NAME
physio —
initiate I/O on raw devices
SYNOPSIS
int
physio(void (*strategy)(struct buf
*), dev_t dev, int flags,
void (*minphys)(struct buf *), struct
uio *uio);
DESCRIPTION
physio()
is a helper function typically called from character device read and write
routines to start I/O on a user process buffer. It calls back on the
provided strategy routine one or more times to
complete the transfer described by uio. The maximum
amount of data to transfer with each call to strategy
is determined by the minphys routine. Since
uio normally describes user space addresses,
physio() needs to lock the appropriate data area
into memory before each transaction with strategy (see
the
uvm_vslock()
and
uvm_vsunlock()
functions in uvm(9)). physio() always awaits the
completion of the entire requested transfer before returning, unless an
error condition is detected earlier.
In all cases, a temporary buffer is allocated from a system pool.
This buffer will have the B_BUSY,
B_PHYS, and B_RAW flags set
when passed to the strategy routine.
A break-down of the arguments follows:
- strategy
- The device strategy routine to call for each chunk of data to initiate device I/O.
- dev
- The device number identifying the device to interact with.
- flags
- Direction of transfer; the only valid settings are
B_READorB_WRITE. - minphys
- A device specific routine called to determine the maximum transfer size that the device's strategy routine can handle.
- uio
- The description of the entire transfer as requested by the user process.
Currently, the results of passing a uio structure
with the ‘uio_segflg’ set to anything other than
UIO_USERSPACE, are undefined.
RETURN VALUES
If successful, physio() returns 0.
EFAULT is returned if the address range described by
uio is not accessible by the requesting process.
physio() will return any error resulting from calls
to the device strategy routine, by examining the
B_ERROR buffer flag and the ‘b_error’
field. Note that the actual transfer size may be less than requested by
uio if the device signals an “end of
file” condition.