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Based on kernel version 2.6.34. Page generated on 2010-05-31 16:02 EST.

	               Dynamic DMA mapping using the generic device
	               ============================================
	
	        James E.J. Bottomley <James.Bottomley[AT]HansenPartnership[DOT]com>
	
	This document describes the DMA API.  For a more gentle introduction
	of the API (and actual examples) see
	Documentation/DMA-API-HOWTO.txt.
	
	This API is split into two pieces.  Part I describes the API.  Part II
	describes the extensions to the API for supporting non-consistent
	memory machines.  Unless you know that your driver absolutely has to
	support non-consistent platforms (this is usually only legacy
	platforms) you should only use the API described in part I.
	
	Part I - dma_ API
	-------------------------------------
	
	To get the dma_ API, you must #include <linux/dma-mapping.h>
	
	
	Part Ia - Using large dma-coherent buffers
	------------------------------------------
	
	void *
	dma_alloc_coherent(struct device *dev, size_t size,
				     dma_addr_t *dma_handle, gfp_t flag)
	
	Consistent memory is memory for which a write by either the device or
	the processor can immediately be read by the processor or device
	without having to worry about caching effects.  (You may however need
	to make sure to flush the processor's write buffers before telling
	devices to read that memory.)
	
	This routine allocates a region of <size> bytes of consistent memory.
	It also returns a <dma_handle> which may be cast to an unsigned
	integer the same width as the bus and used as the physical address
	base of the region.
	
	Returns: a pointer to the allocated region (in the processor's virtual
	address space) or NULL if the allocation failed.
	
	Note: consistent memory can be expensive on some platforms, and the
	minimum allocation length may be as big as a page, so you should
	consolidate your requests for consistent memory as much as possible.
	The simplest way to do that is to use the dma_pool calls (see below).
	
	The flag parameter (dma_alloc_coherent only) allows the caller to
	specify the GFP_ flags (see kmalloc) for the allocation (the
	implementation may choose to ignore flags that affect the location of
	the returned memory, like GFP_DMA).
	
	void
	dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
				   dma_addr_t dma_handle)
	
	Free the region of consistent memory you previously allocated.  dev,
	size and dma_handle must all be the same as those passed into the
	consistent allocate.  cpu_addr must be the virtual address returned by
	the consistent allocate.
	
	Note that unlike their sibling allocation calls, these routines
	may only be called with IRQs enabled.
	
	
	Part Ib - Using small dma-coherent buffers
	------------------------------------------
	
	To get this part of the dma_ API, you must #include <linux/dmapool.h>
	
	Many drivers need lots of small dma-coherent memory regions for DMA
	descriptors or I/O buffers.  Rather than allocating in units of a page
	or more using dma_alloc_coherent(), you can use DMA pools.  These work
	much like a struct kmem_cache, except that they use the dma-coherent allocator,
	not __get_free_pages().  Also, they understand common hardware constraints
	for alignment, like queue heads needing to be aligned on N-byte boundaries.
	
	
		struct dma_pool *
		dma_pool_create(const char *name, struct device *dev,
				size_t size, size_t align, size_t alloc);
	
	The pool create() routines initialize a pool of dma-coherent buffers
	for use with a given device.  It must be called in a context which
	can sleep.
	
	The "name" is for diagnostics (like a struct kmem_cache name); dev and size
	are like what you'd pass to dma_alloc_coherent().  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.
	
	
		void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
				dma_addr_t *dma_handle);
	
	This allocates memory from the pool; the returned memory will meet the size
	and alignment requirements specified at creation time.  Pass GFP_ATOMIC to
	prevent blocking, or if it's permitted (not in_interrupt, not holding SMP locks),
	pass GFP_KERNEL to allow blocking.  Like dma_alloc_coherent(), this returns
	two values:  an address usable by the cpu, and the dma address usable by the
	pool's device.
	
	
		void dma_pool_free(struct dma_pool *pool, void *vaddr,
				dma_addr_t addr);
	
	This puts memory back into the pool.  The pool is what was passed to
	the pool allocation routine; the cpu (vaddr) and dma addresses are what
	were returned when that routine allocated the memory being freed.
	
	
		void dma_pool_destroy(struct dma_pool *pool);
	
	The pool destroy() routines free the resources of the pool.  They must be
	called in a context which can sleep.  Make sure you've freed all allocated
	memory back to the pool before you destroy it.
	
