12062 Convert buf(9S) to mandoc
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 --- EOF ---