Adding upstream version 4.19.249.upstream/4.19.249upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+=========================
+Dynamic DMA mapping Guide
+=========================
+
+:Author: David S. Miller <davem@redhat.com>
+:Author: Richard Henderson <rth@cygnus.com>
+:Author: 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.
+
+CPU and DMA addresses
+=====================
+
+There are several kinds of addresses involved in the DMA API, and it's
+important to understand the differences.
+
+The kernel normally uses virtual addresses.  Any address returned by
+kmalloc(), vmalloc(), and similar interfaces is a virtual address and can
+be stored in a ``void *``.
+
+The virtual memory system (TLB, page tables, etc.) translates virtual
+addresses to CPU physical addresses, which are stored as "phys_addr_t" or
+"resource_size_t".  The kernel manages device resources like registers as
+physical addresses.  These are the addresses in /proc/iomem.  The physical
+address is not directly useful to a driver; it must use ioremap() to map
+the space and produce a virtual address.
+
+I/O devices use a third kind of address: a "bus address".  If a device has
+registers at an MMIO address, or if it performs DMA to read or write system
+memory, the addresses used by the device are bus addresses.  In some
+systems, bus addresses are identical to CPU physical addresses, but in
+general they are not.  IOMMUs and host bridges can produce arbitrary
+mappings between physical and bus addresses.
+
+From a device's point of view, DMA uses the bus address space, but it may
+be restricted to a subset of that space.  For example, even if a system
+supports 64-bit addresses for main memory and PCI BARs, it may use an IOMMU
+so devices only need to use 32-bit DMA addresses.
+
+Here's a picture and some examples::
+
+               CPU                  CPU                  Bus
+             Virtual              Physical             Address
+             Address              Address               Space
+              Space                Space
+
+            +-------+             +------+             +------+
+            |       |             |MMIO  |   Offset    |      |
+            |       |  Virtual    |Space |   applied   |      |
+          C +-------+ --------> B +------+ ----------> +------+ A
+            |       |  mapping    |      |   by host   |      |
+  +-----+   |       |             |      |   bridge    |      |   +--------+
+  |     |   |       |             +------+             |      |   |        |
+  | CPU |   |       |             | RAM  |             |      |   | Device |
+  |     |   |       |             |      |             |      |   |        |
+  +-----+   +-------+             +------+             +------+   +--------+
+            |       |  Virtual    |Buffer|   Mapping   |      |
+          X +-------+ --------> Y +------+ <---------- +------+ Z
+            |       |  mapping    | RAM  |   by IOMMU
+            |       |             |      |
+            |       |             |      |
+            +-------+             +------+
+
+During the enumeration process, the kernel learns about I/O devices and
+their MMIO space and the host bridges that connect them to the system.  For
+example, if a PCI device has a BAR, the kernel reads the bus address (A)
+from the BAR and converts it to a CPU physical address (B).  The address B
+is stored in a struct resource and usually exposed via /proc/iomem.  When a
+driver claims a device, it typically uses ioremap() to map physical address
+B at a virtual address (C).  It can then use, e.g., ioread32(C), to access
+the device registers at bus address A.
+
+If the device supports DMA, the driver sets up a buffer using kmalloc() or
+a similar interface, which returns a virtual address (X).  The virtual
+memory system maps X to a physical address (Y) in system RAM.  The driver
+can use virtual address X to access the buffer, but the device itself
+cannot because DMA doesn't go through the CPU virtual memory system.
+
+In some simple systems, the device can do DMA directly to physical address
+Y.  But in many others, there is IOMMU hardware that translates DMA
+addresses to physical addresses, e.g., it translates Z to Y.  This is part
+of the reason for the DMA API: the driver can give a virtual address X to
+an interface like dma_map_single(), which sets up any required IOMMU
+mapping and returns the DMA address Z.  The driver then tells the device to
+do DMA to Z, and the IOMMU maps it to the buffer at address Y in system
+RAM.
+
+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, i.e., use the dma_map_*() interfaces rather than the
+pci_map_*() interfaces.
+
+First of all, you should make sure::
+
+	#include <linux/dma-mapping.h>
+
+is in your driver, which provides the definition of dma_addr_t.  This type
+can hold any valid DMA address for the platform and should be used
+everywhere you hold a DMA 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_and_coherent()::
+
+	int dma_set_mask_and_coherent(struct device *dev, u64 mask);
+
+which will query the mask for both streaming and coherent APIs together.
+If you have some special requirements, then the following two separate
+queries can be used instead:
+
+	The query for streaming mappings 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, &pdev->dev is a pointer to the
+device struct of a PCI device (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_and_coherent(dev, DMA_BIT_MASK(32))) {
+		dev_warn(dev, "mydev: No suitable DMA available\n");
+		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 {
+		dev_warn(dev, "mydev: No suitable DMA available\n");
+		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_and_coherent(dev, DMA_BIT_MASK(64))) {
+		using_dac = 1;
+		consistent_using_dac = 1;
+	} else if (!dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) {
+		using_dac = 0;
+		consistent_using_dac = 0;
+	} else {
+		dev_warn(dev, "mydev: No suitable DMA available\n");
+		goto ignore_this_device;
+	}
+
+The coherent mask will always be able to set the same or a smaller mask as
+the streaming 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))) {
+		dev_warn(dev, "mydev: 24-bit DMA addressing not available\n");
+		goto ignore_this_device;
+	}
+
+When dma_set_mask() or dma_set_mask_and_coherent() 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;
+		dev_warn(dev, "%s: Playback disabled due to DMA limitations\n",
+		       card->name);
+	}
+	if (!dma_set_mask(dev, RECORD_ADDRESS_BITS)) {
+		card->record_enabled = 1;
+	} else {
+		card->record_enabled = 0;
+		dev_warn(dev, "%s: Record disabled due to DMA limitations\n",
+		       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 DMA 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.