	
	Part Ic - DMA addressing limitations
	------------------------------------
	
	int
	dma_supported(struct device *dev, u64 mask)
	
	Checks to see if the device can support DMA to the memory described by
	mask.
	
	Returns: 1 if it can and 0 if it can't.
	
	Notes: This routine merely tests to see if the mask is possible.  It
	won't change the current mask settings.  It is more intended as an
	internal API for use by the platform than an external API for use by
	driver writers.
	
	int
	dma_set_mask(struct device *dev, u64 mask)
	
	Checks to see if the mask is possible and updates the device
	parameters if it is.
	
	Returns: 0 if successful and a negative error if not.
	
	int
	dma_set_coherent_mask(struct device *dev, u64 mask)
	
	Checks to see if the mask is possible and updates the device
	parameters if it is.
	
	Returns: 0 if successful and a negative error if not.
	
	u64
	dma_get_required_mask(struct device *dev)
	
	This API returns the mask that the platform requires to
	operate efficiently.  Usually this means the returned mask
	is the minimum required to cover all of memory.  Examining the
	required mask gives drivers with variable descriptor sizes the
	opportunity to use smaller descriptors as necessary.
	
	Requesting the required mask does not alter the current mask.  If you
	wish to take advantage of it, you should issue a dma_set_mask()
	call to set the mask to the value returned.
	
	
	Part Id - Streaming DMA mappings
	--------------------------------
	
	dma_addr_t
	dma_map_single(struct device *dev, void *cpu_addr, size_t size,
			      enum dma_data_direction direction)
	
	Maps a piece of processor virtual memory so it can be accessed by the
	device and returns the physical handle of the memory.
	
	The direction for both api's may be converted freely by casting.
	However the dma_ API uses a strongly typed enumerator for its
	direction:
	
	DMA_NONE		no direction (used for debugging)
	DMA_TO_DEVICE		data is going from the memory to the device
	DMA_FROM_DEVICE		data is coming from the device to the memory
	DMA_BIDIRECTIONAL	direction isn't known
	
	Notes:  Not all memory regions in a machine can be mapped by this
	API.  Further, regions that appear to be physically contiguous in
	kernel virtual space may not be contiguous as physical memory.  Since
	this API does not provide any scatter/gather capability, it will fail
	if the user tries to map a non-physically contiguous piece of memory.
	For this reason, it is recommended that memory mapped by this API be
	obtained only from sources which guarantee it to be physically contiguous
	(like kmalloc).
	
	Further, the physical address of the memory must be within the
	dma_mask of the device (the dma_mask represents a bit mask of the
	addressable region for the device.  I.e., if the physical address of
	the memory anded with the dma_mask is still equal to the physical
	address, then the device can perform DMA to the memory).  In order to
	ensure that the memory allocated by kmalloc is within the dma_mask,
	the driver may specify various platform-dependent flags to restrict
	the physical memory range of the allocation (e.g. on x86, GFP_DMA
	guarantees to be within the first 16Mb of available physical memory,
	as required by ISA devices).
	
	Note also that the above constraints on physical contiguity and
	dma_mask may not apply if the platform has an IOMMU (a device which
	supplies a physical to virtual mapping between the I/O memory bus and
	the device).  However, to be portable, device driver writers may *not*
	assume that such an IOMMU exists.
	
	Warnings:  Memory coherency operates at a granularity called the cache
	line width.  In order for memory mapped by this API to operate
	correctly, the mapped region must begin exactly on a cache line
	boundary and end exactly on one (to prevent two separately mapped
	regions from sharing a single cache line).  Since the cache line size
	may not be known at compile time, the API will not enforce this
	requirement.  Therefore, it is recommended that driver writers who
	don't take special care to determine the cache line size at run time
	only map virtual regions that begin and end on page boundaries (which
	are guaranteed also to be cache line boundaries).
	
