Dynamic DMA mapping Guide
  		     =========================

  
  		 David S. Miller <davem@redhat.com>
  		 Richard Henderson <rth@cygnus.com>
  		  Jakub Jelinek <jakub@redhat.com>

  This is a guide to device driver writers on how to use the DMA API
  with example pseudo-code.  For a concise description of the API, see

  DMA-API.txt.
  
  Most of the 64bit platforms have special hardware that translates bus
  addresses (DMA addresses) into physical addresses.  This is similar to
  how page tables and/or a TLB translates virtual addresses to physical
  addresses on a CPU.  This is needed so that e.g. PCI devices can
  access with a Single Address Cycle (32bit DMA address) any page in the
  64bit physical address space.  Previously in Linux those 64bit
  platforms had to set artificial limits on the maximum RAM size in the
  system, so that the virt_to_bus() static scheme works (the DMA address
  translation tables were simply filled on bootup to map each bus
  address to the physical page __pa(bus_to_virt())).
  
  So that Linux can use the dynamic DMA mapping, it needs some help from the
  drivers, namely it has to take into account that DMA addresses should be
  mapped only for the time they are actually used and unmapped after the DMA
  transfer.
  
  The following API will work of course even on platforms where no such

  hardware exists.
  
  Note that the DMA API works with any bus independent of the underlying
  microprocessor architecture. You should use the DMA API rather than
  the bus specific DMA API (e.g. pci_dma_*).

  
  First of all, you should make sure

  #include <linux/dma-mapping.h>

  
  is in your driver. This file will obtain for you the definition of the
  dma_addr_t (which can hold any valid DMA address for the platform)
  type which should be used everywhere you hold a DMA (bus) address
  returned from the DMA mapping functions.
  
  			 What memory is DMA'able?
  
  The first piece of information you must know is what kernel memory can
  be used with the DMA mapping facilities.  There has been an unwritten
  set of rules regarding this, and this text is an attempt to finally
  write them down.
  
  If you acquired your memory via the page allocator
  (i.e. __get_free_page*()) or the generic memory allocators
  (i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
  that memory using the addresses returned from those routines.
  
  This means specifically that you may _not_ use the memory/addresses
  returned from vmalloc() for DMA.  It is possible to DMA to the
  _underlying_ memory mapped into a vmalloc() area, but this requires
  walking page tables to get the physical addresses, and then
  translating each of those pages back to a kernel address using
  something like __va().  [ EDIT: Update this when we integrate
  Gerd Knorr's generic code which does this. ]

  This rule also means that you may use neither kernel image addresses
  (items in data/text/bss segments), nor module image addresses, nor
  stack addresses for DMA.  These could all be mapped somewhere entirely
  different than the rest of physical memory.  Even if those classes of
  memory could physically work with DMA, you'd need to ensure the I/O
  buffers were cacheline-aligned.  Without that, you'd see cacheline
  sharing problems (data corruption) on CPUs with DMA-incoherent caches.
  (The CPU could write to one word, DMA would write to a different one
  in the same cache line, and one of them could be overwritten.)

  
  Also, this means that you cannot take the return of a kmap()
  call and DMA to/from that.  This is similar to vmalloc().
  
  What about block I/O and networking buffers?  The block I/O and
  networking subsystems make sure that the buffers they use are valid
  for you to DMA from/to.
  
  			DMA addressing limitations
  
  Does your device have any DMA addressing limitations?  For example, is

  your device only capable of driving the low order 24-bits of address?
  If so, you need to inform the kernel of this fact.

  
  By default, the kernel assumes that your device can address the full

  32-bits.  For a 64-bit capable device, this needs to be increased.
  And for a device with limitations, as discussed in the previous
  paragraph, it needs to be decreased.
  
  Special note about PCI: PCI-X specification requires PCI-X devices to
  support 64-bit addressing (DAC) for all transactions.  And at least
  one platform (SGI SN2) requires 64-bit consistent allocations to
  operate correctly when the IO bus is in PCI-X mode.
  
  For correct operation, you must interrogate the kernel in your device
  probe routine to see if the DMA controller on the machine can properly
  support the DMA addressing limitation your device has.  It is good
  style to do this even if your device holds the default setting,

  because this shows that you did think about these issues wrt. your
  device.

