Adding upstream version 6.1.76.upstream/6.1.76upstream

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

1 files changed, 846 insertions, 0 deletions

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+============================================
+Dynamic DMA mapping using the generic device
+============================================
+
+:Author: James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
+
+This document describes the DMA API.  For a more gentle introduction
+of the API (and actual examples), see Documentation/core-api/dma-api-howto.rst.
+
+This API is split into two pieces.  Part I describes the basic API.
+Part II describes extensions 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>.  This
+provides dma_addr_t and the interfaces described below.
+
+A dma_addr_t can hold any valid DMA address for the platform.  It can be
+given to a device to use as a DMA source or target.  A CPU cannot reference
+a dma_addr_t directly because there may be translation between its physical
+address space and the DMA address space.
+
+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 returns a pointer to the allocated region (in the processor's virtual
+address space) or NULL if the allocation failed.
+
+It also returns a <dma_handle> which may be cast to an unsigned integer the
+same width as the bus and given to the device as the DMA address base of
+the region.
+
+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 a region of consistent memory you previously allocated.  dev,
+size and dma_handle must all be the same as those passed into
+dma_alloc_coherent().  cpu_addr must be the virtual address returned by
+the dma_alloc_coherent().
+
+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);
+
+dma_pool_create() initializes 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_zalloc(struct dma_pool *pool, gfp_t mem_flags,
+		        dma_addr_t *handle)
+
+Wraps dma_pool_alloc() and also zeroes the returned memory if the
+allocation attempt succeeded.
+
+
+::
+
+	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
+dma_pool_alloc(); 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);
+
+dma_pool_destroy() frees the resources of the pool.  It 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_set_mask_and_coherent(struct device *dev, u64 mask)
+
+Checks to see if the mask is possible and updates the device
+streaming and coherent DMA mask parameters if it is.
+
+Returns: 0 if successful and a negative error if not.
+
+::
+
+	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.
+
+::
+
+	size_t
+	dma_max_mapping_size(struct device *dev);
+
+Returns the maximum size of a mapping for the device. The size parameter
+of the mapping functions like dma_map_single(), dma_map_page() and
+others should not be larger than the returned value.
+
+::
+
+	size_t
+	dma_opt_mapping_size(struct device *dev);
+
+Returns the maximum optimal size of a mapping for the device.
+
+Mapping larger buffers may take much longer in certain scenarios. In
+addition, for high-rate short-lived streaming mappings, the upfront time
+spent on the mapping may account for an appreciable part of the total
+request lifetime. As such, if splitting larger requests incurs no
+significant performance penalty, then device drivers are advised to
+limit total DMA streaming mappings length to the returned value.
+
+::
+
+	bool
+	dma_need_sync(struct device *dev, dma_addr_t dma_addr);
+
+Returns %true if dma_sync_single_for_{device,cpu} calls are required to
+transfer memory ownership.  Returns %false if those calls can be skipped.
+
+::
+
+	unsigned long
+	dma_get_merge_boundary(struct device *dev);
+
+Returns the DMA merge boundary. If the device cannot merge any the DMA address
+segments, the function returns 0.
+
+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 DMA address of the memory.
+
+The direction for both APIs 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
+======================= =============================================
+
+.. note::
+
+	Not all memory regions in a machine can be mapped by this API.
+	Further, contiguous 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, memory to be mapped by
+	this API should be obtained from sources which guarantee it to be
+	physically contiguous (like kmalloc).
+
+	Further, the DMA address of the memory must be within the
+	dma_mask of the device (the dma_mask is a bit mask of the
+	addressable region for the device, i.e., if the DMA address of
+	the memory ANDed with the dma_mask is still equal to the DMA
+	address, then the device can perform DMA to the memory).  To
+	ensure that the memory allocated by kmalloc is within the dma_mask,
+	the driver may specify various platform-dependent flags to restrict
+	the DMA address range of the allocation (e.g., on x86, GFP_DMA
+	guarantees to be within the first 16MB of available DMA addresses,
+	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
+	maps an I/O DMA address to a physical memory address).  However, to be
+	portable, device driver writers may *not* assume that such an IOMMU
+	exists.
+
+.. warning::
+
+	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 device.  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.
+
+::
+
+	dma_addr_t
+	dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size,
+			 enum dma_data_direction dir, unsigned long attrs)
+
+	void
+	dma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size,
+			   enum dma_data_direction dir, unsigned long attrs)
+
+API for mapping and unmapping for MMIO resources. All the notes and
+warnings for the other mapping APIs apply here. The API should only be
+used to map device MMIO resources, mapping of RAM is not permitted.
