xref: /openbmc/linux/drivers/md/dm-zero.c (revision 3664ff82)
1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * Copyright (C) 2003 Jana Saout <jana@saout.de>
4  *
5  * This file is released under the GPL.
6  */
7 
8 #include <linux/device-mapper.h>
9 
10 #include <linux/module.h>
11 #include <linux/init.h>
12 #include <linux/bio.h>
13 
14 #define DM_MSG_PREFIX "zero"
15 
16 /*
17  * Construct a dummy mapping that only returns zeros
18  */
zero_ctr(struct dm_target * ti,unsigned int argc,char ** argv)19 static int zero_ctr(struct dm_target *ti, unsigned int argc, char **argv)
20 {
21 	if (argc != 0) {
22 		ti->error = "No arguments required";
23 		return -EINVAL;
24 	}
25 
26 	/*
27 	 * Silently drop discards, avoiding -EOPNOTSUPP.
28 	 */
29 	ti->num_discard_bios = 1;
30 	ti->discards_supported = true;
31 
32 	return 0;
33 }
34 
35 /*
36  * Return zeros only on reads
37  */
zero_map(struct dm_target * ti,struct bio * bio)38 static int zero_map(struct dm_target *ti, struct bio *bio)
39 {
40 	switch (bio_op(bio)) {
41 	case REQ_OP_READ:
42 		if (bio->bi_opf & REQ_RAHEAD) {
43 			/* readahead of null bytes only wastes buffer cache */
44 			return DM_MAPIO_KILL;
45 		}
46 		zero_fill_bio(bio);
47 		break;
48 	case REQ_OP_WRITE:
49 	case REQ_OP_DISCARD:
50 		/* writes get silently dropped */
51 		break;
52 	default:
53 		return DM_MAPIO_KILL;
54 	}
55 
56 	bio_endio(bio);
57 
58 	/* accepted bio, don't make new request */
59 	return DM_MAPIO_SUBMITTED;
60 }
61 
zero_io_hints(struct dm_target * ti,struct queue_limits * limits)62 static void zero_io_hints(struct dm_target *ti, struct queue_limits *limits)
63 {
64 	limits->max_discard_sectors = UINT_MAX;
65 	limits->max_hw_discard_sectors = UINT_MAX;
66 	limits->discard_granularity = 512;
67 }
68 
69 static struct target_type zero_target = {
70 	.name   = "zero",
71 	.version = {1, 2, 0},
72 	.features = DM_TARGET_NOWAIT,
73 	.module = THIS_MODULE,
74 	.ctr    = zero_ctr,
75 	.map    = zero_map,
76 	.io_hints = zero_io_hints,
77 };
78 module_dm(zero);
79 
80 MODULE_AUTHOR("Jana Saout <jana@saout.de>");
81 MODULE_DESCRIPTION(DM_NAME " dummy target returning zeros");
82 MODULE_LICENSE("GPL");
83