  The query is performed via a call to dma_set_mask():

  	int dma_set_mask(struct device *dev, u64 mask);

  The query for consistent allocations is performed via a call to

  dma_set_coherent_mask():

  	int dma_set_coherent_mask(struct device *dev, u64 mask);

  Here, dev is a pointer to the device struct of your device, and mask
  is a bit mask describing which bits of an address your device
  supports.  It returns zero if your card can perform DMA properly on
  the machine given the address mask you provided.  In general, the
  device struct of your device is embedded in the bus specific device
  struct of your device.  For example, a pointer to the device struct of
  your PCI device is pdev->dev (pdev is a pointer to the PCI device
  struct of your device).

  If it returns non-zero, your device cannot perform DMA properly on

  this platform, and attempting to do so will result in undefined
  behavior.  You must either use a different mask, or not use DMA.
  
  This means that in the failure case, you have three options:
  
  1) Use another DMA mask, if possible (see below).
  2) Use some non-DMA mode for data transfer, if possible.
  3) Ignore this device and do not initialize it.
  
  It is recommended that your driver print a kernel KERN_WARNING message
  when you end up performing either #2 or #3.  In this manner, if a user
  of your driver reports that performance is bad or that the device is not
  even detected, you can ask them for the kernel messages to find out
  exactly why.

  The standard 32-bit addressing device would do something like this:

  	if (dma_set_mask(dev, DMA_BIT_MASK(32))) {

  		printk(KERN_WARNING
  		       "mydev: No suitable DMA available.
  ");
  		goto ignore_this_device;
  	}

  Another common scenario is a 64-bit capable device.  The approach here
  is to try for 64-bit addressing, but back down to a 32-bit mask that
  should not fail.  The kernel may fail the 64-bit mask not because the
  platform is not capable of 64-bit addressing.  Rather, it may fail in
  this case simply because 32-bit addressing is done more efficiently
  than 64-bit addressing.  For example, Sparc64 PCI SAC addressing is
  more efficient than DAC addressing.

  
  Here is how you would handle a 64-bit capable device which can drive
  all 64-bits when accessing streaming DMA:
  
  	int using_dac;

  	if (!dma_set_mask(dev, DMA_BIT_MASK(64))) {

  		using_dac = 1;

  	} else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {

  		using_dac = 0;
  	} else {
  		printk(KERN_WARNING
  		       "mydev: No suitable DMA available.
  ");
  		goto ignore_this_device;
  	}
  
  If a card is capable of using 64-bit consistent allocations as well,
  the case would look like this:
  
  	int using_dac, consistent_using_dac;

  	if (!dma_set_mask(dev, DMA_BIT_MASK(64))) {

  		using_dac = 1;
  	   	consistent_using_dac = 1;

  		dma_set_coherent_mask(dev, DMA_BIT_MASK(64));
  	} else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {

  		using_dac = 0;
  		consistent_using_dac = 0;

  		dma_set_coherent_mask(dev, DMA_BIT_MASK(32));

  	} else {
  		printk(KERN_WARNING
  		       "mydev: No suitable DMA available.
  ");
  		goto ignore_this_device;
  	}

  dma_set_coherent_mask() will always be able to set the same or a
  smaller mask as dma_set_mask(). However for the rare case that a

  device driver only uses consistent allocations, one would have to

  check the return value from dma_set_coherent_mask().

  Finally, if your device can only drive the low 24-bits of

  address you might do something like:

  	if (dma_set_mask(dev, DMA_BIT_MASK(24))) {

  		printk(KERN_WARNING
  		       "mydev: 24-bit DMA addressing not available.
  ");
  		goto ignore_this_device;
  	}

  When dma_set_mask() is successful, and returns zero, the kernel saves
  away this mask you have provided.  The kernel will use this

  information later when you make DMA mappings.
  
  There is a case which we are aware of at this time, which is worth
  mentioning in this documentation.  If your device supports multiple
  functions (for example a sound card provides playback and record
  functions) and the various different functions have _different_
  DMA addressing limitations, you may wish to probe each mask and
  only provide the functionality which the machine can handle.  It

  is important that the last call to dma_set_mask() be for the

  most specific mask.
  
