xref: /dragonfly/share/man/man9/bus_space.9 (revision a68e0df0)

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.\" ``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 FOUNDATION OR CONTRIBUTORS
.\" 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
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.\" $FreeBSD: src/share/man/man9/bus_space.9,v 1.5 2008/03/07 17:02:34 rwatson Exp $
.\" $DragonFly: src/share/man/man9/bus_space.9,v 1.2 2008/04/20 22:24:53 swildner Exp $
.\"
.Dd March 16, 2008
.Dt BUS_SPACE 9
.Os
.Sh NAME
.Nm bus_space ,
.Nm bus_space_barrier ,
.Nm bus_space_copy_region_1 ,
.Nm bus_space_copy_region_2 ,
.Nm bus_space_copy_region_4 ,
.Nm bus_space_copy_region_stream_1 ,
.Nm bus_space_copy_region_stream_2 ,
.Nm bus_space_copy_region_stream_4 ,
.Nm bus_space_free ,
.Nm bus_space_map ,
.Nm bus_space_read_1 ,
.Nm bus_space_read_2 ,
.Nm bus_space_read_4 ,
.Nm bus_space_read_multi_1 ,
.Nm bus_space_read_multi_2 ,
.Nm bus_space_read_multi_4 ,
.Nm bus_space_read_multi_stream_1 ,
.Nm bus_space_read_multi_stream_2 ,
.Nm bus_space_read_multi_stream_4 ,
.Nm bus_space_read_region_1 ,
.Nm bus_space_read_region_2 ,
.Nm bus_space_read_region_4 ,
.Nm bus_space_read_region_stream_1 ,
.Nm bus_space_read_region_stream_2 ,
.Nm bus_space_read_region_stream_4 ,
.Nm bus_space_read_stream_1 ,
.Nm bus_space_read_stream_2 ,
.Nm bus_space_read_stream_4 ,
.Nm bus_space_set_multi_1 ,
.Nm bus_space_set_multi_2 ,
.Nm bus_space_set_multi_4 ,
.Nm bus_space_set_multi_stream_1 ,
.Nm bus_space_set_multi_stream_2 ,
.Nm bus_space_set_multi_stream_4 ,
.Nm bus_space_set_region_1 ,
.Nm bus_space_set_region_2 ,
.Nm bus_space_set_region_4 ,
.Nm bus_space_set_region_stream_1 ,
.Nm bus_space_set_region_stream_2 ,
.Nm bus_space_set_region_stream_4 ,
.Nm bus_space_subregion ,
.Nm bus_space_unmap ,
.Nm bus_space_write_1 ,
.Nm bus_space_write_2 ,
.Nm bus_space_write_4 ,
.Nm bus_space_write_multi_1 ,
.Nm bus_space_write_multi_2 ,
.Nm bus_space_write_multi_4 ,
.Nm bus_space_write_multi_stream_1 ,
.Nm bus_space_write_multi_stream_2 ,
.Nm bus_space_write_multi_stream_4 ,
.Nm bus_space_write_region_1 ,
.Nm bus_space_write_region_2 ,
.Nm bus_space_write_region_4 ,
.Nm bus_space_write_region_stream_1 ,
.Nm bus_space_write_region_stream_2 ,
.Nm bus_space_write_region_stream_4 ,
.Nm bus_space_write_stream_1 ,
.Nm bus_space_write_stream_2 ,
.Nm bus_space_write_stream_4
.Nd "bus space manipulation functions"
.Sh SYNOPSIS
.In sys/bus.h
.Ft int
.Fo bus_space_map
.Fa "bus_space_tag_t space" "bus_addr_t address"
.Fa "bus_size_t size" "int flags" "bus_space_handle_t *handlep"
.Fc
.Ft void
.Fo bus_space_unmap
.Fa "bus_space_tag_t space" "bus_space_handle_t handle" "bus_size_t size"
.Fc
.Ft int
.Fo bus_space_subregion
.Fa "bus_space_tag_t space" "bus_space_handle_t handle"
.Fa "bus_size_t offset" "bus_size_t size" "bus_space_handle_t *nhandlep"
.Fc
.Ft int
.Fo bus_space_alloc
.Fa "bus_space_tag_t space" "bus_addr_t reg_start" "bus_addr_t reg_end"
.Fa "bus_size_t size" "bus_size_t alignment" "bus_size_t boundary"
.Fa "int flags" "bus_addr_t *addrp" "bus_space_handle_t *handlep"
.Fc
.Ft void
.Fo bus_space_free
.Fa "bus_space_tag_t space" "bus_space_handle_t handle" "bus_size_t size"
.Fc
.Ft u_int8_t
.Fo bus_space_read_1
.Fa "bus_space_tag_t space" "bus_space_handle_t handle" "bus_size_t offset"
.Fc
.Ft u_int16_t
.Fo bus_space_read_2
.Fa "bus_space_tag_t space" "bus_space_handle_t handle" "bus_size_t offset"
.Fc
.Ft u_int32_t
.Fo bus_space_read_4
.Fa "bus_space_tag_t space" "bus_space_handle_t handle" "bus_size_t offset"
.Fc
.Ft u_int8_t
.Fo bus_space_read_stream_1
.Fa "bus_space_tag_t space" "bus_space_handle_t handle" "bus_size_t offset"
.Fc
.Ft u_int16_t
.Fo bus_space_read_stream_2
.Fa "bus_space_tag_t space" "bus_space_handle_t handle" "bus_size_t offset"
.Fc
.Ft u_int32_t
.Fo bus_space_read_stream_4
.Fa "bus_space_tag_t space" "bus_space_handle_t handle" "bus_size_t offset"
.Fc
.Ft void
.Fo bus_space_write_1
.Fa "bus_space_tag_t space" "bus_space_handle_t handle"
.Fa "bus_size_t offset" "u_int8_t value"
.Fc
.Ft void
.Fo bus_space_write_2
.Fa "bus_space_tag_t space" "bus_space_handle_t handle"
.Fa "bus_size_t offset" "u_int16_t value"
.Fc
.Ft void
.Fo bus_space_write_4
.Fa "bus_space_tag_t space" "bus_space_handle_t handle"
.Fa "bus_size_t offset" "u_int32_t value"
.Fc
.Ft void
.Fo bus_space_write_stream_1
.Fa "bus_space_tag_t space" "bus_space_handle_t handle"
.Fa "bus_size_t offset" "u_int8_t value"
.Fc
.Ft void
.Fo bus_space_write_stream_2
.Fa "bus_space_tag_t space" "bus_space_handle_t handle"
.Fa "bus_size_t offset" "u_int16_t value"
.Fc
.Ft void
.Fo bus_space_write_stream_4
.Fa "bus_space_tag_t space" "bus_space_handle_t handle"
.Fa "bus_size_t offset" "u_int32_t value"
.Fc
.Ft void
.Fo bus_space_barrier
.Fa "bus_space_tag_t space" "bus_space_handle_t handle"
.Fa "bus_size_t offset" "bus_size_t length" "int flags"
.Fc
.Ft void