+
+  Think 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 virtual address and the DMA 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, boundary);
+
+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 boundary; passing 4096 says memory allocated from this pool
+must not cross 4KByte boundaries (but at that time it may be better to
+use dma_alloc_coherent() directly instead).
+
+Allocate memory from a DMA pool like this::
+
+	cpu_addr = dma_pool_alloc(pool, flags, &dma_handle);
+
+flags are GFP_KERNEL if blocking is permitted (not in_interrupt nor
+holding SMP locks), GFP_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
+
+You 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);
+	if (dma_mapping_error(dev, dma_handle)) {
+		/*
+		 * reduce current DMA mapping usage,
+		 * delay and try again later or
+		 * reset driver.
+		 */
+		goto map_error_handling;
+	}
+
+and to unmap it::
+
+	dma_unmap_single(dev, dma_handle, size, direction);
+
+You should call dma_mapping_error() as dma_map_single() could fail and return
+error.  Doing so will ensure that the mapping code will work correctly on all
+DMA implementations without any dependency on the specifics of the underlying
+implementation. Using the returned address without checking for errors could
+result in failures ranging from panics to silent data corruption.  The same
+applies to dma_map_page() as well.
+
+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);
+	if (dma_mapping_error(dev, dma_handle)) {
+		/*
+		 * reduce current DMA mapping usage,
+		 * delay and try again later or
+		 * reset driver.
+		 */
+		goto map_error_handling;
+	}
+
+	...
+
+	dma_unmap_page(dev, dma_handle, size, direction);
+
+Here, "offset" means byte offset within the given page.
+
+You should call dma_mapping_error() as dma_map_page() could fail and return
+error as outlined under the dma_map_single() discussion.
+
+You should call dma_unmap_page() when the DMA activity is finished, e.g.,
+from the interrupt which told you that the DMA transfer is done.
+
+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.
+
+.. 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 DMA address space is a shared resource and
+you could render the machine unusable by consuming all DMA 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 up-to-date 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.
+
+.. note::
+
+	      The 'nents' argument to dma_sync_sg_for_cpu() and
+	      dma_sync_sg_for_device() must be the same passed to
+	      dma_map_sg(). It is _NOT_ the count returned by
+	      dma_map_sg().
+
+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);
+		if (dma_mapping_error(cp->dev, mapping)) {
+			/*
+			 * reduce current DMA mapping usage,
+			 * delay and try again later or
+			 * reset driver.
+			 */
+			goto map_error_handling;
+		}
+
+		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 dma_addr_t returned from 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.
+		 */
+		goto map_error_handling;
+	}
+
+- unmap pages that are already mapped, when mapping error occurs in the middle
+  of a multiple page mapping attempt. These example are applicable to
+  dma_map_page() as well.
+
+Example 1::
+
+	dma_addr_t dma_handle1;
+	dma_addr_t dma_handle2;
+
+	dma_handle1 = dma_map_single(dev, addr, size, direction);
+	if (dma_mapping_error(dev, dma_handle1)) {
+		/*
+		 * reduce current DMA mapping usage,
+		 * delay and try again later or
+		 * reset driver.
+		 */
+		goto map_error_handling1;
+	}
+	dma_handle2 = dma_map_single(dev, addr, size, direction);
+	if (dma_mapping_error(dev, dma_handle2)) {
+		/*
+		 * reduce current DMA mapping usage,
+		 * delay and try again later or
+		 * reset driver.
+		 */
+		goto map_error_handling2;
+	}
+
+	...
+
+	map_error_handling2:
+		dma_unmap_single(dma_handle1);
+	map_error_handling1:
+
+Example 2::
+
+	/*
+	 * if buffers are allocated in a loop, unmap all mapped buffers when
+	 * mapping error is detected in the middle
+	 */
+
+	dma_addr_t dma_addr;
+	dma_addr_t array[DMA_BUFFERS];
+	int save_index = 0;
+
+	for (i = 0; i < DMA_BUFFERS; i++) {
+
+		...
+
+		dma_addr = dma_map_single(dev, addr, size, direction);
+		if (dma_mapping_error(dev, dma_addr)) {
+			/*
+			 * reduce current DMA mapping usage,
+			 * delay and try again later or
+			 * reset driver.
+			 */
+			goto map_error_handling;
+		}
+		array[i].dma_addr = dma_addr;
+		save_index++;
+	}
+
+	...
+
+	map_error_handling:
+
+	for (i = 0; i < save_index; i++) {
+
+		...
+
+		dma_unmap_single(array[i].dma_addr);
+	}
+
+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.
+
+   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).
+
+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>