1 Dynamic DMA mapping using the generic device 2 ============================================ 3 4 James E.J. Bottomley <James.Bottomley@HansenPartnership.com> 5 6This document describes the DMA API. For a more gentle introduction 7of the API (and actual examples) see 8Documentation/DMA-API-HOWTO.txt. 9 10This API is split into two pieces. Part I describes the API. Part II 11describes the extensions to the API for supporting non-consistent 12memory machines. Unless you know that your driver absolutely has to 13support non-consistent platforms (this is usually only legacy 14platforms) you should only use the API described in part I. 15 16Part I - dma_ API 17------------------------------------- 18 19To get the dma_ API, you must #include <linux/dma-mapping.h> 20 21 22Part Ia - Using large dma-coherent buffers 23------------------------------------------ 24 25void * 26dma_alloc_coherent(struct device *dev, size_t size, 27 dma_addr_t *dma_handle, gfp_t flag) 28 29Consistent memory is memory for which a write by either the device or 30the processor can immediately be read by the processor or device 31without having to worry about caching effects. (You may however need 32to make sure to flush the processor's write buffers before telling 33devices to read that memory.) 34 35This routine allocates a region of <size> bytes of consistent memory. 36It also returns a <dma_handle> which may be cast to an unsigned 37integer the same width as the bus and used as the physical address 38base of the region. 39 40Returns: a pointer to the allocated region (in the processor's virtual 41address space) or NULL if the allocation failed. 42 43Note: consistent memory can be expensive on some platforms, and the 44minimum allocation length may be as big as a page, so you should 45consolidate your requests for consistent memory as much as possible. 46The simplest way to do that is to use the dma_pool calls (see below). 47 48The flag parameter (dma_alloc_coherent only) allows the caller to 49specify the GFP_ flags (see kmalloc) for the allocation (the 50implementation may choose to ignore flags that affect the location of 51the returned memory, like GFP_DMA). 52 53void 54dma_free_coherent(struct device *dev, size_t size, void *cpu_addr, 55 dma_addr_t dma_handle) 56 57Free the region of consistent memory you previously allocated. dev, 58size and dma_handle must all be the same as those passed into the 59consistent allocate. cpu_addr must be the virtual address returned by 60the consistent allocate. 61 62Note that unlike their sibling allocation calls, these routines 63may only be called with IRQs enabled. 64 65 66Part Ib - Using small dma-coherent buffers 67------------------------------------------ 68 69To get this part of the dma_ API, you must #include <linux/dmapool.h> 70 71Many drivers need lots of small dma-coherent memory regions for DMA 72descriptors or I/O buffers. Rather than allocating in units of a page 73or more using dma_alloc_coherent(), you can use DMA pools. These work 74much like a struct kmem_cache, except that they use the dma-coherent allocator, 75not __get_free_pages(). Also, they understand common hardware constraints 76for alignment, like queue heads needing to be aligned on N-byte boundaries. 77 78 79 struct dma_pool * 80 dma_pool_create(const char *name, struct device *dev, 81 size_t size, size_t align, size_t alloc); 82 83The pool create() routines initialize a pool of dma-coherent buffers 84for use with a given device. It must be called in a context which 85can sleep. 86 87The "name" is for diagnostics (like a struct kmem_cache name); dev and size 88are like what you'd pass to dma_alloc_coherent(). The device's hardware 89alignment requirement for this type of data is "align" (which is expressed 90in bytes, and must be a power of two). If your device has no boundary 91crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated 92from this pool must not cross 4KByte boundaries. 93 94 95 void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags, 96 dma_addr_t *dma_handle); 97 98This allocates memory from the pool; the returned memory will meet the size 99and alignment requirements specified at creation time. Pass GFP_ATOMIC to 100prevent blocking, or if it's permitted (not in_interrupt, not holding SMP locks), 101pass GFP_KERNEL to allow blocking. Like dma_alloc_coherent(), this returns 102two values: an address usable by the cpu, and the dma address usable by the 103pool's device. 104 105 106 void dma_pool_free(struct dma_pool *pool, void *vaddr, 107 dma_addr_t addr); 108 109This puts memory back into the pool. The pool is what was passed to 110the pool allocation routine; the cpu (vaddr) and dma addresses are what 111were returned when that routine allocated the memory being freed. 