.Fo bus_space_read_region_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int8_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_read_region_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int16_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_read_region_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int32_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_read_region_stream_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int8_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_read_region_stream_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int16_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_read_region_stream_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int32_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_write_region_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int8_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_write_region_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int16_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_write_region_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int32_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_write_region_stream_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int8_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_write_region_stream_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int16_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_write_region_stream_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int32_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_copy_region_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t srchandle" "bus_size_t srcoffset"
.Fa "bus_space_handle_t dsthandle" "bus_size_t dstoffset" "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_copy_region_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t srchandle" "bus_size_t srcoffset"
.Fa "bus_space_handle_t dsthandle" "bus_size_t dstoffset" "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_copy_region_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t srchandle" "bus_size_t srcoffset"
.Fa "bus_space_handle_t dsthandle" "bus_size_t dstoffset" "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_copy_region_stream_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t srchandle" "bus_size_t srcoffset"
.Fa "bus_space_handle_t dsthandle" "bus_size_t dstoffset" "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_copy_region_stream_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t srchandle" "bus_size_t srcoffset"
.Fa "bus_space_handle_t dsthandle" "bus_size_t dstoffset" "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_copy_region_stream_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t srchandle" "bus_size_t srcoffset"
.Fa "bus_space_handle_t dsthandle" "bus_size_t dstoffset" "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_set_region_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int8_t value"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_set_region_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int16_t value"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_set_region_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int32_t value"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_set_region_stream_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int8_t value"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_set_region_stream_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int16_t value"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_set_region_stream_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int32_t value"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_read_multi_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int8_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_read_multi_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int16_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_read_multi_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int32_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_read_multi_stream_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int8_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_read_multi_stream_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int16_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_read_multi_stream_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int32_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_write_multi_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int8_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_write_multi_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int16_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_write_multi_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int32_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_write_multi_stream_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int8_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_write_multi_stream_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int16_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_write_multi_stream_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int32_t *datap"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_set_multi_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int8_t value"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_set_multi_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int16_t value"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_set_multi_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int32_t value"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_set_multi_stream_1
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int8_t value"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_set_multi_stream_2
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int16_t value"
.Fa "bus_size_t count"
.Fc
.Ft void
.Fo bus_space_set_multi_stream_4
.Fa "bus_space_tag_t space"
.Fa "bus_space_handle_t handle" "bus_size_t offset" "u_int32_t value"
.Fa "bus_size_t count"
.Fc
.Sh DESCRIPTION
The
.Nm
functions exist to allow device drivers
machine-independent access to bus memory and register areas.
