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.\"
.\" Copyright (c) 2002-2015 Mark R V Murray
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
.\" BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
.\" AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" $FreeBSD$
.\"
.Dd August 26, 2018
.Dt RANDOM_HARVEST 9
.Os
.Sh NAME
.Nm random_harvest
.Nd gather entropy from the kernel for the entropy device
.Sh SYNOPSIS
.In sys/types.h
.In sys/random.h
.Ft void
.Fo random_harvest_direct
.Fa "void *entropy"
.Fa "u_int size"
.Fa "enum esource source"
.Fc
.Ft void
.Fo random_harvest_fast
.Fa "void *entropy"
.Fa "u_int size"
.Fa "enum esource source"
.Fc
.Ft void
.Fo random_harvest_queue
.Fa "void *entropy"
.Fa "u_int size"
.Fa "enum esource source"
.Fc
.Sh DESCRIPTION
The
.Fn random_harvest_*
functions are used by device drivers
and other kernel processes to pass data
that is considered (at least partially) stochastic
to the entropy device.
.Pp
The caller should pass
a pointer pointing to the
.Dq random
data in
.Fa entropy .
The argument
.Fa size
contains the number of bytes pointed to.
The
.Fa source
is chosen from one of
the values enumerated in
.Pa sys/dev/random.h .
and is used to indicate the source of the entropy.
.Pp
The
.Fo random_harvest_direct
.Fc
variant is used
for early harvesting
before any multitasking
is enabled.
.Pp
The
.Fn random_harvest_fast
variant is used
by sources that
should not take
a performance hit
from harvesting,
as they are high-rate
sources.
Some entropy is sacrificed,
but the high rate of supply
will compensate for this.
.Pp
The
.Fn random_harvest_queue
variant is used
for general harvesting
and is the default
choice for most entropy sources
such as interrupts
or console events.
.Pp
Interrupt harvesting has been
in part simplified
for the kernel programmer.
If a device driver registers an interrupt handler
with
.Xr BUS_SETUP_INTR 9
or
.Xr bus_setup_intr 9 ,
then it is only necessary to
include the
.Dv INTR_ENTROPY
bit in the
.Fa flags
argument to have that interrupt source
be used for entropy harvesting.
This should be done wherever practicable.
.Sh SEE ALSO
.Xr random 4 ,
.Xr BUS_SETUP_INTR 9
.Sh AUTHORS
The
.Fx
.Xr random 4
entropy device and supporting documentation was written by
.An Mark R V Murray .
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