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Documentation / io-mapping.txt




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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	========================
2	The io_mapping functions
3	========================
4	
5	API
6	===
7	
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.
13	
14	A 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
20	mappable, while 'size' indicates how large a mapping region to
21	enable. Both are in bytes.
22	
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.
25	
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::
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.
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.
38	
39	This _wc variant returns a write-combining map to the
40	page and may only be used with mappings created by
41	io_mapping_create_wc
42	
43	Note that the task may not sleep while holding this page
44	mapped.
45	
46	::
47	
48		void io_mapping_unmap_atomic(void *vaddr)
49	
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.
53	
54	If you need to sleep while holding the lock, you can use the non-atomic
55	variant, although they may be significantly slower.
56	
57	::
58	
59		void *io_mapping_map_wc(struct io_mapping *mapping,
60					unsigned long offset)
61	
62	This works like io_mapping_map_atomic_wc except it allows
63	the task to sleep while holding the page mapped.
64	
65	
66	::
67	
68		void io_mapping_unmap(void *vaddr)
69	
70	This works like io_mapping_unmap_atomic, except it is used
71	for pages mapped with io_mapping_map_wc.
72	
73	At driver close time, the io_mapping object must be freed::
74	
75		void io_mapping_free(struct io_mapping *mapping)
76	
77	Current Implementation
78	======================
79	
80	The initial implementation of these functions uses existing mapping
81	mechanisms and so provides only an abstraction layer and no new
82	functionality.
83	
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.
88	
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.
93	
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.
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