buf - block I/O data transfer structure
#include <sys/ddi.h> #include <sys/sunddi.h>
Architecture independent level 1 (DDI/DKI)
The buf structure is the basic data structure for block I/O transfers. Each block I/O transfer has an associated buffer header. The header contains all the buffer control and status information. For drivers, the buffer header pointer is the sole argument to a block driver strategy(9E) routine. Do not depend on the size of the buf structure when writing a driver.
A buffer header can be linked in multiple lists simultaneously. Because of this, most of the members in the buffer header cannot be changed by the driver, even when the buffer header is in one of the driver's work lists.
Buffer headers are also used by the system for unbuffered or physical I/O for block drivers. In this case, the buffer describes a portion of user data space that is locked into memory.
Block drivers often chain block requests so that overall throughput for the device is maximized. The av_forw and the av_back members of the buf structure can serve as link pointers for chaining block requests.
int b_flags; /* Buffer status */
struct buf *av_forw; /* Driver work list link */
struct buf *av_back; /* Driver work list link */
size_t b_bcount; /* # of bytes to transfer */
union {
caddr_t b_addr; /* Buffer's virtual address */
} b_un;
daddr_t b_blkno; /* Block number on device */
diskaddr_t b_lblkno; /* Expanded block number on dev. */
size_t b_resid; /* # of bytes not xferred */
size_t b_bufsize; /* size of alloc. buffer */
int (*b_iodone)(struct buf *); /* function called */
/* by biodone */
int b_error; /* error number field */
int b_xerror; /* extended error field */
void *b_private; /* "opaque" driver private area */
dev_t b_edev; /* expanded dev field */
The members of the buffer header available to test or set by a driver are as follows:
b_flags stores the buffer status and indicates to the driver whether to read or write to the device. The driver must never clear the b_flags member. If this is done, unpredictable results can occur including loss of disk sanity and the possible failure of other kernel processes.
All b_flags bit values not otherwise specified below are reserved by the kernel and may not be used.
Valid flags are as follows:
Indicates the buffer is in use. The driver must not change this flag unless it allocated the buffer with getrbuf(9F) and no I/O operation is in progress.
Indicates the data transfer has completed. This flag is read-only.
Indicates an I/O transfer error. It is set in conjunction with the b_error and b_xerror fields. bioerror(9F) and bioxerror(9F) should be used in preference to setting the B_ERROR bit. bioerror(9F) and bioerror(9F) should be used in preference to checking the B_ERROR bit and obtaining the b_error and b_xerror field values.
Indicates that data is to be read from the peripheral device into main memory.
Indicates that the data is to be transferred from main memory to the peripheral device. B_WRITE is a pseudo flag and cannot be directly tested; it is only detected as the NOT form of B_READ.
av_forw and av_back can be used by the driver to link the buffer into driver work lists.
b_bcount specifies the number of bytes to be transferred in both a paged and a non-paged I/O request.
b_un.b_addr must only be referenced after calling bp_mapin(9F). After bp_mapin(), b_un.b_addr is the virtual address of the buffer data associated with the I/O request. To efficiently check buffer data alignment, without calling bp_mapin(), a driver should use bioaligned(9F).
b_blkno identifies which logical block on the device (the device is defined by the device number) is to be accessed. The driver might have to convert this logical block number to a physical location such as a cylinder, track, and sector of a disk. This is a 32-bit value. The driver should use b_blkno or b_lblkno, but not both.
b_lblkno identifies which logical block on the device (the device is defined by the device number) is to be accessed. The driver might have to convert this logical block number to a physical location such as a cylinder, track, and sector of a disk. This is a 64-bit value. The driver should use b_lblkno or b_blkno, but not both.
b_resid should be set to the number of bytes not transferred because of an error.
b_bufsize contains the size of the allocated buffer.
b_iodone identifies a specific biodone routine to be called by the driver when the I/O is complete.
b_error can hold an error number that should be passed as a return code from the driver. b_error is set in conjunction with the B_ERROR bit set in the b_flags field. bioerror(9F) should be used in preference to directly setting the B_ERROR bit and b_error field. geterror(9F) should be used in preference to directly checking the B_ERROR bit or obtaining the b_error field value. The b_error values chosen should be constrained by the Intro(2) system calls error numbers associated with the buf operation: see read(2), write(2) for details.
b_xerror can hold an extended error code. b_xerror is set in conjunction with b_error field and the B_ERROR bit in the b_flags field. bioxerror(9F) should be used in preference to directly setting the B_ERROR bit and the b_error and b_xerror fields. getxerror(9F) should be used in preference to directly obtaining the b_xerror field value.
Valid b_xerror values are as follows:
I/O execution succeeded, or I/O execution failed but no extended error information was established via bioxerror(9F).
I/O execution succeeded, but driver retry and recovery operations were necessary to complete the I/O successfully.
I/O execution failed due to an error associated with reading the device medium.
I/O execution failed due to an error associated with writing the device medium.
I/O execution failed due to an error associated with the device medium.
I/O execution failed due to a device error unrelated to device medium.
I/O execution failed due to unexpected protocol data.
I/O execution failed due to unexpected protocol status.
I/O execution failed due to unexpected protocol error.
I/O execution failed due to a transport timeout.
I/O execution failed due to a transport not currently being able to address the device.
I/O execution failed due to some type of transport issue.
I/O execution failed due to device was live suspended.
b_private is for the private use of the device driver.
b_edev contains the major and minor device numbers of the device accessed.
strategy(9E), aphysio(9F), bioaligned(9F), bioclone(9F), biodone(9F), bioerror(9F), bioxerror(9F), bioinit(9F), bp_mapin(9F), clrbuf(9F), geterror(9F), getxerror(9F), getrbuf(9F), physio(9F), iovec(9S), uio(9S)
Writing Device Drivers in Oracle Solaris 11.4
Buffers are a shared resource within the kernel. Drivers should read or write only the members listed in this section. Drivers that attempt to use undocumented members of the buf structure risk corrupting data in the kernel or on the device.