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_zalloc_coherent(struct device *dev, size_t size,
65 dma_addr_t *dma_handle, gfp_t flag)
67 Wraps dma_alloc_coherent() and also zeroes the returned memory if the
68 allocation attempt succeeded.
73 dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
74 dma_addr_t dma_handle)
76 Free a region of consistent memory you previously allocated. dev,
77 size and dma_handle must all be the same as those passed into
78 dma_alloc_coherent(). cpu_addr must be the virtual address returned by
79 the dma_alloc_coherent().
81 Note that unlike their sibling allocation calls, these routines
82 may only be called with IRQs enabled.
85 Part Ib - Using small DMA-coherent buffers
86 ------------------------------------------
88 To get this part of the dma_API, you must #include <linux/dmapool.h>
90 Many drivers need lots of small DMA-coherent memory regions for DMA
91 descriptors or I/O buffers. Rather than allocating in units of a page
92 or more using dma_alloc_coherent(), you can use DMA pools. These work
93 much like a struct kmem_cache, except that they use the DMA-coherent allocator,
94 not __get_free_pages(). Also, they understand common hardware constraints
95 for alignment, like queue heads needing to be aligned on N-byte boundaries.
101 dma_pool_create(const char *name, struct device *dev,
102 size_t size, size_t align, size_t alloc);
104 dma_pool_create() initializes a pool of DMA-coherent buffers
105 for use with a given device. It must be called in a context which
108 The "name" is for diagnostics (like a struct kmem_cache name); dev and size
109 are like what you'd pass to dma_alloc_coherent(). The device's hardware
110 alignment requirement for this type of data is "align" (which is expressed
111 in bytes, and must be a power of two). If your device has no boundary
112 crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated
113 from this pool must not cross 4KByte boundaries.
118 dma_pool_zalloc(struct dma_pool *pool, gfp_t mem_flags,
121 Wraps dma_pool_alloc() and also zeroes the returned memory if the
122 allocation attempt succeeded.
128 dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
129 dma_addr_t *dma_handle);
131 This allocates memory from the pool; the returned memory will meet the
132 size and alignment requirements specified at creation time. Pass
133 GFP_ATOMIC to prevent blocking, or if it's permitted (not
134 in_interrupt, not holding SMP locks), pass GFP_KERNEL to allow
135 blocking. Like dma_alloc_coherent(), this returns two values: an
136 address usable by the CPU, and the DMA address usable by the pool's
142 dma_pool_free(struct dma_pool *pool, void *vaddr,
145 This puts memory back into the pool. The pool is what was passed to
146 dma_pool_alloc(); the CPU (vaddr) and DMA addresses are what
147 were returned when that routine allocated the memory being freed.
152 dma_pool_destroy(struct dma_pool *pool);
154 dma_pool_destroy() frees the resources of the pool. It must be
155 called in a context which can sleep. Make sure you've freed all allocated
156 memory back to the pool before you destroy it.
159 Part Ic - DMA addressing limitations
160 ------------------------------------
165 dma_set_mask_and_coherent(struct device *dev, u64 mask)
167 Checks to see if the mask is possible and updates the device
168 streaming and coherent DMA mask parameters if it is.
170 Returns: 0 if successful and a negative error if not.
175 dma_set_mask(struct device *dev, u64 mask)
177 Checks to see if the mask is possible and updates the device
180 Returns: 0 if successful and a negative error if not.
185 dma_set_coherent_mask(struct device *dev, u64 mask)
187 Checks to see if the mask is possible and updates the device
190 Returns: 0 if successful and a negative error if not.
195 dma_get_required_mask(struct device *dev)
197 This API returns the mask that the platform requires to
198 operate efficiently. Usually this means the returned mask
199 is the minimum required to cover all of memory. Examining the
200 required mask gives drivers with variable descriptor sizes the
201 opportunity to use smaller descriptors as necessary.
203 Requesting the required mask does not alter the current mask. If you
204 wish to take advantage of it, you should issue a dma_set_mask()
205 call to set the mask to the value returned.
208 Part Id - Streaming DMA mappings
209 --------------------------------
214 dma_map_single(struct device *dev, void *cpu_addr, size_t size,
215 enum dma_data_direction direction)
217 Maps a piece of processor virtual memory so it can be accessed by the
218 device and returns the DMA address of the memory.
