1                     Dynamic DMA mapping Guide
   2                     =========================
   4                 David S. Miller <>
   5                 Richard Henderson <>
   6                  Jakub Jelinek <>
   8This is a guide to device driver writers on how to use the DMA API
   9with example pseudo-code.  For a concise description of the API, see
  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())).
  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
  28The following API will work of course even on platforms where no such
  29hardware exists.
  31Note that the DMA API works with any bus independent of the underlying
  32microprocessor architecture. You should use the DMA API rather than
  33the bus specific DMA API (e.g. pci_dma_*).
  35First of all, you should make sure
  37#include <linux/dma-mapping.h>
  39is in your driver. This file will obtain for you the definition of the
  40dma_addr_t (which can hold any valid DMA address for the platform)
  41type which should be used everywhere you hold a DMA (bus) address
  42returned from the DMA mapping functions.
  44                         What memory is DMA'able?
  46The first piece of information you must know is what kernel memory can
  47be used with the DMA mapping facilities.  There has been an unwritten
  48set of rules regarding this, and this text is an attempt to finally
  49write them down.
  51If you acquired your memory via the page allocator
  52(i.e. __get_free_page*()) or the generic memory allocators
  53(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
  54that memory using the addresses returned from those routines.
  56This means specifically that you may _not_ use the memory/addresses
  57returned from vmalloc() for DMA.  It is possible to DMA to the
  58_underlying_ memory mapped into a vmalloc() area, but this requires
  59walking page tables to get the physical addresses, and then
  60translating each of those pages back to a kernel address using
  61something like __va().  [ EDIT: Update this when we integrate
  62Gerd Knorr's generic code which does this. ]
  64This rule also means that you may use neither kernel image addresses
  65(items in data/text/bss segments), nor module image addresses, nor
  66stack addresses for DMA.  These could all be mapped somewhere entirely
  67different than the rest of physical memory.  Even if those classes of
  68memory could physically work with DMA, you'd need to ensure the I/O
  69buffers were cacheline-aligned.  Without that, you'd see cacheline
  70sharing problems (data corruption) on CPUs with DMA-incoherent caches.
  71(The CPU could write to one word, DMA would write to a different one
  72in the same cache line, and one of them could be overwritten.)
  74Also, this means that you cannot take the return of a kmap()
  75call and DMA to/from that.  This is similar to vmalloc().
  77What about block I/O and networking buffers?  The block I/O and
  78networking subsystems make sure that the buffers they use are valid
  79for you to DMA from/to.
  81                        DMA addressing limitations
  83Does your device have any DMA addressing limitations?  For example, is
  84your device only capable of driving the low order 24-bits of address?
  85If so, you need to inform the kernel of this fact.
  87By default, the kernel assumes that your device can address the full
  8832-bits.  For a 64-bit capable device, this needs to be increased.
  89And for a device with limitations, as discussed in the previous
  90paragraph, it needs to be decreased.
  92Special note about PCI: PCI-X specification requires PCI-X devices to
  93support 64-bit addressing (DAC) for all transactions.  And at least
  94one platform (SGI SN2) requires 64-bit consistent allocations to
  95operate correctly when the IO bus is in PCI-X mode.
  97For correct operation, you must interrogate the kernel in your device
  98probe routine to see if the DMA controller on the machine can properly
  99support the DMA addressing limitation your device has.  It is good
 100style to do this even if your device holds the default setting,
 101because this shows that you did think about these issues wrt. your
 104The query is performed via a call to dma_set_mask():
 106        int dma_set_mask(struct device *dev, u64 mask);
 108The query for consistent allocations is performed via a call to
 111        int dma_set_coherent_mask(struct device *dev, u64 mask);
 113Here, dev is a pointer to the device struct of your device, and mask
 114is a bit mask describing which bits of an address your device
 115supports.  It returns zero if your card can perform DMA properly on
 116the machine given the address mask you provided.  In general, the
 117device struct of your device is embedded in the bus specific device
 118struct of your device.  For example, a pointer to the device struct of
 119your PCI device is pdev->dev (pdev is a pointer to the PCI device
 120struct of your device).
