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