220 The direction for both APIs may be converted freely by casting.
221 However the dma_API uses a strongly typed enumerator for its
224 ======================= =============================================
225 DMA_NONE no direction (used for debugging)
226 DMA_TO_DEVICE data is going from the memory to the device
227 DMA_FROM_DEVICE data is coming from the device to the memory
228 DMA_BIDIRECTIONAL direction isn't known
229 ======================= =============================================
233 Not all memory regions in a machine can be mapped by this API.
234 Further, contiguous kernel virtual space may not be contiguous as
235 physical memory. Since this API does not provide any scatter/gather
236 capability, it will fail if the user tries to map a non-physically
237 contiguous piece of memory. For this reason, memory to be mapped by
238 this API should be obtained from sources which guarantee it to be
239 physically contiguous (like kmalloc).
241 Further, the DMA address of the memory must be within the
242 dma_mask of the device (the dma_mask is a bit mask of the
243 addressable region for the device, i.e., if the DMA address of
244 the memory ANDed with the dma_mask is still equal to the DMA
245 address, then the device can perform DMA to the memory). To
246 ensure that the memory allocated by kmalloc is within the dma_mask,
247 the driver may specify various platform-dependent flags to restrict
248 the DMA address range of the allocation (e.g., on x86, GFP_DMA
249 guarantees to be within the first 16MB of available DMA addresses,
250 as required by ISA devices).
252 Note also that the above constraints on physical contiguity and
253 dma_mask may not apply if the platform has an IOMMU (a device which
254 maps an I/O DMA address to a physical memory address). However, to be
255 portable, device driver writers may *not* assume that such an IOMMU
260 Memory coherency operates at a granularity called the cache
261 line width. In order for memory mapped by this API to operate
262 correctly, the mapped region must begin exactly on a cache line
263 boundary and end exactly on one (to prevent two separately mapped
264 regions from sharing a single cache line). Since the cache line size
265 may not be known at compile time, the API will not enforce this
266 requirement. Therefore, it is recommended that driver writers who
267 don't take special care to determine the cache line size at run time
268 only map virtual regions that begin and end on page boundaries (which
269 are guaranteed also to be cache line boundaries).
271 DMA_TO_DEVICE synchronisation must be done after the last modification
272 of the memory region by the software and before it is handed off to
273 the device. Once this primitive is used, memory covered by this
274 primitive should be treated as read-only by the device. If the device
275 may write to it at any point, it should be DMA_BIDIRECTIONAL (see
278 DMA_FROM_DEVICE synchronisation must be done before the driver
279 accesses data that may be changed by the device. This memory should
280 be treated as read-only by the driver. If the driver needs to write
281 to it at any point, it should be DMA_BIDIRECTIONAL (see below).
283 DMA_BIDIRECTIONAL requires special handling: it means that the driver
284 isn't sure if the memory was modified before being handed off to the
285 device and also isn't sure if the device will also modify it. Thus,
286 you must always sync bidirectional memory twice: once before the
287 memory is handed off to the device (to make sure all memory changes
288 are flushed from the processor) and once before the data may be
289 accessed after being used by the device (to make sure any processor
290 cache lines are updated with data that the device may have changed).
295 dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
296 enum dma_data_direction direction)
298 Unmaps the region previously mapped. All the parameters passed in
299 must be identical to those passed in (and returned) by the mapping
305 dma_map_page(struct device *dev, struct page *page,
306 unsigned long offset, size_t size,
307 enum dma_data_direction direction)
310 dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
311 enum dma_data_direction direction)
313 API for mapping and unmapping for pages. All the notes and warnings
314 for the other mapping APIs apply here. Also, although the <offset>
315 and <size> parameters are provided to do partial page mapping, it is
316 recommended that you never use these unless you really know what the
322 dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size,
323 enum dma_data_direction dir, unsigned long attrs)
326 dma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size,
327 enum dma_data_direction dir, unsigned long attrs)
329 API for mapping and unmapping for MMIO resources. All the notes and
330 warnings for the other mapping APIs apply here. The API should only be
331 used to map device MMIO resources, mapping of RAM is not permitted.
