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+The io_mapping functions in linux/io-mapping.h provide an abstraction for
+efficiently mapping small regions of an I/O device to the CPU. The initial
+usage is to support the large graphics aperture on 32-bit processors where
+ioremap_wc cannot be used to statically map the entire aperture to the CPU
+as it would consume too much of the kernel address space.
+
+A mapping object is created during driver initialization using
+
+ struct io_mapping *io_mapping_create_wc(unsigned long base,
+ unsigned long size)
+
+ 'base' is the bus address of the region to be made
+ mappable, while 'size' indicates how large a mapping region to
+ enable. Both are in bytes.
+
+ This _wc variant provides a mapping which may only be used
+ with the io_mapping_map_atomic_wc or io_mapping_map_wc.
+
+With this mapping object, individual pages can be mapped either atomically
+or not, depending on the necessary scheduling environment. Of course, atomic
+maps are more efficient:
+
+ void *io_mapping_map_atomic_wc(struct io_mapping *mapping,
+ unsigned long offset)
+
+ 'offset' is the offset within the defined mapping region.
+ Accessing addresses beyond the region specified in the
+ creation function yields undefined results. Using an offset
+ which is not page aligned yields an undefined result. The
+ return value points to a single page in CPU address space.
+
+ This _wc variant returns a write-combining map to the
+ page and may only be used with mappings created by
+ io_mapping_create_wc
+
+ Note that the task may not sleep while holding this page
+ mapped.
+
+ void io_mapping_unmap_atomic(void *vaddr)
+
+ 'vaddr' must be the the value returned by the last
+ io_mapping_map_atomic_wc call. This unmaps the specified
+ page and allows the task to sleep once again.
+
+If you need to sleep while holding the lock, you can use the non-atomic
+variant, although they may be significantly slower.
+
+ void *io_mapping_map_wc(struct io_mapping *mapping,
+ unsigned long offset)
+
+ This works like io_mapping_map_atomic_wc except it allows
+ the task to sleep while holding the page mapped.
+
+ void io_mapping_unmap(void *vaddr)
+
+ This works like io_mapping_unmap_atomic, except it is used
+ for pages mapped with io_mapping_map_wc.
+
+At driver close time, the io_mapping object must be freed:
+
+ void io_mapping_free(struct io_mapping *mapping)
+
+Current Implementation:
+
+The initial implementation of these functions uses existing mapping
+mechanisms and so provides only an abstraction layer and no new
+functionality.
+
+On 64-bit processors, io_mapping_create_wc calls ioremap_wc for the whole
+range, creating a permanent kernel-visible mapping to the resource. The
+map_atomic and map functions add the requested offset to the base of the
+virtual address returned by ioremap_wc.
+
+On 32-bit processors with HIGHMEM defined, io_mapping_map_atomic_wc uses
+kmap_atomic_pfn to map the specified page in an atomic fashion;
+kmap_atomic_pfn isn't really supposed to be used with device pages, but it
+provides an efficient mapping for this usage.
+
+On 32-bit processors without HIGHMEM defined, io_mapping_map_atomic_wc and
+io_mapping_map_wc both use ioremap_wc, a terribly inefficient function which
+performs an IPI to inform all processors about the new mapping. This results
+in a significant performance penalty.