112 113 114 void dma_pool_destroy(struct dma_pool *pool); 115 116The pool destroy() routines free the resources of the pool. They must be 117called in a context which can sleep. Make sure you've freed all allocated 118memory back to the pool before you destroy it. 119 120 121Part Ic - DMA addressing limitations 122------------------------------------ 123 124int 125dma_supported(struct device *dev, u64 mask) 126 127Checks to see if the device can support DMA to the memory described by 128mask. 129 130Returns: 1 if it can and 0 if it can't. 131 132Notes: This routine merely tests to see if the mask is possible. It 133won't change the current mask settings. It is more intended as an 134internal API for use by the platform than an external API for use by 135driver writers. 136 137int 138dma_set_mask(struct device *dev, u64 mask) 139 140Checks to see if the mask is possible and updates the device 141parameters if it is. 142 143Returns: 0 if successful and a negative error if not. 144 145int 146dma_set_coherent_mask(struct device *dev, u64 mask) 147 148Checks to see if the mask is possible and updates the device 149parameters if it is. 150 151Returns: 0 if successful and a negative error if not. 152 153u64 154dma_get_required_mask(struct device *dev) 155 156This API returns the mask that the platform requires to 157operate efficiently. Usually this means the returned mask 158is the minimum required to cover all of memory. Examining the 159required mask gives drivers with variable descriptor sizes the 160opportunity to use smaller descriptors as necessary. 161 162Requesting the required mask does not alter the current mask. If you 163wish to take advantage of it, you should issue a dma_set_mask() 164call to set the mask to the value returned. 165 166 167Part Id - Streaming DMA mappings 168-------------------------------- 169 170dma_addr_t 171dma_map_single(struct device *dev, void *cpu_addr, size_t size, 172 enum dma_data_direction direction) 173 174Maps a piece of processor virtual memory so it can be accessed by the 175device and returns the physical handle of the memory. 176 177The direction for both api's may be converted freely by casting. 178However the dma_ API uses a strongly typed enumerator for its 179direction: 180 181DMA_NONE no direction (used for debugging) 182DMA_TO_DEVICE data is going from the memory to the device 183DMA_FROM_DEVICE data is coming from the device to the memory 184DMA_BIDIRECTIONAL direction isn't known 185 186Notes: Not all memory regions in a machine can be mapped by this 187API. Further, regions that appear to be physically contiguous in 188kernel virtual space may not be contiguous as physical memory. Since 189this API does not provide any scatter/gather capability, it will fail 190if the user tries to map a non-physically contiguous piece of memory. 191For this reason, it is recommended that memory mapped by this API be 192obtained only from sources which guarantee it to be physically contiguous 193(like kmalloc). 194 195Further, the physical address of the memory must be within the 196dma_mask of the device (the dma_mask represents a bit mask of the 197addressable region for the device. I.e., if the physical address of 198the memory anded with the dma_mask is still equal to the physical 199address, then the device can perform DMA to the memory). In order to 200ensure that the memory allocated by kmalloc is within the dma_mask, 201the driver may specify various platform-dependent flags to restrict 202the physical memory range of the allocation (e.g. on x86, GFP_DMA 203guarantees to be within the first 16Mb of available physical memory, 204as required by ISA devices). 205 206Note also that the above constraints on physical contiguity and 207dma_mask may not apply if the platform has an IOMMU (a device which 208supplies a physical to virtual mapping between the I/O memory bus and 209the device). However, to be portable, device driver writers may *not* 210assume that such an IOMMU exists. 211 212Warnings: Memory coherency operates at a granularity called the cache 213line width. In order for memory mapped by this API to operate 214correctly, the mapped region must begin exactly on a cache line 215boundary and end exactly on one (to prevent two separately mapped 216regions from sharing a single cache line). Since the cache line size 217may not be known at compile time, the API will not enforce this 218requirement. Therefore, it is recommended that driver writers who 219don't take special care to determine the cache line size at run time 220only map virtual regions that begin and end on page boundaries (which 221are guaranteed also to be cache line boundaries). 