|The io_mapping functions
|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
|Note that the task may not sleep while holding this page
| void io_mapping_unmap_atomic(void *vaddr)
|'vaddr' must be 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)
|The initial implementation of these functions uses existing mapping
|mechanisms and so provides only an abstraction layer and no new
|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.