1 Dynamic DMA mapping 2 =================== 3 4 David S. Miller <davem@redhat.com> 5 Richard Henderson <rth@cygnus.com> 6 Jakub Jelinek <jakub@redhat.com> 7 8This document describes the DMA mapping system in terms of the pci_ 9API. For a similar API that works for generic devices, see 10DMA-API.txt. 11 12Most of the 64bit platforms have special hardware that translates bus 13addresses (DMA addresses) into physical addresses. This is similar to 14how page tables and/or a TLB translates virtual addresses to physical 15addresses on a CPU. This is needed so that e.g. PCI devices can 16access with a Single Address Cycle (32bit DMA address) any page in the 1764bit physical address space. Previously in Linux those 64bit 18platforms had to set artificial limits on the maximum RAM size in the 19system, so that the virt_to_bus() static scheme works (the DMA address 20translation tables were simply filled on bootup to map each bus 21address to the physical page __pa(bus_to_virt())). 22 23So that Linux can use the dynamic DMA mapping, it needs some help from the 24drivers, namely it has to take into account that DMA addresses should be 25mapped only for the time they are actually used and unmapped after the DMA 26transfer. 27 28The following API will work of course even on platforms where no such 29hardware exists, see e.g. arch/x86/include/asm/pci.h for how it is implemented on 30top of the virt_to_bus interface. 31 32First of all, you should make sure 33 34#include <linux/pci.h> 35 36is in your driver. This file will obtain for you the definition of the 37dma_addr_t (which can hold any valid DMA address for the platform) 38type which should be used everywhere you hold a DMA (bus) address 39returned from the DMA mapping functions. 40 41 What memory is DMA'able? 42 43The first piece of information you must know is what kernel memory can 44be used with the DMA mapping facilities. There has been an unwritten 45set of rules regarding this, and this text is an attempt to finally 46write them down. 47 48If you acquired your memory via the page allocator 49(i.e. __get_free_page*()) or the generic memory allocators 50(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from 51that memory using the addresses returned from those routines. 52 53This means specifically that you may _not_ use the memory/addresses 54returned from vmalloc() for DMA. It is possible to DMA to the 55_underlying_ memory mapped into a vmalloc() area, but this requires 56walking page tables to get the physical addresses, and then 57translating each of those pages back to a kernel address using 58something like __va(). [ EDIT: Update this when we integrate 59Gerd Knorr's generic code which does this. ] 60 61This rule also means that you may use neither kernel image addresses 62(items in data/text/bss segments), nor module image addresses, nor 63stack addresses for DMA. These could all be mapped somewhere entirely 64different than the rest of physical memory. Even if those classes of 65memory could physically work with DMA, you'd need to ensure the I/O 66buffers were cacheline-aligned. Without that, you'd see cacheline 67sharing problems (data corruption) on CPUs with DMA-incoherent caches. 68(The CPU could write to one word, DMA would write to a different one 69in the same cache line, and one of them could be overwritten.) 70 71Also, this means that you cannot take the return of a kmap() 72call and DMA to/from that. This is similar to vmalloc(). 73 74What about block I/O and networking buffers? The block I/O and 75networking subsystems make sure that the buffers they use are valid 76for you to DMA from/to. 77 78 DMA addressing limitations 79 80Does your device have any DMA addressing limitations? For example, is 81your device only capable of driving the low order 24-bits of address 82on the PCI bus for SAC DMA transfers? If so, you need to inform the 83PCI layer of this fact. 84 85By default, the kernel assumes that your device can address the full 8632-bits in a SAC cycle. For a 64-bit DAC capable device, this needs 87to be increased. And for a device with limitations, as discussed in 88the previous paragraph, it needs to be decreased. 89 90pci_alloc_consistent() by default will return 32-bit DMA addresses. 