  Here is pseudo-code showing how this might be done:

  	#define PLAYBACK_ADDRESS_BITS	DMA_BIT_MASK(32)

  	#define RECORD_ADDRESS_BITS	DMA_BIT_MASK(24)

  
  	struct my_sound_card *card;

  	struct device *dev;

  
  	...

  	if (!dma_set_mask(dev, PLAYBACK_ADDRESS_BITS)) {

  		card->playback_enabled = 1;
  	} else {
  		card->playback_enabled = 0;

  		printk(KERN_WARNING "%s: Playback disabled due to DMA limitations.
  ",

  		       card->name);
  	}

  	if (!dma_set_mask(dev, RECORD_ADDRESS_BITS)) {

  		card->record_enabled = 1;
  	} else {
  		card->record_enabled = 0;

  		printk(KERN_WARNING "%s: Record disabled due to DMA limitations.
  ",

  		       card->name);
  	}
  
  A sound card was used as an example here because this genre of PCI
  devices seems to be littered with ISA chips given a PCI front end,
  and thus retaining the 16MB DMA addressing limitations of ISA.
  
  			Types of DMA mappings
  
  There are two types of DMA mappings:
  
  - Consistent DMA mappings which are usually mapped at driver
    initialization, unmapped at the end and for which the hardware should
    guarantee that the device and the CPU can access the data
    in parallel and will see updates made by each other without any
    explicit software flushing.
  
    Think of "consistent" as "synchronous" or "coherent".
  
    The current default is to return consistent memory in the low 32

    bits of the bus space.  However, for future compatibility you should
    set the consistent mask even if this default is fine for your

    driver.
  
    Good examples of what to use consistent mappings for are:
  
  	- Network card DMA ring descriptors.
  	- SCSI adapter mailbox command data structures.
  	- Device firmware microcode executed out of
  	  main memory.
  
    The invariant these examples all require is that any CPU store
    to memory is immediately visible to the device, and vice
    versa.  Consistent mappings guarantee this.
  
    IMPORTANT: Consistent DMA memory does not preclude the usage of
               proper memory barriers.  The CPU may reorder stores to
  	     consistent memory just as it may normal memory.  Example:
  	     if it is important for the device to see the first word
  	     of a descriptor updated before the second, you must do
  	     something like:
  
  		desc->word0 = address;
  		wmb();
  		desc->word1 = DESC_VALID;
  
               in order to get correct behavior on all platforms.

  	     Also, on some platforms your driver may need to flush CPU write
  	     buffers in much the same way as it needs to flush write buffers
  	     found in PCI bridges (such as by reading a register's value
  	     after writing it).

  - Streaming DMA mappings which are usually mapped for one DMA
    transfer, unmapped right after it (unless you use dma_sync_* below)
    and for which hardware can optimize for sequential accesses.

  
    This of "streaming" as "asynchronous" or "outside the coherency
    domain".
  
    Good examples of what to use streaming mappings for are:
  
  	- Networking buffers transmitted/received by a device.
  	- Filesystem buffers written/read by a SCSI device.
  
    The interfaces for using this type of mapping were designed in
    such a way that an implementation can make whatever performance
    optimizations the hardware allows.  To this end, when using
    such mappings you must be explicit about what you want to happen.

  Neither type of DMA mapping has alignment restrictions that come from
  the underlying bus, although some devices may have such restrictions.

  Also, systems with caches that aren't DMA-coherent will work better
  when the underlying buffers don't share cache lines with other data.

  
  		 Using Consistent DMA mappings.
  
  To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
  you should do:
  
  	dma_addr_t dma_handle;

  	cpu_addr = dma_alloc_coherent(dev, size, &dma_handle, gfp);

  where device is a struct device *. This may be called in interrupt
  context with the GFP_ATOMIC flag.

  
  Size is the length of the region you want to allocate, in bytes.
  
  This routine will allocate RAM for that region, so it acts similarly to
  __get_free_pages (but takes size instead of a page order).  If your
  driver needs regions sized smaller than a page, you may prefer using

  the dma_pool interface, described below.
  