 122If it returns non-zero, your device cannot perform DMA properly on
 123this platform, and attempting to do so will result in undefined
 124behavior.  You must either use a different mask, or not use DMA.
 126This means that in the failure case, you have three options:
 1281) Use another DMA mask, if possible (see below).
 1292) Use some non-DMA mode for data transfer, if possible.
 1303) Ignore this device and do not initialize it.
 132It is recommended that your driver print a kernel KERN_WARNING message
 133when you end up performing either #2 or #3.  In this manner, if a user
 134of your driver reports that performance is bad or that the device is not
 135even detected, you can ask them for the kernel messages to find out
 136exactly why.
 138The standard 32-bit addressing device would do something like this:
 140        if (dma_set_mask(dev, DMA_BIT_MASK(32))) {
 141                printk(KERN_WARNING
 142                       "mydev: No suitable DMA available.\n");
 143                goto ignore_this_device;
 144        }
 146Another common scenario is a 64-bit capable device.  The approach here
 147is to try for 64-bit addressing, but back down to a 32-bit mask that
 148should not fail.  The kernel may fail the 64-bit mask not because the
 149platform is not capable of 64-bit addressing.  Rather, it may fail in
 150this case simply because 32-bit addressing is done more efficiently
 151than 64-bit addressing.  For example, Sparc64 PCI SAC addressing is
 152more efficient than DAC addressing.
 154Here is how you would handle a 64-bit capable device which can drive
 155all 64-bits when accessing streaming DMA:
 157        int using_dac;
 159        if (!dma_set_mask(dev, DMA_BIT_MASK(64))) {
 160                using_dac = 1;
 161        } else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {
 162                using_dac = 0;
 163        } else {
 164                printk(KERN_WARNING
 165                       "mydev: No suitable DMA available.\n");
 166                goto ignore_this_device;
 167        }
 169If a card is capable of using 64-bit consistent allocations as well,
 170the case would look like this:
 172        int using_dac, consistent_using_dac;
 174        if (!dma_set_mask(dev, DMA_BIT_MASK(64))) {
 175                using_dac = 1;
 176                consistent_using_dac = 1;
 177                dma_set_coherent_mask(dev, DMA_BIT_MASK(64));
 178        } else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {
 179                using_dac = 0;
 180                consistent_using_dac = 0;
 181                dma_set_coherent_mask(dev, DMA_BIT_MASK(32));
 182        } else {
 183                printk(KERN_WARNING
 184                       "mydev: No suitable DMA available.\n");
 185                goto ignore_this_device;
 186        }
 188dma_set_coherent_mask() will always be able to set the same or a
 189smaller mask as dma_set_mask(). However for the rare case that a
 190device driver only uses consistent allocations, one would have to
 191check the return value from dma_set_coherent_mask().
 193Finally, if your device can only drive the low 24-bits of
 194address you might do something like:
 196        if (dma_set_mask(dev, DMA_BIT_MASK(24))) {
 197                printk(KERN_WARNING
 198                       "mydev: 24-bit DMA addressing not available.\n");
 199                goto ignore_this_device;
 200        }
 202When dma_set_mask() is successful, and returns zero, the kernel saves
 203away this mask you have provided.  The kernel will use this
 204information later when you make DMA mappings.
 206There is a case which we are aware of at this time, which is worth
 207mentioning in this documentation.  If your device supports multiple
 208functions (for example a sound card provides playback and record
 209functions) and the various different functions have _different_
 210DMA addressing limitations, you may wish to probe each mask and
 211only provide the functionality which the machine can handle.  It
 212is important that the last call to dma_set_mask() be for the
 213most specific mask.
 215Here is pseudo-code showing how this might be done:
 217        #define PLAYBACK_ADDRESS_BITS   DMA_BIT_MASK(32)
 218        #define RECORD_ADDRESS_BITS     DMA_BIT_MASK(24)
 220        struct my_sound_card *card;
 221        struct device *dev;
 223        ...