91PCI-X specification requires PCI-X devices to support 64-bit 92addressing (DAC) for all transactions. And at least one platform (SGI 93SN2) requires 64-bit consistent allocations to operate correctly when 94the IO bus is in PCI-X mode. Therefore, like with pci_set_dma_mask(), 95it's good practice to call pci_set_consistent_dma_mask() to set the 96appropriate mask even if your device only supports 32-bit DMA 97(default) and especially if it's a PCI-X device. 98 99For correct operation, you must interrogate the PCI layer in your 100device probe routine to see if the PCI controller on the machine can 101properly support the DMA addressing limitation your device has. It is 102good style to do this even if your device holds the default setting, 103because this shows that you did think about these issues wrt. your 104device. 105 106The query is performed via a call to pci_set_dma_mask(): 107 108 int pci_set_dma_mask(struct pci_dev *pdev, u64 device_mask); 109 110The query for consistent allocations is performed via a call to 111pci_set_consistent_dma_mask(): 112 113 int pci_set_consistent_dma_mask(struct pci_dev *pdev, u64 device_mask); 114 115Here, pdev is a pointer to the PCI device struct of your device, and 116device_mask is a bit mask describing which bits of a PCI address your 117device supports. It returns zero if your card can perform DMA 118properly on the machine given the address mask you provided. 119 120If it returns non-zero, your device cannot perform DMA properly on 121this platform, and attempting to do so will result in undefined 122behavior. You must either use a different mask, or not use DMA. 123 124This means that in the failure case, you have three options: 125 1261) Use another DMA mask, if possible (see below). 1272) Use some non-DMA mode for data transfer, if possible. 1283) Ignore this device and do not initialize it. 129 130It is recommended that your driver print a kernel KERN_WARNING message 131when you end up performing either #2 or #3. In this manner, if a user 132of your driver reports that performance is bad or that the device is not 133even detected, you can ask them for the kernel messages to find out 134exactly why. 135 136The standard 32-bit addressing PCI device would do something like 137this: 138 139 if (pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) { 140 printk(KERN_WARNING 141 "mydev: No suitable DMA available.\n"); 142 goto ignore_this_device; 143 } 144 145Another common scenario is a 64-bit capable device. The approach 146here is to try for 64-bit DAC addressing, but back down to a 14732-bit mask should that fail. The PCI platform code may fail the 14864-bit mask not because the platform is not capable of 64-bit 149addressing. Rather, it may fail in this case simply because 15032-bit SAC addressing is done more efficiently than DAC addressing. 151Sparc64 is one platform which behaves in this way. 152 153Here is how you would handle a 64-bit capable device which can drive 154all 64-bits when accessing streaming DMA: 155 156 int using_dac; 157 158 if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) { 159 using_dac = 1; 160 } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) { 161 using_dac = 0; 162 } else { 163 printk(KERN_WARNING 164 "mydev: No suitable DMA available.\n"); 165 goto ignore_this_device; 166 } 167 168If a card is capable of using 64-bit consistent allocations as well, 169the case would look like this: 170 171 int using_dac, consistent_using_dac; 172 173 if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) { 174 using_dac = 1; 175 consistent_using_dac = 1; 176 pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(64)); 177 } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) { 178 using_dac = 0; 179 consistent_using_dac = 0; 180 pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(32)); 181 } else { 182 printk(KERN_WARNING 183 "mydev: No suitable DMA available.\n"); 184 goto ignore_this_device; 185 } 186 187pci_set_consistent_dma_mask() will always be able to set the same or a 188smaller mask as pci_set_dma_mask(). However for the rare case that a 189device driver only uses consistent allocations, one would have to 190check the return value from pci_set_consistent_dma_mask(). 