All of the
functions and types described in this document can be used by including
the
.In sys/bus.h
header file.
.Pp
Many common devices are used on multiple architectures, but are accessed
differently on each because of architectural constraints.
For instance, a device which is mapped in one system's I/O space may be
mapped in memory space on a second system.
On a third system, architectural
limitations might change the way registers need to be accessed (e.g.\&
creating a non-linear register space).
In some cases, a single
driver may need to access the same type of device in multiple ways in a
single system or architecture.
The goal of the
.Nm
functions is to allow a single driver source file to manipulate a set
of devices on different system architectures, and to allow a single driver
object file to manipulate a set of devices on multiple bus types on a
single architecture.
.Pp
Not all busses have to implement all functions described in this
document, though that is encouraged if the operations are logically
supported by the bus.
Unimplemented functions should cause
compile-time errors if possible.
.Pp
All of the interface definitions described in this document are shown as
function prototypes and discussed as if they were required to be
functions.
Implementations are encouraged to implement prototyped
(type-checked) versions of these interfaces, but may implement them as
macros if appropriate.
Machine-dependent types, variables, and functions
should be marked clearly
to avoid confusion with the
machine-independent types and functions, and, if possible, should be
given names which make the machine-dependence clear.
.Sh CONCEPTS AND GUIDELINES
Bus spaces are described by bus space tags, which can be created only by
machine-dependent code.
A given machine may have several different types
of bus space (e.g.\& memory space and I/O space), and thus may provide
multiple different bus space tags.
Individual busses or devices on a machine may use more than one bus space
tag.
For instance, ISA devices are
given an ISA memory space tag and an ISA I/O space tag.
Architectures
may have several different tags which represent the same type of
space, for instance because of multiple different host bus interface
chipsets.
.Pp
A range in bus space is described by a bus address and a bus size.
The
bus address describes the start of the range in bus space.
The bus
size describes the size of the range in bytes.
Busses which are not byte
addressable may require use of bus space ranges with appropriately
aligned addresses and properly rounded sizes.
.Pp
Access to regions of bus space is facilitated by use of bus space handles,
which are usually created by mapping a specific range of a bus space.
Handles may also be created by allocating
and mapping a range of bus space, the actual location of which is picked
by the implementation within bounds specified by the caller of the
allocation function.
.Pp
All of the bus space access functions require one bus space tag
argument, at least one handle argument, and at least one offset argument
(a bus size).
The bus space tag specifies the space, each handle specifies a region in
the space, and each offset specifies the offset into the region of the
actual location(s) to be accessed.
Offsets are given in bytes, though busses
may impose alignment constraints.
The offset used to access data
relative to a given handle must be such that all of the data being
accessed is in the mapped region that the handle describes.
Trying to
access data outside that region is an error.
.Pp
Because some architectures' memory systems use buffering to improve
memory and device access performance, there is a mechanism which can be
used to create
.Dq barriers
in the bus space read and write stream.
There
are three types of barriers: read, write, and read/write.
All reads
started to the region before a read barrier must complete before any reads
after the read barrier are started.
(The analogous requirement is true for
write barriers.)
Read/write barriers force all reads and writes started
before the barrier to complete before any reads or writes after the
barrier are started.
Correctly-written drivers will include all
appropriate barriers, and assume only the read/write ordering imposed by
the barrier operations.
.Pp
People trying to write portable drivers with the
.Nm
functions should
try to make minimal assumptions about what the system allows.
In particular,
they should expect that the system requires bus space addresses being
accessed to be naturally aligned (i.e., base address of handle added to
offset is a multiple of the access size), and that the system does
alignment checking on pointers (i.e., pointer to objects being read and
written must point to properly-aligned data).
.Pp
The descriptions of the
.Nm
functions given below all assume that
they are called with proper arguments.
If called with invalid arguments
or arguments that are out of range (e.g.\& trying to access data outside of
the region mapped when a given handle was created), undefined behaviour
results.
In that case, they may cause the
system to halt, either intentionally (via panic) or unintentionally (by
causing a fatal trap of by some other means) or may cause improper
operation which is not immediately fatal.