336 dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
338 In some circumstances dma_map_single(), dma_map_page() and dma_map_resource()
339 will fail to create a mapping. A driver can check for these errors by testing
340 the returned DMA address with dma_mapping_error(). A non-zero return value
341 means the mapping could not be created and the driver should take appropriate
342 action (e.g. reduce current DMA mapping usage or delay and try again later).
347 dma_map_sg(struct device *dev, struct scatterlist *sg,
348 int nents, enum dma_data_direction direction)
350 Returns: the number of DMA address segments mapped (this may be shorter
351 than <nents> passed in if some elements of the scatter/gather list are
352 physically or virtually adjacent and an IOMMU maps them with a single
355 Please note that the sg cannot be mapped again if it has been mapped once.
356 The mapping process is allowed to destroy information in the sg.
358 As with the other mapping interfaces, dma_map_sg() can fail. When it
359 does, 0 is returned and a driver must take appropriate action. It is
360 critical that the driver do something, in the case of a block driver
361 aborting the request or even oopsing is better than doing nothing and
362 corrupting the filesystem.
364 With scatterlists, you use the resulting mapping like this::
366 int i, count = dma_map_sg(dev, sglist, nents, direction);
367 struct scatterlist *sg;
369 for_each_sg(sglist, sg, count, i) {
370 hw_address[i] = sg_dma_address(sg);
371 hw_len[i] = sg_dma_len(sg);
374 where nents is the number of entries in the sglist.
376 The implementation is free to merge several consecutive sglist entries
377 into one (e.g. with an IOMMU, or if several pages just happen to be
378 physically contiguous) and returns the actual number of sg entries it
379 mapped them to. On failure 0, is returned.
381 Then you should loop count times (note: this can be less than nents times)
382 and use sg_dma_address() and sg_dma_len() macros where you previously
383 accessed sg->address and sg->length as shown above.
388 dma_unmap_sg(struct device *dev, struct scatterlist *sg,
389 int nents, enum dma_data_direction direction)
391 Unmap the previously mapped scatter/gather list. All the parameters
392 must be the same as those and passed in to the scatter/gather mapping
395 Note: <nents> must be the number you passed in, *not* the number of
396 DMA address entries returned.
401 dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle,
403 enum dma_data_direction direction)
406 dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle,
408 enum dma_data_direction direction)
411 dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
413 enum dma_data_direction direction)
416 dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
418 enum dma_data_direction direction)
420 Synchronise a single contiguous or scatter/gather mapping for the CPU
421 and device. With the sync_sg API, all the parameters must be the same
422 as those passed into the single mapping API. With the sync_single API,
423 you can use dma_handle and size parameters that aren't identical to
424 those passed into the single mapping API to do a partial sync.
431 - Before reading values that have been written by DMA from the device
432 (use the DMA_FROM_DEVICE direction)
433 - After writing values that will be written to the device using DMA
434 (use the DMA_TO_DEVICE) direction
435 - before *and* after handing memory to the device if the memory is
438 See also dma_map_single().
443 dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
444 enum dma_data_direction dir,
448 dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
449 size_t size, enum dma_data_direction dir,
453 dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
454 int nents, enum dma_data_direction dir,
458 dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
459 int nents, enum dma_data_direction dir,
462 The four functions above are just like the counterpart functions
463 without the _attrs suffixes, except that they pass an optional
466 The interpretation of DMA attributes is architecture-specific, and
467 each attribute should be documented in Documentation/DMA-attributes.txt.
469 If dma_attrs are 0, the semantics of each of these functions
470 is identical to those of the corresponding function
471 without the _attrs suffix. As a result dma_map_single_attrs()
472 can generally replace dma_map_single(), etc.
474 As an example of the use of the ``*_attrs`` functions, here's how
475 you could pass an attribute DMA_ATTR_FOO when mapping memory
478 #include <linux/dma-mapping.h>
479 /* DMA_ATTR_FOO should be defined in linux/dma-mapping.h and
480 * documented in Documentation/DMA-attributes.txt */
484 attr |= DMA_ATTR_FOO;
486 n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, attr);
489 Architectures that care about DMA_ATTR_FOO would check for its
490 presence in their implementations of the mapping and unmapping
493 void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
494 size_t size, enum dma_data_direction dir,
498 if (attrs & DMA_ATTR_FOO)
499 /* twizzle the frobnozzle */
504 Part II - Advanced dma usage
505 ----------------------------
507 Warning: These pieces of the DMA API should not be used in the
508 majority of cases, since they cater for unlikely corner cases that
509 don't belong in usual drivers.