 224        if (!dma_set_mask(dev, PLAYBACK_ADDRESS_BITS)) {
 225                card->playback_enabled = 1;
 226        } else {
 227                card->playback_enabled = 0;
 228                printk(KERN_WARNING "%s: Playback disabled due to DMA limitations.\n",
 229                       card->name);
 230        }
 231        if (!dma_set_mask(dev, RECORD_ADDRESS_BITS)) {
 232                card->record_enabled = 1;
 233        } else {
 234                card->record_enabled = 0;
 235                printk(KERN_WARNING "%s: Record disabled due to DMA limitations.\n",
 236                       card->name);
 237        }
 239A sound card was used as an example here because this genre of PCI
 240devices seems to be littered with ISA chips given a PCI front end,
 241and thus retaining the 16MB DMA addressing limitations of ISA.
 243                        Types of DMA mappings
 245There are two types of DMA mappings:
 247- Consistent DMA mappings which are usually mapped at driver
 248  initialization, unmapped at the end and for which the hardware should
 249  guarantee that the device and the CPU can access the data
 250  in parallel and will see updates made by each other without any
 251  explicit software flushing.
 253  Think of "consistent" as "synchronous" or "coherent".
 255  The current default is to return consistent memory in the low 32
 256  bits of the bus space.  However, for future compatibility you should
 257  set the consistent mask even if this default is fine for your
 258  driver.
 260  Good examples of what to use consistent mappings for are:
 262        - Network card DMA ring descriptors.
 263        - SCSI adapter mailbox command data structures.
 264        - Device firmware microcode executed out of
 265          main memory.
 267  The invariant these examples all require is that any CPU store
 268  to memory is immediately visible to the device, and vice
 269  versa.  Consistent mappings guarantee this.
 271  IMPORTANT: Consistent DMA memory does not preclude the usage of
 272             proper memory barriers.  The CPU may reorder stores to
 273             consistent memory just as it may normal memory.  Example:
 274             if it is important for the device to see the first word
 275             of a descriptor updated before the second, you must do
 276             something like:
 278                desc->word0 = address;
 279                wmb();
 280                desc->word1 = DESC_VALID;
 282             in order to get correct behavior on all platforms.
 284             Also, on some platforms your driver may need to flush CPU write
 285             buffers in much the same way as it needs to flush write buffers
 286             found in PCI bridges (such as by reading a register's value
 287             after writing it).
 289- Streaming DMA mappings which are usually mapped for one DMA
 290  transfer, unmapped right after it (unless you use dma_sync_* below)
 291  and for which hardware can optimize for sequential accesses.
 293  This of "streaming" as "asynchronous" or "outside the coherency
 294  domain".
 296  Good examples of what to use streaming mappings for are:
 298        - Networking buffers transmitted/received by a device.
 299        - Filesystem buffers written/read by a SCSI device.
 301  The interfaces for using this type of mapping were designed in
 302  such a way that an implementation can make whatever performance
 303  optimizations the hardware allows.  To this end, when using
 304  such mappings you must be explicit about what you want to happen.
 306Neither type of DMA mapping has alignment restrictions that come from
 307the underlying bus, although some devices may have such restrictions.
 308Also, systems with caches that aren't DMA-coherent will work better
 309when the underlying buffers don't share cache lines with other data.
 312                 Using Consistent DMA mappings.
 314To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
 315you should do:
 317        dma_addr_t dma_handle;
 319        cpu_addr = dma_alloc_coherent(dev, size, &dma_handle, gfp);
 321where device is a struct device *. This may be called in interrupt
 322context with the GFP_ATOMIC flag.
 324Size is the length of the region you want to allocate, in bytes.
 326This routine will allocate RAM for that region, so it acts similarly to
 327__get_free_pages (but takes size instead of a page order).  If your
 328driver needs regions sized smaller than a page, you may prefer using
 329the dma_pool interface, described below.