191 192Finally, if your device can only drive the low 24-bits of 193address during PCI bus mastering you might do something like: 194 195 if (pci_set_dma_mask(pdev, DMA_BIT_MASK(24))) { 196 printk(KERN_WARNING 197 "mydev: 24-bit DMA addressing not available.\n"); 198 goto ignore_this_device; 199 } 200 201When pci_set_dma_mask() is successful, and returns zero, the PCI layer 202saves away this mask you have provided. The PCI layer will use this 203information later when you make DMA mappings. 204 205There is a case which we are aware of at this time, which is worth 206mentioning in this documentation. If your device supports multiple 207functions (for example a sound card provides playback and record 208functions) and the various different functions have _different_ 209DMA addressing limitations, you may wish to probe each mask and 210only provide the functionality which the machine can handle. It 211is important that the last call to pci_set_dma_mask() be for the 212most specific mask. 213 214Here is pseudo-code showing how this might be done: 215 216 #define PLAYBACK_ADDRESS_BITS DMA_BIT_MASK(32) 217 #define RECORD_ADDRESS_BITS 0x00ffffff 218 219 struct my_sound_card *card; 220 struct pci_dev *pdev; 221 222 ... 223 if (!pci_set_dma_mask(pdev, PLAYBACK_ADDRESS_BITS)) { 224 card->playback_enabled = 1; 225 } else { 226 card->playback_enabled = 0; 227 printk(KERN_WARN "%s: Playback disabled due to DMA limitations.\n", 228 card->name); 229 } 230 if (!pci_set_dma_mask(pdev, RECORD_ADDRESS_BITS)) { 231 card->record_enabled = 1; 232 } else { 233 card->record_enabled = 0; 234 printk(KERN_WARN "%s: Record disabled due to DMA limitations.\n", 235 card->name); 236 } 237 238A sound card was used as an example here because this genre of PCI 239devices seems to be littered with ISA chips given a PCI front end, 240and thus retaining the 16MB DMA addressing limitations of ISA. 241 242 Types of DMA mappings 243 244There are two types of DMA mappings: 245 246- Consistent DMA mappings which are usually mapped at driver 247 initialization, unmapped at the end and for which the hardware should 248 guarantee that the device and the CPU can access the data 249 in parallel and will see updates made by each other without any 250 explicit software flushing. 251 252 Think of "consistent" as "synchronous" or "coherent". 253 254 The current default is to return consistent memory in the low 32 255 bits of the PCI bus space. However, for future compatibility you 256 should set the consistent mask even if this default is fine for your 257 driver. 258 259 Good examples of what to use consistent mappings for are: 260 261 - Network card DMA ring descriptors. 262 - SCSI adapter mailbox command data structures. 263 - Device firmware microcode executed out of 264 main memory. 265 266 The invariant these examples all require is that any CPU store 267 to memory is immediately visible to the device, and vice 268 versa. Consistent mappings guarantee this. 269 270 IMPORTANT: Consistent DMA memory does not preclude the usage of 271 proper memory barriers. The CPU may reorder stores to 272 consistent memory just as it may normal memory. Example: 273 if it is important for the device to see the first word 274 of a descriptor updated before the second, you must do 275 something like: 276 277 desc->word0 = address; 278 wmb(); 279 desc->word1 = DESC_VALID; 280 281 in order to get correct behavior on all platforms. 282 283 Also, on some platforms your driver may need to flush CPU write 284 buffers in much the same way as it needs to flush write buffers 285 found in PCI bridges (such as by reading a register's value 286 after writing it). 287 288- Streaming DMA mappings which are usually mapped for one DMA transfer, 289 unmapped right after it (unless you use pci_dma_sync_* below) and for which 290 hardware can optimize for sequential accesses. 291 292 This of "streaming" as "asynchronous" or "outside the coherency 293 domain". 294 295 Good examples of what to use streaming mappings for are: 296 297 - Networking buffers transmitted/received by a device. 298 - Filesystem buffers written/read by a SCSI device. 