Functions which return
.Ft void
or which return data read from bus space (i.e., functions which
do not obviously return an error code) do not fail.
They could only fail
if given invalid arguments, and in that case their behaviour is undefined.
.Sh TYPES
Several types are defined
to facilitate use of the
.Nm
functions by drivers.
.Ss Vt bus_addr_t
The
.Vt bus_addr_t
type is used to describe bus addresses.
It must be an
unsigned integral type
capable of holding the largest bus address usable by the architecture.
This
type is primarily used when mapping and unmapping bus space.
.Ss Vt bus_size_t
The
.Vt bus_size_t
type is used to describe sizes of ranges in bus space.
It must be an
unsigned integral type capable of holding the size of the largest bus
address range usable on the architecture.
This type is used by virtually all
of the
.Nm
functions, describing sizes when mapping regions and
offsets into regions when performing space access operations.
.Ss Vt bus_space_tag_t
The
.Vt bus_space_tag_t
type is used to describe a particular bus space on a machine.
Its
contents are machine-dependent and should be considered opaque by
machine-independent code.
This type is used by all
.Nm
functions to name the space on which they are operating.
.Ss Vt bus_space_handle_t
The
.Vt bus_space_handle_t
type is used to describe a mapping of a range of bus space.
Its
contents are machine-dependent and should be considered opaque by
machine-independent code.
This type is used when performing bus space
access operations.
.Sh MAPPING AND UNMAPPING BUS SPACE
This section is specific to the
.Nx
version of these functions and may or may not apply to the
.Dx
version.
.Pp
Bus space must be mapped before it can be used, and should be
unmapped when it is no longer needed.
The
.Fn bus_space_map
and
.Fn bus_space_unmap
functions provide these capabilities.
.Pp
Some drivers need to be able to pass a subregion of already-mapped bus
space to another driver or module within a driver.
The
.Fn bus_space_subregion
function allows such subregions to be created.
.Ss Fn bus_space_map space address size flags handlep
The
.Fn bus_space_map
function maps the region of bus space named by the
.Fa space , address ,
and
.Fa size
arguments.
If successful, it returns zero
and fills in the bus space handle pointed to by
.Fa handlep
with the handle
that can be used to access the mapped region.
If unsuccessful,
it will return non-zero and leave the bus space handle pointed
to by
.Fa handlep
in an undefined state.
.Pp
The
.Fa flags
argument controls how the space is to be mapped.
Supported flags include:
.Bl -tag -width ".Dv BUS_SPACE_MAP_CACHEABLE"
.It Dv BUS_SPACE_MAP_CACHEABLE
Try to map the space so that accesses can be cached and/or
prefetched by the system.
If this flag is not specified, the
implementation should map the space so that it will not be cached or
prefetched.
.Pp
This flag must have a value of 1 on all implementations for backward
compatibility.
.It Dv BUS_SPACE_MAP_LINEAR
Try to map the space so that its contents can be accessed linearly via
normal memory access methods (e.g.\& pointer dereferencing and structure
accesses).
This is useful when software wants to do direct access to a memory
device, e.g.\& a frame buffer.
If this flag is specified and linear
mapping is not possible, the
.Fn bus_space_map
call should fail.
If this
flag is not specified, the system may map the space in whatever way is
most convenient.
.El
.Pp
Not all combinations of flags make sense or are supported with all
spaces.
For instance,
.Dv BUS_SPACE_MAP_CACHEABLE
may be meaningless when
used on many systems' I/O port spaces, and on some systems
.Dv BUS_SPACE_MAP_LINEAR
without
.Dv BUS_SPACE_MAP_CACHEABLE
may never work.
When the system hardware or firmware provides hints as to how spaces should be
mapped (e.g.\& the PCI memory mapping registers'
.Dq prefetchable
bit), those
hints should be followed for maximum compatibility.
On some systems,
requesting a mapping that cannot be satisfied (e.g.\& requesting a
non-cacheable mapping when the system can only provide a cacheable one)
will cause the request to fail.
.Pp
Some implementations may keep track of use of bus space for some or all
bus spaces and refuse to allow duplicate allocations.
This is encouraged
for bus spaces which have no notion of slot-specific space addressing,
such as ISA and VME, and for spaces which coexist with those spaces
(e.g.\& EISA and PCI memory and I/O spaces co-existing with ISA memory and
I/O spaces).
.Pp
Mapped regions may contain areas for which there is no device on the
bus.
If space in those areas is accessed, the results are
bus-dependent.
.Ss Fn bus_space_unmap space handle size
The
.Fn bus_space_unmap
function unmaps a region of bus space mapped with
.Fn bus_space_map .
