently counted. This limitation exist to prevent the code
688 from flooding your kernel log. To support debugging a device driver this can
689 be disabled via debugfs. See the debugfs interface documentation below for
692 The debugfs directory for the DMA-API debugging code is called dma-api/. In
693 this directory the following files can currently be found:
695 =============================== ===============================================
696 dma-api/all_errors This file contains a numeric value. If this
697 value is not equal to zero the debugging code
698 will print a warning for every error it finds
699 into the kernel log. Be careful with this
700 option, as it can easily flood your logs.
702 dma-api/disabled This read-only file contains the character 'Y'
703 if the debugging code is disabled. This can
704 happen when it runs out of memory or if it was
705 disabled at boot time
707 dma-api/error_count This file is read-only and shows the total
708 numbers of errors found.
710 dma-api/num_errors The number in this file shows how many
711 warnings will be printed to the kernel log
712 before it stops. This number is initialized to
713 one at system boot and be set by writing into
716 dma-api/min_free_entries This read-only file can be read to get the
717 minimum number of free dma_debug_entries the
718 allocator has ever seen. If this value goes
719 down to zero the code will attempt to increase
720 nr_total_entries to compensate.
722 dma-api/num_free_entries The current number of free dma_debug_entries
725 dma-api/nr_total_entries The total number of dma_debug_entries in the
726 allocator, both free and used.
728 dma-api/driver-filter You can write a name of a driver into this file
729 to limit the debug output to requests from that
730 particular driver. Write an empty string to
731 that file to disable the filter and see
733 =============================== ===============================================
735 If you have this code compiled into your kernel it will be enabled by default.
736 If you want to boot without the bookkeeping anyway you can provide
737 'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
738 Notice that you can not enable it again at runtime. You have to reboot to do
741 If you want to see debug messages only for a special device driver you can
742 specify the dma_debug_driver=<drivername> parameter. This will enable the
743 driver filter at boot time. The debug code will only print errors for that
744 driver afterwards. This filter can be disabled or changed later using debugfs.
746 When the code disables itself at runtime this is most likely because it ran
747 out of dma_debug_entries and was unable to allocate more on-demand. 65536
748 entries are preallocated at boot - if this is too low for you boot with
749 'dma_debug_entries=<your_desired_number>' to overwrite the default. Note
750 that the code allocates entries in batches, so the exact number of
751 preallocated entries may be greater than the actual number requested. The
752 code will print to the kernel log each time it has dynamically allocated
753 as many entries as were initially preallocated. This is to indicate that a
754 larger preallocation size may be appropriate, or if it happens continually
755 that a driver may be leaking mappings.
760 debug_dma_mapping_error(struct device *dev, dma_addr_t dma_addr);
762 dma-debug interface debug_dma_mapping_error() to debug drivers that fail
763 to check DMA mapping errors on addresses returned by dma_map_single() and
764 dma_map_page() interfaces. This interface clears a flag set by
765 debug_dma_map_page() to indicate that dma_mapping_error() has been called by
766 the driver. When driver does unmap, debug_dma_unmap() checks the flag and if
767 this flag is still set, prints warning message that includes call trace that
768 leads up to the unmap. This interface can be called from dma_mapping_error()
769 routines to enable DMA mapping error check debugging.