	DMA_TO_DEVICE synchronisation must be done after the last modification
	of the memory region by the software and before it is handed off to
	the driver.  Once this primitive is used, memory covered by this
	primitive should be treated as read-only by the device.  If the device
	may write to it at any point, it should be DMA_BIDIRECTIONAL (see
	below).
	
	DMA_FROM_DEVICE synchronisation must be done before the driver
	accesses data that may be changed by the device.  This memory should
	be treated as read-only by the driver.  If the driver needs to write
	to it at any point, it should be DMA_BIDIRECTIONAL (see below).
	
	DMA_BIDIRECTIONAL requires special handling: it means that the driver
	isn't sure if the memory was modified before being handed off to the
	device and also isn't sure if the device will also modify it.  Thus,
	you must always sync bidirectional memory twice: once before the
	memory is handed off to the device (to make sure all memory changes
	are flushed from the processor) and once before the data may be
	accessed after being used by the device (to make sure any processor
	cache lines are updated with data that the device may have changed).
	
	void
	dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
			 enum dma_data_direction direction)
	
	Unmaps the region previously mapped.  All the parameters passed in
	must be identical to those passed in (and returned) by the mapping
	API.
	
	dma_addr_t
	dma_map_page(struct device *dev, struct page *page,
			    unsigned long offset, size_t size,
			    enum dma_data_direction direction)
	void
	dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
		       enum dma_data_direction direction)
	
	API for mapping and unmapping for pages.  All the notes and warnings
	for the other mapping APIs apply here.  Also, although the <offset>
	and <size> parameters are provided to do partial page mapping, it is
	recommended that you never use these unless you really know what the
	cache width is.
	
	int
	dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
	
	In some circumstances dma_map_single and dma_map_page will fail to create
	a mapping. A driver can check for these errors by testing the returned
	dma address with dma_mapping_error(). A non-zero return value means the mapping
	could not be created and the driver should take appropriate action (e.g.
	reduce current DMA mapping usage or delay and try again later).
	
		int
		dma_map_sg(struct device *dev, struct scatterlist *sg,
			int nents, enum dma_data_direction direction)
	
	Returns: the number of physical segments mapped (this may be shorter
	than <nents> passed in if some elements of the scatter/gather list are
	physically or virtually adjacent and an IOMMU maps them with a single
	entry).
	
	Please note that the sg cannot be mapped again if it has been mapped once.
	The mapping process is allowed to destroy information in the sg.
	
	As with the other mapping interfaces, dma_map_sg can fail. When it
	does, 0 is returned and a driver must take appropriate action. It is
	critical that the driver do something, in the case of a block driver
	aborting the request or even oopsing is better than doing nothing and
	corrupting the filesystem.
	
	With scatterlists, you use the resulting mapping like this:
	
		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. with an IOMMU, or if several pages just happen to be
	physically contiguous) 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.
	
		void
		dma_unmap_sg(struct device *dev, struct scatterlist *sg,
			int nhwentries, enum dma_data_direction direction)
	
	Unmap the previously mapped scatter/gather list.  All the parameters
	must be the same as those and passed in to the scatter/gather mapping
	API.
	
	Note: <nents> must be the number you passed in, *not* the number of
	physical entries returned.
	
	void
	dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size,
				enum dma_data_direction direction)
	void
	dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size,
				   enum dma_data_direction direction)
	void
	dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nelems,
			    enum dma_data_direction direction)
	void
	dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nelems,
			       enum dma_data_direction direction)
	
	Synchronise a single contiguous or scatter/gather mapping for the cpu
	and device. With the sync_sg API, all the parameters must be the same
	as those passed into the single mapping API. With the sync_single API,
	you can use dma_handle and size parameters that aren't identical to
	those passed into the single mapping API to do a partial sync.
	