299 300 The interfaces for using this type of mapping were designed in 301 such a way that an implementation can make whatever performance 302 optimizations the hardware allows. To this end, when using 303 such mappings you must be explicit about what you want to happen. 304 305Neither type of DMA mapping has alignment restrictions that come 306from PCI, although some devices may have such restrictions. 307Also, systems with caches that aren't DMA-coherent will work better 308when the underlying buffers don't share cache lines with other data. 309 310 311 Using Consistent DMA mappings. 312 313To allocate and map large (PAGE_SIZE or so) consistent DMA regions, 314you should do: 315 316 dma_addr_t dma_handle; 317 318 cpu_addr = pci_alloc_consistent(pdev, size, &dma_handle); 319 320where pdev is a struct pci_dev *. This may be called in interrupt context. 321You should use dma_alloc_coherent (see DMA-API.txt) for buses 322where devices don't have struct pci_dev (like ISA, EISA). 323 324This argument is needed because the DMA translations may be bus 325specific (and often is private to the bus which the device is attached 326to). 327 328Size is the length of the region you want to allocate, in bytes. 329 330This routine will allocate RAM for that region, so it acts similarly to 331__get_free_pages (but takes size instead of a page order). If your 332driver needs regions sized smaller than a page, you may prefer using 333the pci_pool interface, described below. 334 335The consistent DMA mapping interfaces, for non-NULL pdev, will by 336default return a DMA address which is SAC (Single Address Cycle) 337addressable. Even if the device indicates (via PCI dma mask) that it 338may address the upper 32-bits and thus perform DAC cycles, consistent 339allocation will only return > 32-bit PCI addresses for DMA if the 340consistent dma mask has been explicitly changed via 341pci_set_consistent_dma_mask(). This is true of the pci_pool interface 342as well. 343 344pci_alloc_consistent returns two values: the virtual address which you 345can use to access it from the CPU and dma_handle which you pass to the 346card. 347 348The cpu return address and the DMA bus master address are both 349guaranteed to be aligned to the smallest PAGE_SIZE order which 350is greater than or equal to the requested size. This invariant 351exists (for example) to guarantee that if you allocate a chunk 352which is smaller than or equal to 64 kilobytes, the extent of the 353buffer you receive will not cross a 64K boundary. 354 355To unmap and free such a DMA region, you call: 356 357 pci_free_consistent(pdev, size, cpu_addr, dma_handle); 358 359where pdev, size are the same as in the above call and cpu_addr and 360dma_handle are the values pci_alloc_consistent returned to you. 361This function may not be called in interrupt context. 362 363If your driver needs lots of smaller memory regions, you can write 364custom code to subdivide pages returned by pci_alloc_consistent, 365or you can use the pci_pool API to do that. A pci_pool is like 366a kmem_cache, but it uses pci_alloc_consistent not __get_free_pages. 367Also, it understands common hardware constraints for alignment, 368like queue heads needing to be aligned on N byte boundaries. 369 370Create a pci_pool like this: 371 372 struct pci_pool *pool; 373 374 pool = pci_pool_create(name, pdev, size, align, alloc); 375 376The "name" is for diagnostics (like a kmem_cache name); pdev and size 377are as above. The device's hardware alignment requirement for this 378type of data is "align" (which is expressed in bytes, and must be a 379power of two). If your device has no boundary crossing restrictions, 380pass 0 for alloc; passing 4096 says memory allocated from this pool 381must not cross 4KByte boundaries (but at that time it may be better to 382go for pci_alloc_consistent directly instead). 383 384Allocate memory from a pci pool like this: 385 386 cpu_addr = pci_pool_alloc(pool, flags, &dma_handle); 387 388flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor 389holding SMP locks), SLAB_ATOMIC otherwise. Like pci_alloc_consistent, 390this returns two values, cpu_addr and dma_handle. 391 392Free memory that was allocated from a pci_pool like this: 393 394 pci_pool_free(pool, cpu_addr, dma_handle); 395 396where pool is what you passed to pci_pool_alloc, and cpu_addr and 397dma_handle are the values pci_pool_alloc returned. This function 398may be called in interrupt context. 