511 If you don't understand how cache line coherency works between a
512 processor and an I/O device, you should not be using this part of the
518 dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
519 gfp_t flag, unsigned long attrs)
521 Identical to dma_alloc_coherent() except that when the
522 DMA_ATTR_NON_CONSISTENT flags is passed in the attrs argument, the
523 platform will choose to return either consistent or non-consistent memory
524 as it sees fit. By using this API, you are guaranteeing to the platform
525 that you have all the correct and necessary sync points for this memory
526 in the driver should it choose to return non-consistent memory.
528 Note: where the platform can return consistent memory, it will
529 guarantee that the sync points become nops.
531 Warning: Handling non-consistent memory is a real pain. You should
532 only use this API if you positively know your driver will be
533 required to work on one of the rare (usually non-PCI) architectures
534 that simply cannot make consistent memory.
539 dma_free_attrs(struct device *dev, size_t size, void *cpu_addr,
540 dma_addr_t dma_handle, unsigned long attrs)
542 Free memory allocated by the dma_alloc_attrs(). All parameters common
543 parameters must identical to those otherwise passed to dma_fre_coherent,
544 and the attrs argument must be identical to the attrs passed to
550 dma_get_cache_alignment(void)
552 Returns the processor cache alignment. This is the absolute minimum
553 alignment *and* width that you must observe when either mapping
554 memory or doing partial flushes.
558 This API may return a number *larger* than the actual cache
559 line, but it will guarantee that one or more cache lines fit exactly
560 into the width returned by this call. It will also always be a power
561 of two for easy alignment.
566 dma_cache_sync(struct device *dev, void *vaddr, size_t size,
567 enum dma_data_direction direction)
569 Do a partial sync of memory that was allocated by dma_alloc_attrs() with
570 the DMA_ATTR_NON_CONSISTENT flag starting at virtual address vaddr and
571 continuing on for size. Again, you *must* observe the cache line
572 boundaries when doing this.
577 dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
578 dma_addr_t device_addr, size_t size, int
581 Declare region of memory to be handed out by dma_alloc_coherent() when
582 it's asked for coherent memory for this device.
584 phys_addr is the CPU physical address to which the memory is currently
585 assigned (this will be ioremapped so the CPU can access the region).
587 device_addr is the DMA address the device needs to be programmed
588 with to actually address this memory (this will be handed out as the
589 dma_addr_t in dma_alloc_coherent()).
591 size is the size of the area (must be multiples of PAGE_SIZE).
593 flags can be ORed together and are:
595 - DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions.
596 Do not allow dma_alloc_coherent() to fall back to system memory when
597 it's out of memory in the declared region.
599 As a simplification for the platforms, only *one* such region of
600 memory may be declared per device.
602 For reasons of efficiency, most platforms choose to track the declared
603 region only at the granularity of a page. For smaller allocations,
604 you should use the dma_pool() API.
609 dma_release_declared_memory(struct device *dev)
611 Remove the memory region previously declared from the system. This
612 API performs *no* in-use checking for this region and will return
613 unconditionally having removed all the required structures. It is the
614 driver's job to ensure that no parts of this memory region are
620 dma_mark_declared_memory_occupied(struct device *dev,
621 dma_addr_t device_addr, size_t size)
623 This is used to occupy specific regions of the declared space
624 (dma_alloc_coherent() will hand out the first free region it finds).
626 device_addr is the *device* address of the region requested.
628 size is the size (and should be a page-sized multiple).
630 The return value will be either a pointer to the processor virtual
631 address of the memory, or an error (via PTR_ERR()) if any part of the
634 Part III - Debug drivers use of the DMA-API
635 -------------------------------------------
637 The DMA-API as described above has some constraints. DMA addresses must be
638 released with the corresponding function with the same size for example. With
639 the advent of hardware IOMMUs it becomes more and more important that drivers
640 do not violate those constraints. In the worst case such a violation can
641 result in data corruption up to destroyed filesystems.
643 To debug drivers and find bugs in the usage of the DMA-API checking code can
644 be compiled into the kernel which will tell the developer about those
645 violations. If your architecture supports it you can select the "Enable
646 debugging of DMA-API usage" option in your kernel configuration. Enabling this
647 option has a performance impact. Do not enable it in production kernels.