222 223DMA_TO_DEVICE synchronisation must be done after the last modification 224of the memory region by the software and before it is handed off to 225the driver. Once this primitive is used, memory covered by this 226primitive should be treated as read-only by the device. If the device 227may write to it at any point, it should be DMA_BIDIRECTIONAL (see 228below). 229 230DMA_FROM_DEVICE synchronisation must be done before the driver 231accesses data that may be changed by the device. This memory should 232be treated as read-only by the driver. If the driver needs to write 233to it at any point, it should be DMA_BIDIRECTIONAL (see below). 234 235DMA_BIDIRECTIONAL requires special handling: it means that the driver 236isn't sure if the memory was modified before being handed off to the 237device and also isn't sure if the device will also modify it. Thus, 238you must always sync bidirectional memory twice: once before the 239memory is handed off to the device (to make sure all memory changes 240are flushed from the processor) and once before the data may be 241accessed after being used by the device (to make sure any processor 242cache lines are updated with data that the device may have changed). 243 244void 245dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size, 246 enum dma_data_direction direction) 247 248Unmaps the region previously mapped. All the parameters passed in 249must be identical to those passed in (and returned) by the mapping 250API. 251 252dma_addr_t 253dma_map_page(struct device *dev, struct page *page, 254 unsigned long offset, size_t size, 255 enum dma_data_direction direction) 256void 257dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size, 258 enum dma_data_direction direction) 259 260API for mapping and unmapping for pages. All the notes and warnings 261for the other mapping APIs apply here. Also, although the <offset> 262and <size> parameters are provided to do partial page mapping, it is 263recommended that you never use these unless you really know what the 264cache width is. 265 266int 267dma_mapping_error(struct device *dev, dma_addr_t dma_addr) 268 269In some circumstances dma_map_single and dma_map_page will fail to create 270a mapping. A driver can check for these errors by testing the returned 271dma address with dma_mapping_error(). A non-zero return value means the mapping 272could not be created and the driver should take appropriate action (e.g. 273reduce current DMA mapping usage or delay and try again later). 274 275 int 276 dma_map_sg(struct device *dev, struct scatterlist *sg, 277 int nents, enum dma_data_direction direction) 278 279Returns: the number of physical segments mapped (this may be shorter 280than <nents> passed in if some elements of the scatter/gather list are 281physically or virtually adjacent and an IOMMU maps them with a single 282entry). 283 284Please note that the sg cannot be mapped again if it has been mapped once. 285The mapping process is allowed to destroy information in the sg. 286 287As with the other mapping interfaces, dma_map_sg can fail. When it 288does, 0 is returned and a driver must take appropriate action. It is 289critical that the driver do something, in the case of a block driver 290aborting the request or even oopsing is better than doing nothing and 291corrupting the filesystem. 292 293With scatterlists, you use the resulting mapping like this: 294 295 int i, count = dma_map_sg(dev, sglist, nents, direction); 296 struct scatterlist *sg; 297 298 for_each_sg(sglist, sg, count, i) { 299 hw_address[i] = sg_dma_address(sg); 300 hw_len[i] = sg_dma_len(sg); 301 } 302 303where nents is the number of entries in the sglist. 304 305The implementation is free to merge several consecutive sglist entries 306into one (e.g. with an IOMMU, or if several pages just happen to be 307physically contiguous) and returns the actual number of sg entries it 308mapped them to. On failure 0, is returned. 309 310Then you should loop count times (note: this can be less than nents times) 311and use sg_dma_address() and sg_dma_len() macros where you previously 312accessed sg->address and sg->length as shown above. 313 314 void 315 dma_unmap_sg(struct device *dev, struct scatterlist *sg, 316 int nhwentries, enum dma_data_direction direction) 317 318Unmap the previously mapped scatter/gather list. All the parameters 319must be the same as those and passed in to the scatter/gather mapping 320API. 321 322Note: <nents> must be the number you passed in, *not* the number of 323physical entries returned. 