399 400Destroy a pci_pool by calling: 401 402 pci_pool_destroy(pool); 403 404Make sure you've called pci_pool_free for all memory allocated 405from a pool before you destroy the pool. This function may not 406be called in interrupt context. 407 408 DMA Direction 409 410The interfaces described in subsequent portions of this document 411take a DMA direction argument, which is an integer and takes on 412one of the following values: 413 414 PCI_DMA_BIDIRECTIONAL 415 PCI_DMA_TODEVICE 416 PCI_DMA_FROMDEVICE 417 PCI_DMA_NONE 418 419One should provide the exact DMA direction if you know it. 420 421PCI_DMA_TODEVICE means "from main memory to the PCI device" 422PCI_DMA_FROMDEVICE means "from the PCI device to main memory" 423It is the direction in which the data moves during the DMA 424transfer. 425 426You are _strongly_ encouraged to specify this as precisely 427as you possibly can. 428 429If you absolutely cannot know the direction of the DMA transfer, 430specify PCI_DMA_BIDIRECTIONAL. It means that the DMA can go in 431either direction. The platform guarantees that you may legally 432specify this, and that it will work, but this may be at the 433cost of performance for example. 434 435The value PCI_DMA_NONE is to be used for debugging. One can 436hold this in a data structure before you come to know the 437precise direction, and this will help catch cases where your 438direction tracking logic has failed to set things up properly. 439 440Another advantage of specifying this value precisely (outside of 441potential platform-specific optimizations of such) is for debugging. 442Some platforms actually have a write permission boolean which DMA 443mappings can be marked with, much like page protections in the user 444program address space. Such platforms can and do report errors in the 445kernel logs when the PCI controller hardware detects violation of the 446permission setting. 447 448Only streaming mappings specify a direction, consistent mappings 449implicitly have a direction attribute setting of 450PCI_DMA_BIDIRECTIONAL. 451 452The SCSI subsystem tells you the direction to use in the 453'sc_data_direction' member of the SCSI command your driver is 454working on. 455 456For Networking drivers, it's a rather simple affair. For transmit 457packets, map/unmap them with the PCI_DMA_TODEVICE direction 458specifier. For receive packets, just the opposite, map/unmap them 459with the PCI_DMA_FROMDEVICE direction specifier. 460 461 Using Streaming DMA mappings 462 463The streaming DMA mapping routines can be called from interrupt 464context. There are two versions of each map/unmap, one which will 465map/unmap a single memory region, and one which will map/unmap a 466scatterlist. 467 468To map a single region, you do: 469 470 struct pci_dev *pdev = mydev->pdev; 471 dma_addr_t dma_handle; 472 void *addr = buffer->ptr; 473 size_t size = buffer->len; 474 475 dma_handle = pci_map_single(pdev, addr, size, direction); 476 477and to unmap it: 478 479 pci_unmap_single(pdev, dma_handle, size, direction); 480 481You should call pci_unmap_single when the DMA activity is finished, e.g. 482from the interrupt which told you that the DMA transfer is done. 483 484Using cpu pointers like this for single mappings has a disadvantage, 485you cannot reference HIGHMEM memory in this way. Thus, there is a 486map/unmap interface pair akin to pci_{map,unmap}_single. These 487interfaces deal with page/offset pairs instead of cpu pointers. 488Specifically: 489 490 struct pci_dev *pdev = mydev->pdev; 491 dma_addr_t dma_handle; 492 struct page *page = buffer->page; 493 unsigned long offset = buffer->offset; 494 size_t size = buffer->len; 495 496 dma_handle = pci_map_page(pdev, page, offset, size, direction); 497 498 ... 499 500 pci_unmap_page(pdev, dma_handle, size, direction); 501 502Here, "offset" means byte offset within the given page. 503 504With scatterlists, you map a region gathered from several regions by: 505 506 int i, count = pci_map_sg(pdev, sglist, nents, direction); 507 struct scatterlist *sg; 508 509 for_each_sg(sglist, sg, count, i) { 510 hw_address[i] = sg_dma_address(sg); 511 hw_len[i] = sg_dma_len(sg); 512 } 513 514where nents is the number of entries in the sglist. 