1 dm-io
2 =====
3
4 Dm-io provides synchronous and asynchronous I/O services. There are three
5 types of I/O services available, and each type has a sync and an async
6 version.
7
8 The user must set up an io_region structure to describe the desired location
9 of the I/O. Each io_region indicates a block-device along with the starting
10 sector and size of the region.
11
12 struct io_region {
13 struct block_device *bdev;
14 sector_t sector;
15 sector_t count;
16 };
17
18 Dm-io can read from one io_region or write to one or more io_regions. Writes
19 to multiple regions are specified by an array of io_region structures.
20
21 The first I/O service type takes a list of memory pages as the data buffer for
22 the I/O, along with an offset into the first page.
23
24 struct page_list {
25 struct page_list *next;
26 struct page *page;
27 };
28
29 int dm_io_sync(unsigned int num_regions, struct io_region *where, int rw,
30 struct page_list *pl, unsigned int offset,
31 unsigned long *error_bits);
32 int dm_io_async(unsigned int num_regions, struct io_region *where, int rw,
33 struct page_list *pl, unsigned int offset,
34 io_notify_fn fn, void *context);
35
36 The second I/O service type takes an array of bio vectors as the data buffer
37 for the I/O. This service can be handy if the caller has a pre-assembled bio,
38 but wants to direct different portions of the bio to different devices.
39
40 int dm_io_sync_bvec(unsigned int num_regions, struct io_region *where,
41 int rw, struct bio_vec *bvec,
42 unsigned long *error_bits);
43 int dm_io_async_bvec(unsigned int num_regions, struct io_region *where,
44 int rw, struct bio_vec *bvec,
45 io_notify_fn fn, void *context);
46
47 The third I/O service type takes a pointer to a vmalloc'd memory buffer as the
48 data buffer for the I/O. This service can be handy if the caller needs to do
49 I/O to a large region but doesn't want to allocate a large number of individual
50 memory pages.
51
52 int dm_io_sync_vm(unsigned int num_regions, struct io_region *where, int rw,
53 void *data, unsigned long *error_bits);
54 int dm_io_async_vm(unsigned int num_regions, struct io_region *where, int rw,
55 void *data, io_notify_fn fn, void *context);
56
57 Callers of the asynchronous I/O services must include the name of a completion
58 callback routine and a pointer to some context data for the I/O.
59
60 typedef void (*io_notify_fn)(unsigned long error, void *context);
61
62 The "error" parameter in this callback, as well as the "*error" parameter in
63 all of the synchronous versions, is a bitset (instead of a simple error value).
64 In the case of an write-I/O to multiple regions, this bitset allows dm-io to
65 indicate success or failure on each individual region.
66
67 Before using any of the dm-io services, the user should call dm_io_get()
68 and specify the number of pages they expect to perform I/O on concurrently.
69 Dm-io will attempt to resize its mempool to make sure enough pages are
70 always available in order to avoid unnecessary waiting while performing I/O.
71
72 When the user is finished using the dm-io services, they should call
73 dm_io_put() and specify the same number of pages that were given on the
74 dm_io_get() call.
75
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