	Notes:  You must do this:
	
	- Before reading values that have been written by DMA from the device
	  (use the DMA_FROM_DEVICE direction)
	- After writing values that will be written to the device using DMA
	  (use the DMA_TO_DEVICE) direction
	- before *and* after handing memory to the device if the memory is
	  DMA_BIDIRECTIONAL
	
	See also dma_map_single().
	
	dma_addr_t
	dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
			     enum dma_data_direction dir,
			     struct dma_attrs *attrs)
	
	void
	dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
			       size_t size, enum dma_data_direction dir,
			       struct dma_attrs *attrs)
	
	int
	dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
			 int nents, enum dma_data_direction dir,
			 struct dma_attrs *attrs)
	
	void
	dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
			   int nents, enum dma_data_direction dir,
			   struct dma_attrs *attrs)
	
	The four functions above are just like the counterpart functions
	without the _attrs suffixes, except that they pass an optional
	struct dma_attrs*.
	
	struct dma_attrs encapsulates a set of "dma attributes". For the
	definition of struct dma_attrs see linux/dma-attrs.h.
	
	The interpretation of dma attributes is architecture-specific, and
	each attribute should be documented in Documentation/DMA-attributes.txt.
	
	If struct dma_attrs* is NULL, the semantics of each of these
	functions is identical to those of the corresponding function
	without the _attrs suffix. As a result dma_map_single_attrs()
	can generally replace dma_map_single(), etc.
	
	As an example of the use of the *_attrs functions, here's how
	you could pass an attribute DMA_ATTR_FOO when mapping memory
	for DMA:
	
	#include <linux/dma-attrs.h>
	/* DMA_ATTR_FOO should be defined in linux/dma-attrs.h and
	 * documented in Documentation/DMA-attributes.txt */
	...
	
		DEFINE_DMA_ATTRS(attrs);
		dma_set_attr(DMA_ATTR_FOO, &attrs);
		....
		n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, &attr);
		....
	
	Architectures that care about DMA_ATTR_FOO would check for its
	presence in their implementations of the mapping and unmapping
	routines, e.g.:
	
	void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
				     size_t size, enum dma_data_direction dir,
				     struct dma_attrs *attrs)
	{
		....
		int foo =  dma_get_attr(DMA_ATTR_FOO, attrs);
		....
		if (foo)
			/* twizzle the frobnozzle */
		....
	
	
	Part II - Advanced dma_ usage
	-----------------------------
	
	Warning: These pieces of the DMA API should not be used in the
	majority of cases, since they cater for unlikely corner cases that
	don't belong in usual drivers.
	
	If you don't understand how cache line coherency works between a
	processor and an I/O device, you should not be using this part of the
	API at all.
	
	void *
	dma_alloc_noncoherent(struct device *dev, size_t size,
				       dma_addr_t *dma_handle, gfp_t flag)
	
	Identical to dma_alloc_coherent() except that the platform will
	choose to return either consistent or non-consistent memory as it sees
	fit.  By using this API, you are guaranteeing to the platform that you
	have all the correct and necessary sync points for this memory in the
	driver should it choose to return non-consistent memory.
	
	Note: where the platform can return consistent memory, it will
	guarantee that the sync points become nops.
	
	Warning:  Handling non-consistent memory is a real pain.  You should
	only ever use this API if you positively know your driver will be
	required to work on one of the rare (usually non-PCI) architectures
	that simply cannot make consistent memory.
	
	void
	dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
				      dma_addr_t dma_handle)
	
	Free memory allocated by the nonconsistent API.  All parameters must
	be identical to those passed in (and returned by
	dma_alloc_noncoherent()).
	
	int
	dma_is_consistent(struct device *dev, dma_addr_t dma_handle)
	
	Returns true if the device dev is performing consistent DMA on the memory
	area pointed to by the dma_handle.
	
	int
	dma_get_cache_alignment(void)
	
	Returns the processor cache alignment.  This is the absolute minimum
	alignment *and* width that you must observe when either mapping
	memory or doing partial flushes.
	