515 516The implementation is free to merge several consecutive sglist entries 517into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any 518consecutive sglist entries can be merged into one provided the first one 519ends and the second one starts on a page boundary - in fact this is a huge 520advantage for cards which either cannot do scatter-gather or have very 521limited number of scatter-gather entries) and returns the actual number 522of sg entries it mapped them to. On failure 0 is returned. 523 524Then you should loop count times (note: this can be less than nents times) 525and use sg_dma_address() and sg_dma_len() macros where you previously 526accessed sg->address and sg->length as shown above. 527 528To unmap a scatterlist, just call: 529 530 pci_unmap_sg(pdev, sglist, nents, direction); 531 532Again, make sure DMA activity has already finished. 533 534PLEASE NOTE: The 'nents' argument to the pci_unmap_sg call must be 535 the _same_ one you passed into the pci_map_sg call, 536 it should _NOT_ be the 'count' value _returned_ from the 537 pci_map_sg call. 538 539Every pci_map_{single,sg} call should have its pci_unmap_{single,sg} 540counterpart, because the bus address space is a shared resource (although 541in some ports the mapping is per each BUS so less devices contend for the 542same bus address space) and you could render the machine unusable by eating 543all bus addresses. 544 545If you need to use the same streaming DMA region multiple times and touch 546the data in between the DMA transfers, the buffer needs to be synced 547properly in order for the cpu and device to see the most uptodate and 548correct copy of the DMA buffer. 549 550So, firstly, just map it with pci_map_{single,sg}, and after each DMA 551transfer call either: 552 553 pci_dma_sync_single_for_cpu(pdev, dma_handle, size, direction); 554 555or: 556 557 pci_dma_sync_sg_for_cpu(pdev, sglist, nents, direction); 558 559as appropriate. 560 561Then, if you wish to let the device get at the DMA area again, 562finish accessing the data with the cpu, and then before actually 563giving the buffer to the hardware call either: 564 565 pci_dma_sync_single_for_device(pdev, dma_handle, size, direction); 566 567or: 568 569 pci_dma_sync_sg_for_device(dev, sglist, nents, direction); 570 571as appropriate. 572 573After the last DMA transfer call one of the DMA unmap routines 574pci_unmap_{single,sg}. If you don't touch the data from the first pci_map_* 575call till pci_unmap_*, then you don't have to call the pci_dma_sync_* 576routines at all. 577 578Here is pseudo code which shows a situation in which you would need 579to use the pci_dma_sync_*() interfaces. 580 581 my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len) 582 { 583 dma_addr_t mapping; 584 585 mapping = pci_map_single(cp->pdev, buffer, len, PCI_DMA_FROMDEVICE); 586 587 cp->rx_buf = buffer; 588 cp->rx_len = len; 589 cp->rx_dma = mapping; 590 591 give_rx_buf_to_card(cp); 592 } 593 594 ... 595 596 my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs) 597 { 598 struct my_card *cp = devid; 599 600 ... 601 if (read_card_status(cp) == RX_BUF_TRANSFERRED) { 602 struct my_card_header *hp; 603 604 /* Examine the header to see if we wish 605 * to accept the data. But synchronize 606 * the DMA transfer with the CPU first 607 * so that we see updated contents. 608 */ 609 pci_dma_sync_single_for_cpu(cp->pdev, cp->rx_dma, 610 cp->rx_len, 611 PCI_DMA_FROMDEVICE); 612 613 /* Now it is safe to examine the buffer. */ 614 hp = (struct my_card_header *) cp->rx_buf; 615 if (header_is_ok(hp)) { 616 pci_unmap_single(cp->pdev, cp->rx_dma, cp->rx_len, 617 PCI_DMA_FROMDEVICE); 618 pass_to_upper_layers(cp->rx_buf); 619 make_and_setup_new_rx_buf(cp); 620 } else { 621 /* Just sync the buffer and give it back 622 * to the card. 623 */ 624 pci_dma_sync_single_for_device(cp->pdev, 625 cp->rx_dma, 626 cp->rx_len, 627 PCI_DMA_FROMDEVICE); 628 give_rx_buf_to_card(cp); 629 } 630 } 631 } 632 633Drivers converted fully to this interface should not use virt_to_bus any 634longer, nor should they use bus_to_virt. Some drivers have to be changed a 635little bit, because there is no longer an equivalent to bus_to_virt in the 636dynamic DMA mapping scheme - you have to always store the DMA addresses 637returned by the pci_alloc_consistent, pci_pool_alloc, and pci_map_single 638calls (pci_map_sg stores them in the scatterlist itself if the platform 639supports dynamic DMA mapping in hardware) in your driver structures and/or 640in the card registers. 