  The consistent DMA mapping interfaces, for non-NULL dev, will by
  default return a DMA address which is 32-bit addressable.  Even if the
  device indicates (via DMA mask) that it may address the upper 32-bits,
  consistent allocation will only return > 32-bit addresses for DMA if
  the consistent DMA mask has been explicitly changed via
  dma_set_coherent_mask().  This is true of the dma_pool interface as
  well.
  
  dma_alloc_coherent returns two values: the virtual address which you

  can use to access it from the CPU and dma_handle which you pass to the
  card.
  
  The cpu return address and the DMA bus master address are both
  guaranteed to be aligned to the smallest PAGE_SIZE order which
  is greater than or equal to the requested size.  This invariant
  exists (for example) to guarantee that if you allocate a chunk
  which is smaller than or equal to 64 kilobytes, the extent of the
  buffer you receive will not cross a 64K boundary.
  
  To unmap and free such a DMA region, you call:

  	dma_free_coherent(dev, size, cpu_addr, dma_handle);

  where dev, size are the same as in the above call and cpu_addr and
  dma_handle are the values dma_alloc_coherent returned to you.

  This function may not be called in interrupt context.
  
  If your driver needs lots of smaller memory regions, you can write

  custom code to subdivide pages returned by dma_alloc_coherent,
  or you can use the dma_pool API to do that.  A dma_pool is like
  a kmem_cache, but it uses dma_alloc_coherent not __get_free_pages.

  Also, it understands common hardware constraints for alignment,
  like queue heads needing to be aligned on N byte boundaries.

  Create a dma_pool like this:

  	struct dma_pool *pool;

  	pool = dma_pool_create(name, dev, size, align, alloc);

  The "name" is for diagnostics (like a kmem_cache name); dev and size

  are as above.  The device's hardware alignment requirement for this
  type of data is "align" (which is expressed in bytes, and must be a
  power of two).  If your device has no boundary crossing restrictions,
  pass 0 for alloc; passing 4096 says memory allocated from this pool
  must not cross 4KByte boundaries (but at that time it may be better to

  go for dma_alloc_coherent directly instead).

  Allocate memory from a dma pool like this:

  	cpu_addr = dma_pool_alloc(pool, flags, &dma_handle);

  
  flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor

  holding SMP locks), SLAB_ATOMIC otherwise.  Like dma_alloc_coherent,

  this returns two values, cpu_addr and dma_handle.

  Free memory that was allocated from a dma_pool like this:

  	dma_pool_free(pool, cpu_addr, dma_handle);

  where pool is what you passed to dma_pool_alloc, and cpu_addr and
  dma_handle are the values dma_pool_alloc returned. This function

  may be called in interrupt context.

  Destroy a dma_pool by calling:

  	dma_pool_destroy(pool);

  Make sure you've called dma_pool_free for all memory allocated

  from a pool before you destroy the pool. This function may not
  be called in interrupt context.
  
  			DMA Direction
  
  The interfaces described in subsequent portions of this document
  take a DMA direction argument, which is an integer and takes on
  one of the following values:

   DMA_BIDIRECTIONAL
   DMA_TO_DEVICE
   DMA_FROM_DEVICE
   DMA_NONE

  
  One should provide the exact DMA direction if you know it.

  DMA_TO_DEVICE means "from main memory to the device"
  DMA_FROM_DEVICE means "from the device to main memory"

  It is the direction in which the data moves during the DMA
  transfer.
  
  You are _strongly_ encouraged to specify this as precisely
  as you possibly can.
  
  If you absolutely cannot know the direction of the DMA transfer,

  specify DMA_BIDIRECTIONAL.  It means that the DMA can go in

  either direction.  The platform guarantees that you may legally
  specify this, and that it will work, but this may be at the
  cost of performance for example.

  The value DMA_NONE is to be used for debugging.  One can

  hold this in a data structure before you come to know the
  precise direction, and this will help catch cases where your
  direction tracking logic has failed to set things up properly.
  
  Another advantage of specifying this value precisely (outside of
  potential platform-specific optimizations of such) is for debugging.
  Some platforms actually have a write permission boolean which DMA
  mappings can be marked with, much like page protections in the user
  program address space.  Such platforms can and do report errors in the

  kernel logs when the DMA controller hardware detects violation of the

  permission setting.
  
