freebsd-skq/share/man/man9/physio.9

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.Dd January 19, 2012
.Dt PHYSIO 9
.Os
.Sh NAME
.Nm physio
.Nd initiate I/O on raw devices
.Sh SYNOPSIS
.In sys/param.h
.In sys/systm.h
.In sys/bio.h
.In sys/buf.h
.Ft int
.Fn physio "struct cdev *dev" "struct uio *uio" "int ioflag"
.Sh DESCRIPTION
The
.Fn physio
is a helper function typically called from character device
.Fn read
and
.Fn write
routines to start I/O on a user process buffer.
The maximum amount of data to transfer with each call
is determined by
.Fa dev->si_iosize_max .
The
.Fn physio
call converts the I/O request into a
.Fn strategy
request and passes the new request to the driver's
.Fn strategy
routine for processing.
.Pp
Since
.Fa uio
normally describes user space addresses,
.Fn physio
needs to lock those pages into memory.
This is done by calling
.Fn vmapbuf
for the appropriate pages.
.Fn physio
always awaits the completion of the entire requested transfer before
returning, unless an error condition is detected earlier.
.Pp
A break-down of the arguments follows:
.Bl -tag -width indent
.It Fa dev
The device number identifying the device to interact with.
.It Fa uio
The description of the entire transfer as requested by the user process.
Currently, the results of passing a
.Fa uio
structure with the
.Va uio_segflg
set to anything other than
.Dv UIO_USERSPACE
are undefined.
.It Fa ioflag
The ioflag argument from the
.Fn read
or
.Fn write
function calling
.Fn physio .
.El
.Sh RETURN VALUES
If successful
.Fn physio
returns 0.
.Er EFAULT
is returned if the address range described by
.Fa uio
is not accessible by the requesting process.
.Fn physio
will return any error resulting from calls to the device strategy routine,
by examining the
.Dv B_ERROR
buffer flag and the
.Va b_error
field.
Note that the actual transfer size may be less than requested by
.Fa uio
if the device signals an
.Dq "end of file"
condition.
.Sh SEE ALSO
.Xr read 2 ,
.Xr write 2
.Sh HISTORY
The
.Nm
manual page is originally from
.Nx
with minor changes for applicability with
.Fx .
.Pp
The
.Nm
call has been completely re-written for providing higher
I/O and paging performance.