324 325void 326dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size, 327 enum dma_data_direction direction) 328void 329dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size, 330 enum dma_data_direction direction) 331void 332dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nelems, 333 enum dma_data_direction direction) 334void 335dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nelems, 336 enum dma_data_direction direction) 337 338Synchronise a single contiguous or scatter/gather mapping for the cpu 339and device. With the sync_sg API, all the parameters must be the same 340as those passed into the single mapping API. With the sync_single API, 341you can use dma_handle and size parameters that aren't identical to 342those passed into the single mapping API to do a partial sync. 343 344Notes: You must do this: 345 346- Before reading values that have been written by DMA from the device 347 (use the DMA_FROM_DEVICE direction) 348- After writing values that will be written to the device using DMA 349 (use the DMA_TO_DEVICE) direction 350- before *and* after handing memory to the device if the memory is 351 DMA_BIDIRECTIONAL 352 353See also dma_map_single(). 354 355dma_addr_t 356dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size, 357 enum dma_data_direction dir, 358 struct dma_attrs *attrs) 359 360void 361dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr, 362 size_t size, enum dma_data_direction dir, 363 struct dma_attrs *attrs) 364 365int 366dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl, 367 int nents, enum dma_data_direction dir, 368 struct dma_attrs *attrs) 369 370void 371dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl, 372 int nents, enum dma_data_direction dir, 373 struct dma_attrs *attrs) 374 375The four functions above are just like the counterpart functions 376without the _attrs suffixes, except that they pass an optional 377struct dma_attrs*. 378 379struct dma_attrs encapsulates a set of "dma attributes". For the 380definition of struct dma_attrs see linux/dma-attrs.h. 381 382The interpretation of dma attributes is architecture-specific, and 383each attribute should be documented in Documentation/DMA-attributes.txt. 384 385If struct dma_attrs* is NULL, the semantics of each of these 386functions is identical to those of the corresponding function 387without the _attrs suffix. As a result dma_map_single_attrs() 388can generally replace dma_map_single(), etc. 389 390As an example of the use of the *_attrs functions, here's how 391you could pass an attribute DMA_ATTR_FOO when mapping memory 392for DMA: 393 394#include <linux/dma-attrs.h> 395/* DMA_ATTR_FOO should be defined in linux/dma-attrs.h and 396 * documented in Documentation/DMA-attributes.txt */ 397... 398 399 DEFINE_DMA_ATTRS(attrs); 400 dma_set_attr(DMA_ATTR_FOO, &attrs); 401 .... 402 n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, &attr); 403 .... 404 405Architectures that care about DMA_ATTR_FOO would check for its 406presence in their implementations of the mapping and unmapping 407routines, e.g.: 408 409void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr, 410 size_t size, enum dma_data_direction dir, 411 struct dma_attrs *attrs) 412{ 413 .... 414 int foo = dma_get_attr(DMA_ATTR_FOO, attrs); 415 .... 416 if (foo) 417 /* twizzle the frobnozzle */ 418 .... 419 420 421Part II - Advanced dma_ usage 422----------------------------- 423 424Warning: These pieces of the DMA API should not be used in the 425majority of cases, since they cater for unlikely corner cases that 426don't belong in usual drivers. 427 428If you don't understand how cache line coherency works between a 429processor and an I/O device, you should not be using this part of the 430API at all. 431 432void * 433dma_alloc_noncoherent(struct device *dev, size_t size, 434 dma_addr_t *dma_handle, gfp_t flag) 435 436Identical to dma_alloc_coherent() except that the platform will 437choose to return either consistent or non-consistent memory as it sees 438fit. By using this API, you are guaranteeing to the platform that you 439have all the correct and necessary sync points for this memory in the 440driver should it choose to return non-consistent memory. 441 442Note: where the platform can return consistent memory, it will 443guarantee that the sync points become nops. 444 445Warning: Handling non-consistent memory is a real pain. You should 446only ever use this API if you positively know your driver will be 447required to work on one of the rare (usually non-PCI) architectures 448that simply cannot make consistent memory. 449 450void 451dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr, 452 dma_addr_t dma_handle) 453 454Free memory allocated by the nonconsistent API. All parameters must 455be identical to those passed in (and returned by 456dma_alloc_noncoherent()). 457 458int 459dma_get_cache_alignment(void) 460 461Returns the processor cache alignment. This is the absolute minimum 462alignment *and* width that you must observe when either mapping 463memory or doing partial flushes. 