Writing Device Drivers

Vectored I/O

In character drivers, transfers are described by a uio(9S) structure. The uio(9S) structure contains information about the direction and size of the transfer, plus an array of buffers for one end of the transfer. The other end is the device.

The uio(9S) structure contains the following members:

iovec_t       *uio_iov;       /* base address of the iovec */
                              /* buffer description array */
int           uio_iovcnt;     /* the number of iovec structures */
off_t         uio_offset;     /* 32-bit offset into file where */
                              /* data is transferred from or to */
offset_t      uio_loffset;    /* 64-bit offset into file where */
                              /* data is transferred from or to */
uio_seg_t     uio_segflg;     /* identifies the type of I/O transfer */
                              /* UIO_SYSSPACE:  kernel <-> kernel */
                              /* UIO_USERSPACE: kernel <-> user */
short         uio_fmode;      /* file mode flags (not driver setTable) */
daddr_t       uio_limit;      /* 32-bit ulimit for file (maximum */
                              /* block offset). not driver settable. */
diskaddr_t    uio_llimit;     /* 64-bit ulimit for file (maximum block */
                              /* block offset). not driver settable. */
int           uio_resid;      /* amount (in bytes) not */
                              /* transferred on completion */

A uio(9S) structure is passed to the driver read(9E) and write(9E) entry points. This structure is generalized to support what is called gather-write and scatter-read. When writing to a device, the data buffers to be written do not have to be contiguous in application memory. Similarly, data that is transferred from a device into memory comes off in a contiguous stream but can go into noncontiguous areas of application memory. See the readv(2), writev(2), pread(2), and pwrite(2) man pages for more information on scatter-gather I/O.

Each buffer is described by an iovec(9S) structure. This structure contains a pointer to the data area and the number of bytes to be transferred.

caddr_t    iov_base;    /* address of buffer */
int        iov_len;     /* amount to transfer */

The uio structure contains a pointer to an array of iovec(9S) structures. The base address of this array is held in uio_iov, and the number of elements is stored in uio_iovcnt.

The uio_offset field contains the 32-bit offset into the device at which the application needs to begin the transfer. uio_loffset is used for 64-bit file offsets. If the device does not support the notion of an offset, these fields can be safely ignored. The driver should interpret either uio_offset or uio_loffset, but not both. If the driver has set the D_64BIT flag in the cb_ops(9S) structure, that driver should use uio_loffset.

The uio_resid field starts out as the number of bytes to be transferred, that is, the sum of all the iov_len fields in uio_iov. This field must be set by the driver to the number of bytes that were not transferred before returning. The read(2) and write(2) system calls use the return value from the read(9E) and write(9E) entry points to determine failed transfers. If a failure occurs, these routines return -1. If the return value indicates success, the system calls return the number of bytes requested minus uio_resid. If uio_resid is not changed by the driver, the read(2) and write(2) calls return 0. A return value of 0 indicates end-of-file, even though all the data has been transferred.

The support routines uiomove(9F), physio(9F), and aphysio(9F) update the uio(9S) structure directly. These support routines update the device offset to account for the data transfer. Neither the uio_offset or uio_loffset fields need to be adjusted when the driver is used with a seekable device that uses the concept of position. I/O performed to a device in this manner is constrained by the maximum possible value of uio_offset or uio_loffset. An example of such a usage is raw I/O on a disk.

If the device has no concept of position, the driver can take the following steps:

  1. Save uio_offset or uio_loffset.

  2. Perform the I/O operation.

  3. Restore uio_offset or uio_loffset to the field's initial value.

I/O that is performed to a device in this manner is not constrained by the maximum possible value of uio_offset or uio_loffset. An example of this type of usage is I/O on a serial line.

The following example shows one way to preserve uio_loffset in the read(9E) function.

static int
xxread(dev_t dev, struct uio *uio_p, cred_t *cred_p)
    offset_t off;
    /* ... */
    off = uio_p->uio_loffset;  /* save the offset */
    /* do the transfer */
    uio_p->uio_loffset = off;  /* restore it */