 331The consistent DMA mapping interfaces, for non-NULL dev, will by
 332default return a DMA address which is 32-bit addressable.  Even if the
 333device indicates (via DMA mask) that it may address the upper 32-bits,
 334consistent allocation will only return > 32-bit addresses for DMA if
 335the consistent DMA mask has been explicitly changed via
 336dma_set_coherent_mask().  This is true of the dma_pool interface as
 339dma_alloc_coherent returns two values: the virtual address which you
 340can use to access it from the CPU and dma_handle which you pass to the
 343The cpu return address and the DMA bus master address are both
 344guaranteed to be aligned to the smallest PAGE_SIZE order which
 345is greater than or equal to the requested size.  This invariant
 346exists (for example) to guarantee that if you allocate a chunk
 347which is smaller than or equal to 64 kilobytes, the extent of the
 348buffer you receive will not cross a 64K boundary.
 350To unmap and free such a DMA region, you call:
 352        dma_free_coherent(dev, size, cpu_addr, dma_handle);
 354where dev, size are the same as in the above call and cpu_addr and
 355dma_handle are the values dma_alloc_coherent returned to you.
 356This function may not be called in interrupt context.
 358If your driver needs lots of smaller memory regions, you can write
 359custom code to subdivide pages returned by dma_alloc_coherent,
 360or you can use the dma_pool API to do that.  A dma_pool is like
 361a kmem_cache, but it uses dma_alloc_coherent not __get_free_pages.
 362Also, it understands common hardware constraints for alignment,
 363like queue heads needing to be aligned on N byte boundaries.
 365Create a dma_pool like this:
 367        struct dma_pool *pool;
 369        pool = dma_pool_create(name, dev, size, align, alloc);
 371The "name" is for diagnostics (like a kmem_cache name); dev and size
 372are as above.  The device's hardware alignment requirement for this
 373type of data is "align" (which is expressed in bytes, and must be a
 374power of two).  If your device has no boundary crossing restrictions,
 375pass 0 for alloc; passing 4096 says memory allocated from this pool
 376must not cross 4KByte boundaries (but at that time it may be better to
 377go for dma_alloc_coherent directly instead).
 379Allocate memory from a dma pool like this:
 381        cpu_addr = dma_pool_alloc(pool, flags, &dma_handle);
 383flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
 384holding SMP locks), SLAB_ATOMIC otherwise.  Like dma_alloc_coherent,
 385this returns two values, cpu_addr and dma_handle.
 387Free memory that was allocated from a dma_pool like this:
 389        dma_pool_free(pool, cpu_addr, dma_handle);
 391where pool is what you passed to dma_pool_alloc, and cpu_addr and
 392dma_handle are the values dma_pool_alloc returned. This function
 393may be called in interrupt context.
 395Destroy a dma_pool by calling:
 397        dma_pool_destroy(pool);
 399Make sure you've called dma_pool_free for all memory allocated
 400from a pool before you destroy the pool. This function may not
 401be called in interrupt context.
 403                        DMA Direction
 405The interfaces described in subsequent portions of this document
 406take a DMA direction argument, which is an integer and takes on
 407one of the following values:
 414One should provide the exact DMA direction if you know it.
 416DMA_TO_DEVICE means "from main memory to the device"
 417DMA_FROM_DEVICE means "from the device to main memory"
 418It is the direction in which the data moves during the DMA
 421You are _strongly_ encouraged to specify this as precisely
 422as you possibly can.
 424If you absolutely cannot know the direction of the DMA transfer,
 425specify DMA_BIDIRECTIONAL.  It means that the DMA can go in
 426either direction.  The platform guarantees that you may legally
 427specify this, and that it will work, but this may be at the
 428cost of performance for example.
 430The value DMA_NONE is to be used for debugging.  One can
 431hold this in a data structure before you come to know the
 432precise direction, and this will help catch cases where your
 433direction tracking logic has failed to set things up properly.
 435Another advantage of specifying this value precisely (outside of
 436potential platform-specific optimizations of such) is for debugging.
 437Some platforms actually have a write permission boolean which DMA
 438mappings can be marked with, much like page protections in the user
 439program address space.  Such platforms can and do report errors in the
 440kernel logs when the DMA controller hardware detects violation of the
 441permission setting.
 443Only streaming mappings specify a direction, consistent mappings
 444implicitly have a direction attribute setting of
 447The SCSI subsystem tells you the direction to use in the
 448'sc_data_direction' member of the SCSI command your driver is
 449working on.