	Notes: This API may return a number *larger* than the actual cache
	line, but it will guarantee that one or more cache lines fit exactly
	into the width returned by this call.  It will also always be a power
	of two for easy alignment.
	
	void
	dma_cache_sync(struct device *dev, void *vaddr, size_t size,
		       enum dma_data_direction direction)
	
	Do a partial sync of memory that was allocated by
	dma_alloc_noncoherent(), starting at virtual address vaddr and
	continuing on for size.  Again, you *must* observe the cache line
	boundaries when doing this.
	
	int
	dma_declare_coherent_memory(struct device *dev, dma_addr_t bus_addr,
				    dma_addr_t device_addr, size_t size, int
				    flags)
	
	Declare region of memory to be handed out by dma_alloc_coherent when
	it's asked for coherent memory for this device.
	
	bus_addr is the physical address to which the memory is currently
	assigned in the bus responding region (this will be used by the
	platform to perform the mapping).
	
	device_addr is the physical address the device needs to be programmed
	with actually to address this memory (this will be handed out as the
	dma_addr_t in dma_alloc_coherent()).
	
	size is the size of the area (must be multiples of PAGE_SIZE).
	
	flags can be or'd together and are:
	
	DMA_MEMORY_MAP - request that the memory returned from
	dma_alloc_coherent() be directly writable.
	
	DMA_MEMORY_IO - request that the memory returned from
	dma_alloc_coherent() be addressable using read/write/memcpy_toio etc.
	
	One or both of these flags must be present.
	
	DMA_MEMORY_INCLUDES_CHILDREN - make the declared memory be allocated by
	dma_alloc_coherent of any child devices of this one (for memory residing
	on a bridge).
	
	DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions. 
	Do not allow dma_alloc_coherent() to fall back to system memory when
	it's out of memory in the declared region.
	
	The return value will be either DMA_MEMORY_MAP or DMA_MEMORY_IO and
	must correspond to a passed in flag (i.e. no returning DMA_MEMORY_IO
	if only DMA_MEMORY_MAP were passed in) for success or zero for
	failure.
	
	Note, for DMA_MEMORY_IO returns, all subsequent memory returned by
	dma_alloc_coherent() may no longer be accessed directly, but instead
	must be accessed using the correct bus functions.  If your driver
	isn't prepared to handle this contingency, it should not specify
	DMA_MEMORY_IO in the input flags.
	
	As a simplification for the platforms, only *one* such region of
	memory may be declared per device.
	
	For reasons of efficiency, most platforms choose to track the declared
	region only at the granularity of a page.  For smaller allocations,
	you should use the dma_pool() API.
	
	void
	dma_release_declared_memory(struct device *dev)
	
	Remove the memory region previously declared from the system.  This
	API performs *no* in-use checking for this region and will return
	unconditionally having removed all the required structures.  It is the
	driver's job to ensure that no parts of this memory region are
	currently in use.
	
	void *
	dma_mark_declared_memory_occupied(struct device *dev,
					  dma_addr_t device_addr, size_t size)
	
	This is used to occupy specific regions of the declared space
	(dma_alloc_coherent() will hand out the first free region it finds).
	
	device_addr is the *device* address of the region requested.
	
	size is the size (and should be a page-sized multiple).
	
	The return value will be either a pointer to the processor virtual
	address of the memory, or an error (via PTR_ERR()) if any part of the
	region is occupied.
	
	Part III - Debug drivers use of the DMA-API
	-------------------------------------------
	
	The DMA-API as described above as some constraints. DMA addresses must be
	released with the corresponding function with the same size for example. With
	the advent of hardware IOMMUs it becomes more and more important that drivers
	do not violate those constraints. In the worst case such a violation can
	result in data corruption up to destroyed filesystems.
	
	To debug drivers and find bugs in the usage of the DMA-API checking code can
	be compiled into the kernel which will tell the developer about those
	violations. If your architecture supports it you can select the "Enable
	debugging of DMA-API usage" option in your kernel configuration. Enabling this
	option has a performance impact. Do not enable it in production kernels.
	