464 465Notes: This API may return a number *larger* than the actual cache 466line, but it will guarantee that one or more cache lines fit exactly 467into the width returned by this call. It will also always be a power 468of two for easy alignment. 469 470void 471dma_cache_sync(struct device *dev, void *vaddr, size_t size, 472 enum dma_data_direction direction) 473 474Do a partial sync of memory that was allocated by 475dma_alloc_noncoherent(), starting at virtual address vaddr and 476continuing on for size. Again, you *must* observe the cache line 477boundaries when doing this. 478 479int 480dma_declare_coherent_memory(struct device *dev, dma_addr_t bus_addr, 481 dma_addr_t device_addr, size_t size, int 482 flags) 483 484Declare region of memory to be handed out by dma_alloc_coherent when 485it's asked for coherent memory for this device. 486 487bus_addr is the physical address to which the memory is currently 488assigned in the bus responding region (this will be used by the 489platform to perform the mapping). 490 491device_addr is the physical address the device needs to be programmed 492with actually to address this memory (this will be handed out as the 493dma_addr_t in dma_alloc_coherent()). 494 495size is the size of the area (must be multiples of PAGE_SIZE). 496 497flags can be or'd together and are: 498 499DMA_MEMORY_MAP - request that the memory returned from 500dma_alloc_coherent() be directly writable. 501 502DMA_MEMORY_IO - request that the memory returned from 503dma_alloc_coherent() be addressable using read/write/memcpy_toio etc. 504 505One or both of these flags must be present. 506 507DMA_MEMORY_INCLUDES_CHILDREN - make the declared memory be allocated by 508dma_alloc_coherent of any child devices of this one (for memory residing 509on a bridge). 510 511DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions. 512Do not allow dma_alloc_coherent() to fall back to system memory when 513it's out of memory in the declared region. 514 515The return value will be either DMA_MEMORY_MAP or DMA_MEMORY_IO and 516must correspond to a passed in flag (i.e. no returning DMA_MEMORY_IO 517if only DMA_MEMORY_MAP were passed in) for success or zero for 518failure. 519 520Note, for DMA_MEMORY_IO returns, all subsequent memory returned by 521dma_alloc_coherent() may no longer be accessed directly, but instead 522must be accessed using the correct bus functions. If your driver 523isn't prepared to handle this contingency, it should not specify 524DMA_MEMORY_IO in the input flags. 525 526As a simplification for the platforms, only *one* such region of 527memory may be declared per device. 528 529For reasons of efficiency, most platforms choose to track the declared 530region only at the granularity of a page. For smaller allocations, 531you should use the dma_pool() API. 532 533void 534dma_release_declared_memory(struct device *dev) 535 536Remove the memory region previously declared from the system. This 537API performs *no* in-use checking for this region and will return 538unconditionally having removed all the required structures. It is the 539driver's job to ensure that no parts of this memory region are 540currently in use. 541 542void * 543dma_mark_declared_memory_occupied(struct device *dev, 544 dma_addr_t device_addr, size_t size) 545 546This is used to occupy specific regions of the declared space 547(dma_alloc_coherent() will hand out the first free region it finds). 548 549device_addr is the *device* address of the region requested. 550 551size is the size (and should be a page-sized multiple). 552 553The return value will be either a pointer to the processor virtual 554address of the memory, or an error (via PTR_ERR()) if any part of the 555region is occupied. 556 557Part III - Debug drivers use of the DMA-API 558------------------------------------------- 559 560The DMA-API as described above as some constraints. DMA addresses must be 561released with the corresponding function with the same size for example. With 562the advent of hardware IOMMUs it becomes more and more important that drivers 563do not violate those constraints. In the worst case such a violation can 564result in data corruption up to destroyed filesystems. 565 566To debug drivers and find bugs in the usage of the DMA-API checking code can 567be compiled into the kernel which will tell the developer about those 568violations. If your architecture supports it you can select the "Enable 569debugging of DMA-API usage" option in your kernel configuration. Enabling this 570option has a performance impact. Do not enable it in production kernels. 