 451For Networking drivers, it's a rather simple affair.  For transmit
 452packets, map/unmap them with the DMA_TO_DEVICE direction
 453specifier.  For receive packets, just the opposite, map/unmap them
 454with the DMA_FROM_DEVICE direction specifier.
 456                  Using Streaming DMA mappings
 458The streaming DMA mapping routines can be called from interrupt
 459context.  There are two versions of each map/unmap, one which will
 460map/unmap a single memory region, and one which will map/unmap a
 463To map a single region, you do:
 465        struct device *dev = &my_dev->dev;
 466        dma_addr_t dma_handle;
 467        void *addr = buffer->ptr;
 468        size_t size = buffer->len;
 470        dma_handle = dma_map_single(dev, addr, size, direction);
 472and to unmap it:
 474        dma_unmap_single(dev, dma_handle, size, direction);
 476You should call dma_unmap_single when the DMA activity is finished, e.g.
 477from the interrupt which told you that the DMA transfer is done.
 479Using cpu pointers like this for single mappings has a disadvantage,
 480you cannot reference HIGHMEM memory in this way.  Thus, there is a
 481map/unmap interface pair akin to dma_{map,unmap}_single.  These
 482interfaces deal with page/offset pairs instead of cpu pointers.
 485        struct device *dev = &my_dev->dev;
 486        dma_addr_t dma_handle;
 487        struct page *page = buffer->page;
 488        unsigned long offset = buffer->offset;
 489        size_t size = buffer->len;
 491        dma_handle = dma_map_page(dev, page, offset, size, direction);
 493        ...
 495        dma_unmap_page(dev, dma_handle, size, direction);
 497Here, "offset" means byte offset within the given page.
 499With scatterlists, you map a region gathered from several regions by:
 501        int i, count = dma_map_sg(dev, sglist, nents, direction);
 502        struct scatterlist *sg;
 504        for_each_sg(sglist, sg, count, i) {
 505                hw_address[i] = sg_dma_address(sg);
 506                hw_len[i] = sg_dma_len(sg);
 507        }
 509where nents is the number of entries in the sglist.
 511The implementation is free to merge several consecutive sglist entries
 512into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
 513consecutive sglist entries can be merged into one provided the first one
 514ends and the second one starts on a page boundary - in fact this is a huge
 515advantage for cards which either cannot do scatter-gather or have very
 516limited number of scatter-gather entries) and returns the actual number
 517of sg entries it mapped them to. On failure 0 is returned.
 519Then you should loop count times (note: this can be less than nents times)
 520and use sg_dma_address() and sg_dma_len() macros where you previously
 521accessed sg->address and sg->length as shown above.
 523To unmap a scatterlist, just call:
 525        dma_unmap_sg(dev, sglist, nents, direction);
 527Again, make sure DMA activity has already finished.
 529PLEASE NOTE:  The 'nents' argument to the dma_unmap_sg call must be
 530              the _same_ one you passed into the dma_map_sg call,
 531              it should _NOT_ be the 'count' value _returned_ from the
 532              dma_map_sg call.
 534Every dma_map_{single,sg} call should have its dma_unmap_{single,sg}
 535counterpart, because the bus address space is a shared resource (although
 536in some ports the mapping is per each BUS so less devices contend for the
 537same bus address space) and you could render the machine unusable by eating
 538all bus addresses.
 540If you need to use the same streaming DMA region multiple times and touch
 541the data in between the DMA transfers, the buffer needs to be synced
 542properly in order for the cpu and device to see the most uptodate and
 543correct copy of the DMA buffer.
 545So, firstly, just map it with dma_map_{single,sg}, and after each DMA
 546transfer call either:
 548        dma_sync_single_for_cpu(dev, dma_handle, size, direction);
 552        dma_sync_sg_for_cpu(dev, sglist, nents, direction);
 554as appropriate.