	If you boot the resulting kernel will contain code which does some bookkeeping
	about what DMA memory was allocated for which device. If this code detects an
	error it prints a warning message with some details into your kernel log. An
	example warning message may look like this:
	
	------------[ cut here ]------------
	WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
		check_unmap+0x203/0x490()
	Hardware name:
	forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
		function [device address=0x00000000640444be] [size=66 bytes] [mapped as
	single] [unmapped as page]
	Modules linked in: nfsd exportfs bridge stp llc r8169
	Pid: 0, comm: swapper Tainted: G        W  2.6.28-dmatest-09289-g8bb99c0 #1
	Call Trace:
	 <IRQ>  [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
	 [<ffffffff80647b70>] _spin_unlock+0x10/0x30
	 [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
	 [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
	 [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
	 [<ffffffff80252f96>] queue_work+0x56/0x60
	 [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
	 [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
	 [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
	 [<ffffffff80235177>] find_busiest_group+0x207/0x8a0
	 [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
	 [<ffffffff803c7ea3>] check_unmap+0x203/0x490
	 [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
	 [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
	 [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
	 [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
	 [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
	 [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
	 [<ffffffff8020c093>] ret_from_intr+0x0/0xa
	 <EOI> <4>---[ end trace f6435a98e2a38c0e ]---
	
	The driver developer can find the driver and the device including a stacktrace
	of the DMA-API call which caused this warning.
	
	Per default only the first error will result in a warning message. All other
	errors will only silently counted. This limitation exist to prevent the code
	from flooding your kernel log. To support debugging a device driver this can
	be disabled via debugfs. See the debugfs interface documentation below for
	details.
	
	The debugfs directory for the DMA-API debugging code is called dma-api/. In
	this directory the following files can currently be found:
	
		dma-api/all_errors	This file contains a numeric value. If this
					value is not equal to zero the debugging code
					will print a warning for every error it finds
					into the kernel log. Be careful with this
					option, as it can easily flood your logs.
	
		dma-api/disabled	This read-only file contains the character 'Y'
					if the debugging code is disabled. This can
					happen when it runs out of memory or if it was
					disabled at boot time
	
		dma-api/error_count	This file is read-only and shows the total
					numbers of errors found.
	
		dma-api/num_errors	The number in this file shows how many
					warnings will be printed to the kernel log
					before it stops. This number is initialized to
					one at system boot and be set by writing into
					this file
	
		dma-api/min_free_entries
					This read-only file can be read to get the
					minimum number of free dma_debug_entries the
					allocator has ever seen. If this value goes
					down to zero the code will disable itself
					because it is not longer reliable.
	
		dma-api/num_free_entries
					The current number of free dma_debug_entries
					in the allocator.
	
		dma-api/driver-filter
					You can write a name of a driver into this file
					to limit the debug output to requests from that
					particular driver. Write an empty string to
					that file to disable the filter and see
					all errors again.
	
	If you have this code compiled into your kernel it will be enabled by default.
	If you want to boot without the bookkeeping anyway you can provide
	'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
	Notice that you can not enable it again at runtime. You have to reboot to do
	so.
	
	If you want to see debug messages only for a special device driver you can
	specify the dma_debug_driver=<drivername> parameter. This will enable the
	driver filter at boot time. The debug code will only print errors for that
	driver afterwards. This filter can be disabled or changed later using debugfs.
	
	When the code disables itself at runtime this is most likely because it ran
	out of dma_debug_entries. These entries are preallocated at boot. The number
	of preallocated entries is defined per architecture. If it is too low for you
	boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
	architectural default.

