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Diffstat (limited to 'Documentation/io-mapping.txt')
| -rw-r--r-- | Documentation/io-mapping.txt | 82 |
1 files changed, 0 insertions, 82 deletions
diff --git a/Documentation/io-mapping.txt b/Documentation/io-mapping.txt deleted file mode 100644 index 473e43b2d58..00000000000 --- a/Documentation/io-mapping.txt +++ /dev/null @@ -1,82 +0,0 @@ -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. |