 556Then, if you wish to let the device get at the DMA area again,
 557finish accessing the data with the cpu, and then before actually
 558giving the buffer to the hardware call either:
 560        dma_sync_single_for_device(dev, dma_handle, size, direction);
 564        dma_sync_sg_for_device(dev, sglist, nents, direction);
 566as appropriate.
 568After the last DMA transfer call one of the DMA unmap routines
 569dma_unmap_{single,sg}. If you don't touch the data from the first dma_map_*
 570call till dma_unmap_*, then you don't have to call the dma_sync_*
 571routines at all.
 573Here is pseudo code which shows a situation in which you would need
 574to use the dma_sync_*() interfaces.
 576        my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len)
 577        {
 578                dma_addr_t mapping;
 580                mapping = dma_map_single(cp->dev, buffer, len, DMA_FROM_DEVICE);
 582                cp->rx_buf = buffer;
 583                cp->rx_len = len;
 584                cp->rx_dma = mapping;
 586                give_rx_buf_to_card(cp);
 587        }
 589        ...
 591        my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs)
 592        {
 593                struct my_card *cp = devid;
 595                ...
 596                if (read_card_status(cp) == RX_BUF_TRANSFERRED) {
 597                        struct my_card_header *hp;
 599                        /* Examine the header to see if we wish
 600                         * to accept the data.  But synchronize
 601                         * the DMA transfer with the CPU first
 602                         * so that we see updated contents.
 603                         */
 604                        dma_sync_single_for_cpu(&cp->dev, cp->rx_dma,
 605                                                cp->rx_len,
 606                                                DMA_FROM_DEVICE);
 608                        /* Now it is safe to examine the buffer. */
 609                        hp = (struct my_card_header *) cp->rx_buf;
 610                        if (header_is_ok(hp)) {
 611                                dma_unmap_single(&cp->dev, cp->rx_dma, cp->rx_len,
 612                                                 DMA_FROM_DEVICE);
 613                                pass_to_upper_layers(cp->rx_buf);
 614                                make_and_setup_new_rx_buf(cp);
 615                        } else {
 616                                /* Just sync the buffer and give it back
 617                                 * to the card.
 618                                 */
 619                                dma_sync_single_for_device(&cp->dev,
 620                                                           cp->rx_dma,
 621                                                           cp->rx_len,
 622                                                           DMA_FROM_DEVICE);
 623                                give_rx_buf_to_card(cp);
 624                        }
 625                }
 626        }
 628Drivers converted fully to this interface should not use virt_to_bus any
 629longer, nor should they use bus_to_virt. Some drivers have to be changed a
 630little bit, because there is no longer an equivalent to bus_to_virt in the
 631dynamic DMA mapping scheme - you have to always store the DMA addresses
 632returned by the dma_alloc_coherent, dma_pool_alloc, and dma_map_single
 633calls (dma_map_sg stores them in the scatterlist itself if the platform
 634supports dynamic DMA mapping in hardware) in your driver structures and/or
 635in the card registers.
 637All drivers should be using these interfaces with no exceptions.  It
 638is planned to completely remove virt_to_bus() and bus_to_virt() as
 639they are entirely deprecated.  Some ports already do not provide these
 640as it is impossible to correctly support them.
 642                        Handling Errors
 644DMA address space is limited on some architectures and an allocation
 645failure can be determined by:
 647- checking if dma_alloc_coherent returns NULL or dma_map_sg returns 0
 649- checking the returned dma_addr_t of dma_map_single and dma_map_page
 650  by using dma_mapping_error():
 652        dma_addr_t dma_handle;
 654        dma_handle = dma_map_single(dev, addr, size, direction);
 655        if (dma_mapping_error(dev, dma_handle)) {
 656                /*
 657                 * reduce current DMA mapping usage,
 658                 * delay and try again later or
 659                 * reset driver.
 660                 */
 661        }
 663Networking drivers must call dev_kfree_skb to free the socket buffer
 664and return NETDEV_TX_OK if the DMA mapping fails on the transmit hook
 665(ndo_start_xmit). This means that the socket buffer is just dropped in
 666the failure case.
 668SCSI drivers must return SCSI_MLQUEUE_HOST_BUSY if the DMA mapping
 669fails in the queuecommand hook. This means that the SCSI subsystem
 670passes the command to the driver again later.