641 642All PCI drivers should be using these interfaces with no exceptions. 643It is planned to completely remove virt_to_bus() and bus_to_virt() as 644they are entirely deprecated. Some ports already do not provide these 645as it is impossible to correctly support them. 646 647 Optimizing Unmap State Space Consumption 648 649On many platforms, pci_unmap_{single,page}() is simply a nop. 650Therefore, keeping track of the mapping address and length is a waste 651of space. Instead of filling your drivers up with ifdefs and the like 652to "work around" this (which would defeat the whole purpose of a 653portable API) the following facilities are provided. 654 655Actually, instead of describing the macros one by one, we'll 656transform some example code. 657 6581) Use DECLARE_PCI_UNMAP_{ADDR,LEN} in state saving structures. 659 Example, before: 660 661 struct ring_state { 662 struct sk_buff *skb; 663 dma_addr_t mapping; 664 __u32 len; 665 }; 666 667 after: 668 669 struct ring_state { 670 struct sk_buff *skb; 671 DECLARE_PCI_UNMAP_ADDR(mapping) 672 DECLARE_PCI_UNMAP_LEN(len) 673 }; 674 675 NOTE: DO NOT put a semicolon at the end of the DECLARE_*() 676 macro. 677 6782) Use pci_unmap_{addr,len}_set to set these values. 679 Example, before: 680 681 ringp->mapping = FOO; 682 ringp->len = BAR; 683 684 after: 685 686 pci_unmap_addr_set(ringp, mapping, FOO); 687 pci_unmap_len_set(ringp, len, BAR); 688 6893) Use pci_unmap_{addr,len} to access these values. 690 Example, before: 691 692 pci_unmap_single(pdev, ringp->mapping, ringp->len, 693 PCI_DMA_FROMDEVICE); 694 695 after: 696 697 pci_unmap_single(pdev, 698 pci_unmap_addr(ringp, mapping), 699 pci_unmap_len(ringp, len), 700 PCI_DMA_FROMDEVICE); 701 702It really should be self-explanatory. We treat the ADDR and LEN 703separately, because it is possible for an implementation to only 704need the address in order to perform the unmap operation. 705 706 Platform Issues 707 708If you are just writing drivers for Linux and do not maintain 709an architecture port for the kernel, you can safely skip down 710to "Closing". 711 7121) Struct scatterlist requirements. 713 714 Struct scatterlist must contain, at a minimum, the following 715 members: 716 717 struct page *page; 718 unsigned int offset; 719 unsigned int length; 720 721 The base address is specified by a "page+offset" pair. 722 723 Previous versions of struct scatterlist contained a "void *address" 724 field that was sometimes used instead of page+offset. As of Linux 725 2.5., page+offset is always used, and the "address" field has been 726 deleted. 727 7282) More to come... 729 730 Handling Errors 731 732DMA address space is limited on some architectures and an allocation 733failure can be determined by: 734 735- checking if pci_alloc_consistent returns NULL or pci_map_sg returns 0 736 737- checking the returned dma_addr_t of pci_map_single and pci_map_page 738 by using pci_dma_mapping_error(): 739 740 dma_addr_t dma_handle; 741 742 dma_handle = pci_map_single(pdev, addr, size, direction); 743 if (pci_dma_mapping_error(pdev, dma_handle)) { 744 /* 745 * reduce current DMA mapping usage, 746 * delay and try again later or 747 * reset driver. 748 */ 749 } 750 751 Closing 752 753This document, and the API itself, would not be in it's current 754form without the feedback and suggestions from numerous individuals. 755We would like to specifically mention, in no particular order, the 756following people: 757 758 Russell King <rmk@arm.linux.org.uk> 759 Leo Dagum <dagum@barrel.engr.sgi.com> 760 Ralf Baechle <ralf@oss.sgi.com> 761 Grant Grundler <grundler@cup.hp.com> 762 Jay Estabrook <Jay.Estabrook@compaq.com> 763 Thomas Sailer <sailer@ife.ee.ethz.ch> 764 Andrea Arcangeli <andrea@suse.de> 765 Jens Axboe <jens.axboe@oracle.com> 766 David Mosberger-Tang <davidm@hpl.hp.com> 767