+
+::
+
+	int
+	dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
+
+In some circumstances dma_map_single(), dma_map_page() and dma_map_resource()
+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 DMA address 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 nents, 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
+DMA address 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 nents,
+			    enum dma_data_direction direction)
+
+	void
+	dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
+			       int nents,
+			       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.
+
+
+.. note::
+
+   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,
+			     unsigned long attrs)
+
+	void
+	dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
+			       size_t size, enum dma_data_direction dir,
+			       unsigned long attrs)
+
+	int
+	dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
+			 int nents, enum dma_data_direction dir,
+			 unsigned long attrs)
+
+	void
+	dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
+			   int nents, enum dma_data_direction dir,
+			   unsigned long attrs)
+
+The four functions above are just like the counterpart functions
+without the _attrs suffixes, except that they pass an optional
+dma_attrs.
+
+The interpretation of DMA attributes is architecture-specific, and
+each attribute should be documented in
+Documentation/core-api/dma-attributes.rst.
+
+If dma_attrs are 0, 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-mapping.h>
+	/* DMA_ATTR_FOO should be defined in linux/dma-mapping.h and
+	* documented in Documentation/core-api/dma-attributes.rst */
+	...
+
+		unsigned long attr;
+		attr |= DMA_ATTR_FOO;
+		....
+		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,
+				     unsigned long attrs)
+	{
+		....
+		if (attrs & DMA_ATTR_FOO)
+			/* twizzle the frobnozzle */
+		....
+	}
+
+
+Part II - Non-coherent DMA allocations
+--------------------------------------
+
+These APIs allow to allocate pages that are guaranteed to be DMA addressable
+by the passed in device, but which need explicit management of memory ownership
+for the kernel vs the device.
+
+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.
+
+::
+
+	struct page *
+	dma_alloc_pages(struct device *dev, size_t size, dma_addr_t *dma_handle,
+			enum dma_data_direction dir, gfp_t gfp)
+
+This routine allocates a region of <size> bytes of non-coherent memory.  It
+returns a pointer to first struct page for the region, or NULL if the
+allocation failed. The resulting struct page can be used for everything a
+struct page is suitable for.
+
+It also returns a <dma_handle> which may be cast to an unsigned integer the
+same width as the bus and given to the device as the DMA address base of
+the region.
+
+The dir parameter specified if data is read and/or written by the device,
+see dma_map_single() for details.
+
+The gfp parameter allows the caller to specify the ``GFP_`` flags (see
+kmalloc()) for the allocation, but rejects flags used to specify a memory
+zone such as GFP_DMA or GFP_HIGHMEM.
+
+Before giving the memory to the device, dma_sync_single_for_device() needs
+to be called, and before reading memory written by the device,
+dma_sync_single_for_cpu(), just like for streaming DMA mappings that are
+reused.
+
+::
+
+	void
+	dma_free_pages(struct device *dev, size_t size, struct page *page,
+			dma_addr_t dma_handle, enum dma_data_direction dir)
+
+Free a region of memory previously allocated using dma_alloc_pages().
+dev, size, dma_handle and dir must all be the same as those passed into
+dma_alloc_pages().  page must be the pointer returned by dma_alloc_pages().
+
+::
+
+	int
+	dma_mmap_pages(struct device *dev, struct vm_area_struct *vma,
+		       size_t size, struct page *page)
+
+Map an allocation returned from dma_alloc_pages() into a user address space.
+dev and size must be the same as those passed into dma_alloc_pages().
+page must be the pointer returned by dma_alloc_pages().
+
+::
+
+	void *
+	dma_alloc_noncoherent(struct device *dev, size_t size,
+			dma_addr_t *dma_handle, enum dma_data_direction dir,
+			gfp_t gfp)
+
+This routine is a convenient wrapper around dma_alloc_pages that returns the
+kernel virtual address for the allocated memory instead of the page structure.
+
+::
+
+	void
+	dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
+			dma_addr_t dma_handle, enum dma_data_direction dir)
+
+Free a region of memory previously allocated using dma_alloc_noncoherent().
+dev, size, dma_handle and dir must all be the same as those passed into
+dma_alloc_noncoherent().  cpu_addr must be the virtual address returned by
+dma_alloc_noncoherent().