• [ Documentation ]
• 00-INDEX
• [ ABI ]
• [ accounting ]
• [ acpi ]
• [ aoe ]
• applying-patches.txt
• [ arm ]
• atomic_ops.txt
• [ auxdisplay ]
• bad_memory.txt
• basic_profiling.txt
• binfmt_misc.txt
• [ blackfin ]
• [ block ]
• [ blockdev ]
• braille-console.txt
• bt8xxgpio.txt
• btmrvl.txt
• BUG-HUNTING
• cachetlb.txt
• [ cdrom ]
• [ cgroups ]
• Changes
• circular-buffers.txt
• CodingStyle
• [ connector ]
• [ console ]
• [ cpu-freq ]
• cpu-hotplug.txt
• cpu-load.txt
• [ cpuidle ]
• cputopology.txt
• credentials.txt
• [ cris ]
• [ crypto ]
• dcdbas.txt
• debugging-modules.txt
• debugging-via-ohci1394.txt
• dell_rbu.txt
• [ development-process ]
• [ device-mapper ]
• devices.txt
• DMA-API-HOWTO.txt
• DMA-API.txt
• DMA-attributes.txt
• DMA-ISA-LPC.txt
• dmaengine.txt
• [ DocBook ]
• dontdiff
• [ driver-model ]
• [ dvb ]
• dynamic-debug-howto.txt
• [ early-userspace ]
• edac.txt
• eisa.txt
• email-clients.txt
• [ fault-injection ]
• [ fb ]
• feature-removal-schedule.txt
• [ filesystems ]
• [ firmware_class ]
• flexible-arrays.txt
• [ frv ]
• futex-requeue-pi.txt
• gcov.txt
• gpio.txt
• highuid.txt
• HOWTO
• hw_random.txt
• [ hwmon ]
• [ i2c ]
• [ i2o ]
• [ ia64 ]
• [ ide ]
• [ infiniband ]
• init.txt
• initrd.txt
• [ input ]
• Intel-IOMMU.txt
• intel_txt.txt
• IO-mapping.txt
• io-mapping.txt
• io_ordering.txt
• [ ioctl ]
• iostats.txt
• IPMI.txt
• IRQ-affinity.txt
• IRQ.txt
• irqflags-tracing.txt
• isapnp.txt
• [ isdn ]
• [ ja_JP ]
• java.txt
• [ kbuild ]
• [ kdump ]
• kernel-doc-nano-HOWTO.txt
• kernel-docs.txt
• kernel-parameters.txt
• keys-request-key.txt
• keys.txt
• kmemcheck.txt
• kmemleak.txt
• [ ko_KR ]
• kobject.txt
• kprobes.txt
• kref.txt
• [ kvm ]
• [ laptops ]
• ldm.txt
• leds-class.txt
• leds-lp3944.txt
• [ lguest ]
• local_ops.txt
• lockdep-design.txt
• lockstat.txt
• logo.gif
• logo.txt
• [ m68k ]
• magic-number.txt
• [ make ]
• Makefile
• ManagementStyle
• mca.txt
• md.txt
• memory-barriers.txt
• memory-hotplug.txt
• memory.txt
• [ mips ]
• [ misc-devices ]
• [ mn10300 ]
• mono.txt
• [ mtd ]
• mutex-design.txt
• [ namespaces ]
• [ netlabel ]
• [ networking ]
• nmi_watchdog.txt
• nommu-mmap.txt
• numastat.txt
• oops-tracing.txt
• [ parisc ]
• parport-lowlevel.txt
• parport.txt
• [ PCI ]
• [ pcmcia ]
• pi-futex.txt
• pnp.txt
• [ power ]
• [ powerpc ]
• [ pps ]
• [ prctl ]
• preempt-locking.txt
• printk-formats.txt
• prio_tree.txt
• rbtree.txt
• [ RCU ]
• rfkill.txt
• robust-futex-ABI.txt
• robust-futexes.txt
• rt-mutex-design.txt
• rt-mutex.txt
• rtc.txt
• [ s390 ]
• SAK.txt
• [ scheduler ]
• [ scsi ]
• SecurityBugs
• SELinux.txt
• [ serial ]
• serial-console.txt
• sgi-ioc4.txt
• sgi-visws.txt
• [ sh ]
• slow-work.txt
• SM501.txt
• Smack.txt
• [ sound ]
• [ sparc ]
• sparse.txt
• [ spi ]
• spinlocks.txt
• stable_api_nonsense.txt
• stable_kernel_rules.txt
• SubmitChecklist
• SubmittingDrivers
• SubmittingPatches
• svga.txt
• [ sysctl ]
• sysfs-rules.txt
• sysrq.txt
• [ telephony ]
• [ thermal ]
• [ timers ]
• tomoyo.txt
• [ trace ]
• [ uml ]
• unaligned-memory-access.txt
• unicode.txt
• unshare.txt
• [ usb ]
• VGA-softcursor.txt
• vgaarbiter.txt
• video-output.txt
• [ video4linux ]
• [ vm ]
• volatile-considered-harmful.txt
• [ w1 ]
• [ watchdog ]
• [ wimax ]
• [ x86 ]
• [ zh_CN ]
• zorro.txt
•