 672                Optimizing Unmap State Space Consumption
 674On many platforms, dma_unmap_{single,page}() is simply a nop.
 675Therefore, keeping track of the mapping address and length is a waste
 676of space.  Instead of filling your drivers up with ifdefs and the like
 677to "work around" this (which would defeat the whole purpose of a
 678portable API) the following facilities are provided.
 680Actually, instead of describing the macros one by one, we'll
 681transform some example code.
 6831) Use DEFINE_DMA_UNMAP_{ADDR,LEN} in state saving structures.
 684   Example, before:
 686        struct ring_state {
 687                struct sk_buff *skb;
 688                dma_addr_t mapping;
 689                __u32 len;
 690        };
 692   after:
 694        struct ring_state {
 695                struct sk_buff *skb;
 696                DEFINE_DMA_UNMAP_ADDR(mapping);
 697                DEFINE_DMA_UNMAP_LEN(len);
 698        };
 7002) Use dma_unmap_{addr,len}_set to set these values.
 701   Example, before:
 703        ringp->mapping = FOO;
 704        ringp->len = BAR;
 706   after:
 708        dma_unmap_addr_set(ringp, mapping, FOO);
 709        dma_unmap_len_set(ringp, len, BAR);
 7113) Use dma_unmap_{addr,len} to access these values.
 712   Example, before:
 714        dma_unmap_single(dev, ringp->mapping, ringp->len,
 715                         DMA_FROM_DEVICE);
 717   after:
 719        dma_unmap_single(dev,
 720                         dma_unmap_addr(ringp, mapping),
 721                         dma_unmap_len(ringp, len),
 722                         DMA_FROM_DEVICE);
 724It really should be self-explanatory.  We treat the ADDR and LEN
 725separately, because it is possible for an implementation to only
 726need the address in order to perform the unmap operation.
 728                        Platform Issues
 730If you are just writing drivers for Linux and do not maintain
 731an architecture port for the kernel, you can safely skip down
 732to "Closing".
 7341) Struct scatterlist requirements.
 736   Don't invent the architecture specific struct scatterlist; just use
 737   <asm-generic/scatterlist.h>. You need to enable
 738   CONFIG_NEED_SG_DMA_LENGTH if the architecture supports IOMMUs
 739   (including software IOMMU).
 743   Architectures must ensure that kmalloc'ed buffer is
 744   DMA-safe. Drivers and subsystems depend on it. If an architecture
 745   isn't fully DMA-coherent (i.e. hardware doesn't ensure that data in
 746   the CPU cache is identical to data in main memory),
 747   ARCH_DMA_MINALIGN must be set so that the memory allocator
 748   makes sure that kmalloc'ed buffer doesn't share a cache line with
 749   the others. See arch/arm/include/asm/cache.h as an example.
 751   Note that ARCH_DMA_MINALIGN is about DMA memory alignment
 752   constraints. You don't need to worry about the architecture data
 753   alignment constraints (e.g. the alignment constraints about 64-bit
 754   objects).
 7563) Supporting multiple types of IOMMUs
 758   If your architecture needs to support multiple types of IOMMUs, you
 759   can use include/linux/asm-generic/dma-mapping-common.h. It's a
 760   library to support the DMA API with multiple types of IOMMUs. Lots
 761   of architectures (x86, powerpc, sh, alpha, ia64, microblaze and
 762   sparc) use it. Choose one to see how it can be used. If you need to
 763   support multiple types of IOMMUs in a single system, the example of
 764   x86 or powerpc helps.
 766                           Closing
 768This document, and the API itself, would not be in its current
 769form without the feedback and suggestions from numerous individuals.
 770We would like to specifically mention, in no particular order, the
 771following people:
 773        Russell King <>
 774        Leo Dagum <>
 775        Ralf Baechle <>
 776        Grant Grundler <>
 777        Jay Estabrook <>
 778        Thomas Sailer <>
 779        Andrea Arcangeli <>
 780        Jens Axboe <>
 781        David Mosberger-Tang <>
 782 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.