1======================== 2The io_mapping functions 3======================== 4 5API 6=== 7 8The io_mapping functions in linux/io-mapping.h provide an abstraction for 9efficiently mapping small regions of an I/O device to the CPU. The initial 10usage is to support the large graphics aperture on 32-bit processors where 11ioremap_wc cannot be used to statically map the entire aperture to the CPU 12as it would consume too much of the kernel address space. 13 14A mapping object is created during driver initialization using:: 15 16 struct io_mapping *io_mapping_create_wc(unsigned long base, 17 unsigned long size) 18 19'base' is the bus address of the region to be made 20mappable, while 'size' indicates how large a mapping region to 21enable. Both are in bytes. 22 23This _wc variant provides a mapping which may only be used 24with the io_mapping_map_atomic_wc or io_mapping_map_wc. 25 26With this mapping object, individual pages can be mapped either atomically 27or not, depending on the necessary scheduling environment. Of course, atomic 28maps are more efficient:: 29 30 void *io_mapping_map_atomic_wc(struct io_mapping *mapping, 31 unsigned long offset) 32 33'offset' is the offset within the defined mapping region. 34Accessing addresses beyond the region specified in the 35creation function yields undefined results. Using an offset 36which is not page aligned yields an undefined result. The 37return value points to a single page in CPU address space. 38 39This _wc variant returns a write-combining map to the 40page and may only be used with mappings created by 41io_mapping_create_wc 42 43Note that the task may not sleep while holding this page 44mapped. 45 46:: 47 48 void io_mapping_unmap_atomic(void *vaddr) 49 50'vaddr' must be the value returned by the last 51io_mapping_map_atomic_wc call. This unmaps the specified 52page and allows the task to sleep once again. 53 54If you need to sleep while holding the lock, you can use the non-atomic 55variant, although they may be significantly slower. 56 57:: 58 59 void *io_mapping_map_wc(struct io_mapping *mapping, 60 unsigned long offset) 61 62This works like io_mapping_map_atomic_wc except it allows 63the task to sleep while holding the page mapped. 64 65 66:: 67 68 void io_mapping_unmap(void *vaddr) 69 70This works like io_mapping_unmap_atomic, except it is used 71for pages mapped with io_mapping_map_wc. 72 73At driver close time, the io_mapping object must be freed:: 74 75 void io_mapping_free(struct io_mapping *mapping) 76 77Current Implementation 78====================== 79 80The initial implementation of these functions uses existing mapping 81mechanisms and so provides only an abstraction layer and no new 82functionality. 83 84On 64-bit processors, io_mapping_create_wc calls ioremap_wc for the whole 85range, creating a permanent kernel-visible mapping to the resource. The 86map_atomic and map functions add the requested offset to the base of the 87virtual address returned by ioremap_wc. 88 89On 32-bit processors with HIGHMEM defined, io_mapping_map_atomic_wc uses 90kmap_atomic_pfn to map the specified page in an atomic fashion; 91kmap_atomic_pfn isn't really supposed to be used with device pages, but it 92provides an efficient mapping for this usage. 93 94On 32-bit processors without HIGHMEM defined, io_mapping_map_atomic_wc and 95io_mapping_map_wc both use ioremap_wc, a terribly inefficient function which 96performs an IPI to inform all processors about the new mapping. This results 97in a significant performance penalty. 98