1 BUF(9S) Data Structures for Drivers BUF(9S)
2
3 NAME
4 buf - block I/O data transfer structure
5
6 SYNOPSIS
7 #include <sys/ddi.h>
8 #include <sys/sunddi.h>
9
10 INTERFACE LEVEL
11 Architecture independent level 1 (DDI/DKI)
12
13 DESCRIPTION
14 The buf structure is the basic data structure for block I/O transfers.
15 Each block I/O transfer has an associated buffer header. The header
16 contains all the buffer control and status information. For drivers, the
17 buffer header pointer is the sole argument to a block driver strategy(9E)
18 routine. Do not depend on the size of the buf structure when writing a
19 driver.
20
21 A buffer header can be linked in multiple lists simultaneously. Because
22 of this, most of the members in the buffer header cannot be changed by
23 the driver, even when the buffer header is in one of the driver's work
24 lists.
25
26 Buffer headers are also used by the system for unbuffered or physical I/O
27 for block drivers. In this case, the buffer describes a portion of user
28 data space that is locked into memory.
29
30 Block drivers often chain block requests so that overall throughput for
31 the device is maximized. The av_forw and the av_back members of the buf
32 structure can serve as link pointers for chaining block requests.
33
34 STRUCTURE MEMBERS
35 int b_flags; /* Buffer status */
36 struct buf *av_forw; /* Driver work list link */
37 struct buf *av_back; /* Driver work list link */
38 size_t b_bcount; /* # of bytes to transfer */
39 union {
40 caddr_t b_addr; /* Buffer's virtual address */
41 } b_un;
42 daddr_t b_blkno; /* Block number on device */
43 diskaddr_t b_lblkno; /* Expanded block number on dev. */
44 size_t b_resid; /* # of bytes not xferred */
45 size_t b_bufsize; /* size of alloc. buffer */
46 int (*b_iodone)(struct buf *); /* function called */
47 /* by biodone */
48 int b_error; /* expanded error field */
49 void *b_private; /* "opaque" driver private area */
50 dev_t b_edev; /* expanded dev field */
51
52 The members of the buffer header available to test or set by a driver are
53 as follows:
54
55 b_flags stores the buffer status and indicates to the driver whether to
56 read or write to the device. The driver must never clear the b_flags
57 member. If this is done, unpredictable results can occur including loss
58 of disk sanity and the possible failure of other kernel processes.
59
60 All b_flags bit values not otherwise specified above are reserved by the
61 kernel and may not be used.
62
63 Valid flags are as follows:
64
65 B_BUSY Indicates the buffer is in use. The driver must not change
66 this flag unless it allocated the buffer with getrbuf(9F) and
67 no I/O operation is in progress.
68
69 B_DONE Indicates the data transfer has completed. This flag is read-
70 only.
71
72 B_ERROR Indicates an I/O transfer error. It is set in conjunction with
73 the b_error field. bioerror(9F) should be used in preference
74 to setting the B_ERROR bit.
75
76 B_PAGEIO Indicates the buffer is being used in a paged I/O request. See
77 the description of the b_un.b_addr field for more information.
78 This flag is read-only.
79
80 B_PHYS indicates the buffer header is being used for physical (direct)
81 I/O to a user data area. See the description of the
82 b_un.b_addr field for more information. This flag is read-
83 only.
84
85 B_READ Indicates that data is to be read from the peripheral device
86 into main memory.
87
88 B_WRITE Indicates that the data is to be transferred from main memory
89 to the peripheral device. B_WRITE is a pseudo flag and cannot
90 be directly tested; it is only detected as the NOT form of
91 B_READ.
92
93 av_forw and av_back can be used by the driver to link the buffer into
94 driver work lists.
95
96 b_bcount specifies the number of bytes to be transferred in both a paged
97 and a non-paged I/O request.
98
99 b_un.b_addr is the virtual address of the I/O request, unless B_PAGEIO is
100 set. The address is a kernel virtual address, unless B_PHYS is set, in
101 which case it is a user virtual address. If B_PAGEIO is set, b_un.b_addr
102 contains kernel private data. Note that either one of B_PHYS and
103 B_PAGEIO, or neither, can be set, but not both.
104
105 b_blkno identifies which logical block on the device (the device is
106 defined by the device number) is to be accessed. The driver might have
107 to convert this logical block number to a physical location such as a
108 cylinder, track, and sector of a disk. This is a 32-bit value. The
109 driver should use b_blkno or b_lblkno, but not both.
110
111 b_lblkno identifies which logical block on the device (the device is
112 defined by the device number) is to be accessed. The driver might have
113 to convert this logical block number to a physical location such as a
114 cylinder, track, and sector of a disk. This is a 64-bit value. The
115 driver should use b_lblkno or b_blkno, but not both.
116
117 b_resid should be set to the number of bytes not transferred because of
118 an error.
119
120 b_bufsize contains the size of the allocated buffer.
121
122 b_iodone identifies a specific biodone(9F) routine to be called by the
123 driver when the I/O is complete.
124
125 b_error can hold an error code that should be passed as a return code
126 from the driver. b_error is set in conjunction with the B_ERROR bit set
127 in the b_flags member. bioerror(9F) should be used in preference to
128 setting the b_error field.
129
130 b_private is for the private use of the device driver.
131
132 b_edev contains the major and minor device numbers of the device
133 accessed.
134
135 SEE ALSO
136 strategy(9E), aphysio(9F), bioclone(9F), biodone(9F), bioerror(9F),
137 bioinit(9F), clrbuf(9F), getrbuf(9F), physio(9F), iovec(9S), uio(9S)
138
139 Writing Device Drivers.
140
141 WARNINGS
142 Buffers are a shared resource within the kernel. Drivers should read or
143 write only the members listed in this section. Drivers that attempt to
144 use undocumented members of the buf structure risk corrupting data in the
145 kernel or on the device.
146
147 illumos July 9, 2018 illumos