  Only streaming mappings specify a direction, consistent mappings
  implicitly have a direction attribute setting of

  DMA_BIDIRECTIONAL.

  The SCSI subsystem tells you the direction to use in the
  'sc_data_direction' member of the SCSI command your driver is
  working on.

  
  For Networking drivers, it's a rather simple affair.  For transmit

  packets, map/unmap them with the DMA_TO_DEVICE direction

  specifier.  For receive packets, just the opposite, map/unmap them

  with the DMA_FROM_DEVICE direction specifier.

  
  		  Using Streaming DMA mappings
  
  The streaming DMA mapping routines can be called from interrupt
  context.  There are two versions of each map/unmap, one which will
  map/unmap a single memory region, and one which will map/unmap a
  scatterlist.
  
  To map a single region, you do:

  	struct device *dev = &my_dev->dev;

  	dma_addr_t dma_handle;
  	void *addr = buffer->ptr;
  	size_t size = buffer->len;

  	dma_handle = dma_map_single(dev, addr, size, direction);

  
  and to unmap it:

  	dma_unmap_single(dev, dma_handle, size, direction);

  You should call dma_unmap_single when the DMA activity is finished, e.g.

  from the interrupt which told you that the DMA transfer is done.
  
  Using cpu pointers like this for single mappings has a disadvantage,
  you cannot reference HIGHMEM memory in this way.  Thus, there is a

  map/unmap interface pair akin to dma_{map,unmap}_single.  These

  interfaces deal with page/offset pairs instead of cpu pointers.
  Specifically:

  	struct device *dev = &my_dev->dev;

  	dma_addr_t dma_handle;
  	struct page *page = buffer->page;
  	unsigned long offset = buffer->offset;
  	size_t size = buffer->len;

  	dma_handle = dma_map_page(dev, page, offset, size, direction);

  
  	...

  	dma_unmap_page(dev, dma_handle, size, direction);

  
  Here, "offset" means byte offset within the given page.
  
  With scatterlists, you map a region gathered from several regions by:

  	int i, count = dma_map_sg(dev, sglist, nents, direction);

  	struct scatterlist *sg;

  	for_each_sg(sglist, sg, count, i) {

  		hw_address[i] = sg_dma_address(sg);
  		hw_len[i] = sg_dma_len(sg);
  	}
  
  where nents is the number of entries in the sglist.
  
  The implementation is free to merge several consecutive sglist entries
  into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
  consecutive sglist entries can be merged into one provided the first one
  ends and the second one starts on a page boundary - in fact this is a huge
  advantage for cards which either cannot do scatter-gather or have very
  limited number of scatter-gather entries) and returns the actual number
  of sg entries it mapped them to. On failure 0 is returned.
  
  Then you should loop count times (note: this can be less than nents times)
  and use sg_dma_address() and sg_dma_len() macros where you previously
  accessed sg->address and sg->length as shown above.
  
  To unmap a scatterlist, just call:

  	dma_unmap_sg(dev, sglist, nents, direction);

  
  Again, make sure DMA activity has already finished.

  PLEASE NOTE:  The 'nents' argument to the dma_unmap_sg call must be
                the _same_ one you passed into the dma_map_sg call,

  	      it should _NOT_ be the 'count' value _returned_ from the

                dma_map_sg call.

  Every dma_map_{single,sg} call should have its dma_unmap_{single,sg}

  counterpart, because the bus address space is a shared resource (although
  in some ports the mapping is per each BUS so less devices contend for the
  same bus address space) and you could render the machine unusable by eating
  all bus addresses.
  
  If you need to use the same streaming DMA region multiple times and touch
  the data in between the DMA transfers, the buffer needs to be synced
  properly in order for the cpu and device to see the most uptodate and
  correct copy of the DMA buffer.

  So, firstly, just map it with dma_map_{single,sg}, and after each DMA

  transfer call either:

  	dma_sync_single_for_cpu(dev, dma_handle, size, direction);

  
  or:

  	dma_sync_sg_for_cpu(dev, sglist, nents, direction);

  
  as appropriate.
  