571 572If you boot the resulting kernel will contain code which does some bookkeeping 573about what DMA memory was allocated for which device. If this code detects an 574error it prints a warning message with some details into your kernel log. An 575example warning message may look like this: 576 577------------[ cut here ]------------ 578WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448 579 check_unmap+0x203/0x490() 580Hardware name: 581forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong 582 function [device address=0x00000000640444be] [size=66 bytes] [mapped as 583single] [unmapped as page] 584Modules linked in: nfsd exportfs bridge stp llc r8169 585Pid: 0, comm: swapper Tainted: G W 2.6.28-dmatest-09289-g8bb99c0 #1 586Call Trace: 587 <IRQ> [<ffffffff80240b22>] warn_slowpath+0xf2/0x130 588 [<ffffffff80647b70>] _spin_unlock+0x10/0x30 589 [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0 590 [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40 591 [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0 592 [<ffffffff80252f96>] queue_work+0x56/0x60 593 [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50 594 [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0 595 [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40 596 [<ffffffff80235177>] find_busiest_group+0x207/0x8a0 597 [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50 598 [<ffffffff803c7ea3>] check_unmap+0x203/0x490 599 [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50 600 [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0 601 [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0 602 [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70 603 [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150 604 [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0 605 [<ffffffff8020c093>] ret_from_intr+0x0/0xa 606 <EOI> <4>---[ end trace f6435a98e2a38c0e ]--- 607 608The driver developer can find the driver and the device including a stacktrace 609of the DMA-API call which caused this warning. 610 611Per default only the first error will result in a warning message. All other 612errors will only silently counted. This limitation exist to prevent the code 613from flooding your kernel log. To support debugging a device driver this can 614be disabled via debugfs. See the debugfs interface documentation below for 615details. 616 617The debugfs directory for the DMA-API debugging code is called dma-api/. In 618this directory the following files can currently be found: 619 620 dma-api/all_errors This file contains a numeric value. If this 621 value is not equal to zero the debugging code 622 will print a warning for every error it finds 623 into the kernel log. Be careful with this 624 option, as it can easily flood your logs. 625 626 dma-api/disabled This read-only file contains the character 'Y' 627 if the debugging code is disabled. This can 628 happen when it runs out of memory or if it was 629 disabled at boot time 630 631 dma-api/error_count This file is read-only and shows the total 632 numbers of errors found. 633 634 dma-api/num_errors The number in this file shows how many 635 warnings will be printed to the kernel log 636 before it stops. This number is initialized to 637 one at system boot and be set by writing into 638 this file 639 640 dma-api/min_free_entries 641 This read-only file can be read to get the 642 minimum number of free dma_debug_entries the 643 allocator has ever seen. If this value goes 644 down to zero the code will disable itself 645 because it is not longer reliable. 646 647 dma-api/num_free_entries 648 The current number of free dma_debug_entries 649 in the allocator. 650 651 dma-api/driver-filter 652 You can write a name of a driver into this file 653 to limit the debug output to requests from that 654 particular driver. Write an empty string to 655 that file to disable the filter and see 656 all errors again. 657 658If you have this code compiled into your kernel it will be enabled by default. 659If you want to boot without the bookkeeping anyway you can provide 660'dma_debug=off' as a boot parameter. This will disable DMA-API debugging. 661Notice that you can not enable it again at runtime. You have to reboot to do 662so. 663 664If you want to see debug messages only for a special device driver you can 665specify the dma_debug_driver=<drivername> parameter. This will enable the 666driver filter at boot time. The debug code will only print errors for that 667driver afterwards. This filter can be disabled or changed later using debugfs. 668 669When the code disables itself at runtime this is most likely because it ran 670out of dma_debug_entries. These entries are preallocated at boot. The number 671of preallocated entries is defined per architecture. If it is too low for you 672boot with 'dma_debug_entries=<your_desired_number>' to overwrite the 673architectural default. 674