+
+::
+
+	struct sg_table *
+	dma_alloc_noncontiguous(struct device *dev, size_t size,
+				enum dma_data_direction dir, gfp_t gfp,
+				unsigned long attrs);
+
+This routine allocates  <size> bytes of non-coherent and possibly non-contiguous
+memory.  It returns a pointer to struct sg_table that describes the allocated
+and DMA mapped memory, or NULL if the allocation failed. The resulting memory
+can be used for struct page mapped into a scatterlist are suitable for.
+
+The return sg_table is guaranteed to have 1 single DMA mapped segment as
+indicated by sgt->nents, but it might have multiple CPU side segments as
+indicated by sgt->orig_nents.
+
+The dir parameter specified if data is read and/or written by the device,
+see dma_map_single() for details.
+
+The gfp parameter allows the caller to specify the ``GFP_`` flags (see
+kmalloc()) for the allocation, but rejects flags used to specify a memory
+zone such as GFP_DMA or GFP_HIGHMEM.
+
+The attrs argument must be either 0 or DMA_ATTR_ALLOC_SINGLE_PAGES.
+
+Before giving the memory to the device, dma_sync_sgtable_for_device() needs
+to be called, and before reading memory written by the device,
+dma_sync_sgtable_for_cpu(), just like for streaming DMA mappings that are
+reused.
+
+::
+
+	void
+	dma_free_noncontiguous(struct device *dev, size_t size,
+			       struct sg_table *sgt,
+			       enum dma_data_direction dir)
+
+Free memory previously allocated using dma_alloc_noncontiguous().  dev, size,
+and dir must all be the same as those passed into dma_alloc_noncontiguous().
+sgt must be the pointer returned by dma_alloc_noncontiguous().
+
+::
+
+	void *
+	dma_vmap_noncontiguous(struct device *dev, size_t size,
+		struct sg_table *sgt)
+
+Return a contiguous kernel mapping for an allocation returned from
+dma_alloc_noncontiguous().  dev and size must be the same as those passed into
+dma_alloc_noncontiguous().  sgt must be the pointer returned by
+dma_alloc_noncontiguous().
+
+Once a non-contiguous allocation is mapped using this function, the
+flush_kernel_vmap_range() and invalidate_kernel_vmap_range() APIs must be used
+to manage the coherency between the kernel mapping, the device and user space
+mappings (if any).
+
+::
+
+	void
+	dma_vunmap_noncontiguous(struct device *dev, void *vaddr)
+
+Unmap a kernel mapping returned by dma_vmap_noncontiguous().  dev must be the
+same the one passed into dma_alloc_noncontiguous().  vaddr must be the pointer
+returned by dma_vmap_noncontiguous().
+
+
+::
+
+	int
+	dma_mmap_noncontiguous(struct device *dev, struct vm_area_struct *vma,
+			       size_t size, struct sg_table *sgt)
+
+Map an allocation returned from dma_alloc_noncontiguous() into a user address
+space.  dev and size must be the same as those passed into
+dma_alloc_noncontiguous().  sgt must be the pointer returned by
+dma_alloc_noncontiguous().
+
+::
+
+	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.
+
+.. note::
+
+	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.
+
+
+Part III - Debug drivers use of the DMA-API
+-------------------------------------------
+
+The DMA-API as described above has 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::
+
+	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/dump			This read-only file contains current DMA
+				mappings.
+
+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 attempt to increase
+				nr_total_entries to compensate.
+
+dma-api/num_free_entries	The current number of free dma_debug_entries
+				in the allocator.
+
+dma-api/nr_total_entries	The total number of dma_debug_entries in the
+				allocator, both free and used.
+
+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 and was unable to allocate more on-demand. 65536
+entries are preallocated at boot - if this is too low for you boot with
+'dma_debug_entries=<your_desired_number>' to overwrite the default. Note
+that the code allocates entries in batches, so the exact number of
+preallocated entries may be greater than the actual number requested. The
+code will print to the kernel log each time it has dynamically allocated
+as many entries as were initially preallocated. This is to indicate that a
+larger preallocation size may be appropriate, or if it happens continually
+that a driver may be leaking mappings.
+
+::
+
+	void
+	debug_dma_mapping_error(struct device *dev, dma_addr_t dma_addr);
+
+dma-debug interface debug_dma_mapping_error() to debug drivers that fail
+to check DMA mapping errors on addresses returned by dma_map_single() and
+dma_map_page() interfaces. This interface clears a flag set by
+debug_dma_map_page() to indicate that dma_mapping_error() has been called by
+the driver. When driver does unmap, debug_dma_unmap() checks the flag and if
+this flag is still set, prints warning message that includes call trace that
+leads up to the unmap. This interface can be called from dma_mapping_error()
+routines to enable DMA mapping error check debugging.