  Then, if you wish to let the device get at the DMA area again,
  finish accessing the data with the cpu, and then before actually
  giving the buffer to the hardware call either:

  	dma_sync_single_for_device(dev, dma_handle, size, direction);

  
  or:

  	dma_sync_sg_for_device(dev, sglist, nents, direction);

  
  as appropriate.
  
  After the last DMA transfer call one of the DMA unmap routines

  dma_unmap_{single,sg}. If you don't touch the data from the first dma_map_*
  call till dma_unmap_*, then you don't have to call the dma_sync_*

  routines at all.
  
  Here is pseudo code which shows a situation in which you would need

  to use the dma_sync_*() interfaces.

  
  	my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len)
  	{
  		dma_addr_t mapping;

  		mapping = dma_map_single(cp->dev, buffer, len, DMA_FROM_DEVICE);

  
  		cp->rx_buf = buffer;
  		cp->rx_len = len;
  		cp->rx_dma = mapping;
  
  		give_rx_buf_to_card(cp);
  	}
  
  	...
  
  	my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs)
  	{
  		struct my_card *cp = devid;
  
  		...
  		if (read_card_status(cp) == RX_BUF_TRANSFERRED) {
  			struct my_card_header *hp;
  
  			/* Examine the header to see if we wish
  			 * to accept the data.  But synchronize
  			 * the DMA transfer with the CPU first
  			 * so that we see updated contents.
  			 */

  			dma_sync_single_for_cpu(&cp->dev, cp->rx_dma,
  						cp->rx_len,
  						DMA_FROM_DEVICE);

  
  			/* Now it is safe to examine the buffer. */
  			hp = (struct my_card_header *) cp->rx_buf;
  			if (header_is_ok(hp)) {

  				dma_unmap_single(&cp->dev, cp->rx_dma, cp->rx_len,
  						 DMA_FROM_DEVICE);

  				pass_to_upper_layers(cp->rx_buf);
  				make_and_setup_new_rx_buf(cp);
  			} else {

  				/* CPU should not write to
  				 * DMA_FROM_DEVICE-mapped area,
  				 * so dma_sync_single_for_device() is
  				 * not needed here. It would be required
  				 * for DMA_BIDIRECTIONAL mapping if
  				 * the memory was modified.

  				 */

  				give_rx_buf_to_card(cp);
  			}
  		}
  	}
  
  Drivers converted fully to this interface should not use virt_to_bus any
  longer, nor should they use bus_to_virt. Some drivers have to be changed a
  little bit, because there is no longer an equivalent to bus_to_virt in the
  dynamic DMA mapping scheme - you have to always store the DMA addresses

  returned by the dma_alloc_coherent, dma_pool_alloc, and dma_map_single
  calls (dma_map_sg stores them in the scatterlist itself if the platform

  supports dynamic DMA mapping in hardware) in your driver structures and/or
  in the card registers.

  All drivers should be using these interfaces with no exceptions.  It
  is planned to completely remove virt_to_bus() and bus_to_virt() as

  they are entirely deprecated.  Some ports already do not provide these
  as it is impossible to correctly support them.

  			Handling Errors
  
  DMA address space is limited on some architectures and an allocation
  failure can be determined by:
  
  - checking if dma_alloc_coherent returns NULL or dma_map_sg returns 0
  
  - checking the returned dma_addr_t of dma_map_single and dma_map_page
    by using dma_mapping_error():
  
  	dma_addr_t dma_handle;
  
  	dma_handle = dma_map_single(dev, addr, size, direction);
  	if (dma_mapping_error(dev, dma_handle)) {
  		/*
  		 * reduce current DMA mapping usage,
  		 * delay and try again later or
  		 * reset driver.
  		 */
  	}
  
  Networking drivers must call dev_kfree_skb to free the socket buffer
  and return NETDEV_TX_OK if the DMA mapping fails on the transmit hook
  (ndo_start_xmit). This means that the socket buffer is just dropped in
  the failure case.
  
  SCSI drivers must return SCSI_MLQUEUE_HOST_BUSY if the DMA mapping
  fails in the queuecommand hook. This means that the SCSI subsystem
  passes the command to the driver again later.