649 If you boot the resulting kernel will contain code which does some bookkeeping
650 about what DMA memory was allocated for which device. If this code detects an
651 error it prints a warning message with some details into your kernel log. An
652 example warning message may look like this::
654 WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
655 check_unmap+0x203/0x490()
657 forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
658 function [device address=0x00000000640444be] [size=66 bytes] [mapped as
659 single] [unmapped as page]
660 Modules linked in: nfsd exportfs bridge stp llc r8169
661 Pid: 0, comm: swapper Tainted: G W 2.6.28-dmatest-09289-g8bb99c0 #1
663 <IRQ> [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
664 [<ffffffff80647b70>] _spin_unlock+0x10/0x30
665 [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
666 [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
667 [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
668 [<ffffffff80252f96>] queue_work+0x56/0x60
669 [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
670 [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
671 [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
672 [<ffffffff80235177>] find_busiest_group+0x207/0x8a0
673 [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
674 [<ffffffff803c7ea3>] check_unmap+0x203/0x490
675 [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
676 [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
677 [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
678 [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
679 [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
680 [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
681 [<ffffffff8020c093>] ret_from_intr+0x0/0xa
682 <EOI> <4>---[ end trace f6435a98e2a38c0e ]---
684 The driver developer can find the driver and the device including a stacktrace
685 of the DMA-API call which caused this warning.
687 Per default only the first error will result in a warning message. All other
688 errors will only silently counted. This limitation exist to prevent the code
689 from flooding your kernel log. To support debugging a device driver this can
690 be disabled via debugfs. See the debugfs interface documentation below for
693 The debugfs directory for the DMA-API debugging code is called dma-api/. In
694 this directory the following files can currently be found:
696 =============================== ===============================================
697 dma-api/all_errors This file contains a numeric value. If this
698 value is not equal to zero the debugging code
699 will print a warning for every error it finds
700 into the kernel log. Be careful with this
701 option, as it can easily flood your logs.
703 dma-api/disabled This read-only file contains the character 'Y'
704 if the debugging code is disabled. This can
705 happen when it runs out of memory or if it was
706 disabled at boot time
708 dma-api/error_count This file is read-only and shows the total
709 numbers of errors found.
711 dma-api/num_errors The number in this file shows how many
712 warnings will be printed to the kernel log
713 before it stops. This number is initialized to
714 one at system boot and be set by writing into
717 dma-api/min_free_entries This read-only file can be read to get the
718 minimum number of free dma_debug_entries the
719 allocator has ever seen. If this value goes
720 down to zero the code will disable itself
721 because it is not longer reliable.
723 dma-api/num_free_entries The current number of free dma_debug_entries
726 dma-api/driver-filter You can write a name of a driver into this file
727 to limit the debug output to requests from that
728 particular driver. Write an empty string to
729 that file to disable the filter and see
731 =============================== ===============================================
733 If you have this code compiled into your kernel it will be enabled by default.
734 If you want to boot without the bookkeeping anyway you can provide
735 'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
736 Notice that you can not enable it again at runtime. You have to reboot to do
739 If you want to see debug messages only for a special device driver you can
740 specify the dma_debug_driver=<drivername> parameter. This will enable the
741 driver filter at boot time. The debug code will only print errors for that
742 driver afterwards. This filter can be disabled or changed later using debugfs.
744 When the code disables itself at runtime this is most likely because it ran
745 out of dma_debug_entries. These entries are preallocated at boot. The number
746 of preallocated entries is defined per architecture. If it is too low for you
747 boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
748 architectural default.
753 debug_dma_mapping_error(struct device *dev, dma_addr_t dma_addr);
755 dma-debug interface debug_dma_mapping_error() to debug drivers that fail
756 to check DMA mapping errors on addresses returned by dma_map_single() and
757 dma_map_page() interfaces. This interface clears a flag set by
758 debug_dma_map_page() to indicate that dma_mapping_error() has been called by
759 the driver. When driver does unmap, debug_dma_unmap() checks the flag and if
760 this flag is still set, prints warning message that includes call trace that
761 leads up to the unmap. This interface can be called from dma_mapping_error()
762 routines to enable DMA mapping error check debugging.