  		Optimizing Unmap State Space Consumption

  On many platforms, dma_unmap_{single,page}() is simply a nop.

  Therefore, keeping track of the mapping address and length is a waste
  of space.  Instead of filling your drivers up with ifdefs and the like
  to "work around" this (which would defeat the whole purpose of a
  portable API) the following facilities are provided.
  
  Actually, instead of describing the macros one by one, we'll
  transform some example code.

  1) Use DEFINE_DMA_UNMAP_{ADDR,LEN} in state saving structures.

     Example, before:
  
  	struct ring_state {
  		struct sk_buff *skb;
  		dma_addr_t mapping;
  		__u32 len;
  	};
  
     after:
  
  	struct ring_state {
  		struct sk_buff *skb;

  		DEFINE_DMA_UNMAP_ADDR(mapping);
  		DEFINE_DMA_UNMAP_LEN(len);

  	};

  2) Use dma_unmap_{addr,len}_set to set these values.

     Example, before:
  
  	ringp->mapping = FOO;
  	ringp->len = BAR;
  
     after:

  	dma_unmap_addr_set(ringp, mapping, FOO);
  	dma_unmap_len_set(ringp, len, BAR);

  3) Use dma_unmap_{addr,len} to access these values.

     Example, before:

  	dma_unmap_single(dev, ringp->mapping, ringp->len,
  			 DMA_FROM_DEVICE);

  
     after:

  	dma_unmap_single(dev,
  			 dma_unmap_addr(ringp, mapping),
  			 dma_unmap_len(ringp, len),
  			 DMA_FROM_DEVICE);

  
  It really should be self-explanatory.  We treat the ADDR and LEN
  separately, because it is possible for an implementation to only
  need the address in order to perform the unmap operation.
  
  			Platform Issues
  
  If you are just writing drivers for Linux and do not maintain
  an architecture port for the kernel, you can safely skip down
  to "Closing".
  
  1) Struct scatterlist requirements.

     Don't invent the architecture specific struct scatterlist; just use
     <asm-generic/scatterlist.h>. You need to enable
     CONFIG_NEED_SG_DMA_LENGTH if the architecture supports IOMMUs
     (including software IOMMU).

  2) ARCH_DMA_MINALIGN

  
     Architectures must ensure that kmalloc'ed buffer is
     DMA-safe. Drivers and subsystems depend on it. If an architecture
     isn't fully DMA-coherent (i.e. hardware doesn't ensure that data in
     the CPU cache is identical to data in main memory),

     ARCH_DMA_MINALIGN must be set so that the memory allocator

     makes sure that kmalloc'ed buffer doesn't share a cache line with
     the others. See arch/arm/include/asm/cache.h as an example.

     Note that ARCH_DMA_MINALIGN is about DMA memory alignment

     constraints. You don't need to worry about the architecture data
     alignment constraints (e.g. the alignment constraints about 64-bit
     objects).

  3) Supporting multiple types of IOMMUs
  
     If your architecture needs to support multiple types of IOMMUs, you
     can use include/linux/asm-generic/dma-mapping-common.h. It's a
     library to support the DMA API with multiple types of IOMMUs. Lots
     of architectures (x86, powerpc, sh, alpha, ia64, microblaze and
     sparc) use it. Choose one to see how it can be used. If you need to
     support multiple types of IOMMUs in a single system, the example of
     x86 or powerpc helps.

  			   Closing

  This document, and the API itself, would not be in its current

  form without the feedback and suggestions from numerous individuals.
  We would like to specifically mention, in no particular order, the
  following people:
  
  	Russell King <rmk@arm.linux.org.uk>
  	Leo Dagum <dagum@barrel.engr.sgi.com>
  	Ralf Baechle <ralf@oss.sgi.com>
  	Grant Grundler <grundler@cup.hp.com>
  	Jay Estabrook <Jay.Estabrook@compaq.com>
  	Thomas Sailer <sailer@ife.ee.ethz.ch>
  	Andrea Arcangeli <andrea@suse.de>

  	Jens Axboe <jens.axboe@oracle.com>

  	David Mosberger-Tang <davidm@hpl.hp.com>