When unmapping a region, the
.Fa size
specified should be
the same as the size given to
.Fn bus_space_map
when mapping that region.
.Pp
After
.Fn bus_space_unmap
is called on a handle, that handle is no longer
valid.
(If copies were made of the handle they are no longer valid,
either.)
.Pp
This function will never fail.
If it would fail (e.g.\& because of an
argument error), that indicates a software bug which should cause a
panic.
In that case,
.Fn bus_space_unmap
will never return.
.Ss Fn bus_space_subregion space handle offset size nhandlep
The
.Fn bus_space_subregion
function is a convenience function which makes a
new handle to some subregion of an already-mapped region of bus space.
The subregion described by the new handle starts at byte offset
.Fa offset
into the region described by
.Fa handle ,
with the size give by
.Fa size ,
and must be wholly contained within the original region.
.Pp
If successful,
.Fn bus_space_subregion
returns zero and fills in the bus
space handle pointed to by
.Fa nhandlep .
If unsuccessful, it returns non-zero and leaves the bus space handle
pointed to by
.Fa nhandlep
in an
undefined state.
In either case, the handle described by
.Fa handle
remains valid and is unmodified.
.Pp
When done with a handle created by
.Fn bus_space_subregion ,
the handle should
be thrown away.
Under no circumstances should
.Fn bus_space_unmap
be used on the handle.
Doing so may confuse any resource management
being done on the space, and will result in undefined behaviour.
When
.Fn bus_space_unmap
or
.Fn bus_space_free
is called on a handle, all subregions of that handle become invalid.
.Sh ALLOCATING AND FREEING BUS SPACE
This section is specific to the
.Nx
version of these functions and may or may not apply to the
.Dx
version.
.Pp
Some devices require or allow bus space to be allocated by the operating
system for device use.
When the devices no longer need the space, the
operating system should free it for use by other devices.
The
.Fn bus_space_alloc
and
.Fn bus_space_free
functions provide these capabilities.
.Ss Fn bus_space_alloc space reg_start reg_end size alignment boundary \
flags addrp handlep
The
.Fn bus_space_alloc
function allocates and maps a region of bus space with the size given by
.Fa size ,
corresponding to the given constraints.
If successful, it returns
zero, fills in the bus address pointed to by
.Fa addrp
with the bus space address of the allocated region, and fills in
the bus space handle pointed to by
.Fa handlep
with the handle that can be used to access that region.
If unsuccessful, it returns non-zero and leaves the bus address pointed to by
.Fa addrp
and the bus space handle pointed to by
.Fa handlep
in an undefined state.
.Pp
Constraints on the allocation are given by the
.Fa reg_start , reg_end , alignment ,
and
.Fa boundary
parameters.
The allocated region will start at or after
.Fa reg_start
and end before or at
.Fa reg_end .
The
.Fa alignment
constraint must be a power of two, and the allocated region will start at
an address that is an even multiple of that power of two.
The
.Fa boundary
constraint, if non-zero, ensures that the region is allocated so that
.Fa "first address in region"
/
.Fa boundary
has the same value as
.Fa "last address in region"
/
.Fa boundary .
If the constraints cannot be met,
.Fn bus_space_alloc
will fail.
It is an error to specify a set of
constraints that can never be met
(for example,
.Fa size
greater than
.Fa boundary ) .
.Pp
The
.Fa flags
parameter is the same as the like-named parameter to
.Fn bus_space_map ,
the same flag values should be used, and they have the
same meanings.
.Pp
Handles created by
.Fn bus_space_alloc
should only be freed with
.Fn bus_space_free .
Trying to use
.Fn bus_space_unmap
on them causes undefined behaviour.
The
.Fn bus_space_subregion
function can be used on
handles created by
.Fn bus_space_alloc .
.Ss Fn bus_space_free space handle size
The
.Fn bus_space_free
function unmaps and frees a region of bus space mapped
and allocated with
.Fn bus_space_alloc .
When unmapping a region, the
.Fa size
specified should be the same as the size given to
.Fn bus_space_alloc
when allocating the region.
.Pp
After
.Fn bus_space_free
is called on a handle, that handle is no longer valid.
(If copies were
made of the handle, they are no longer valid, either.)
.Pp
This function will never fail.
If it would fail (e.g.\& because of an
argument error), that indicates a software bug which should cause a
panic.
In that case,
.Fn bus_space_free
will never return.
.Sh READING AND WRITING SINGLE DATA ITEMS
The simplest way to access bus space is to read or write a single data
item.
The
.Fn bus_space_read_N
and
.Fn bus_space_write_N
families of functions provide
the ability to read and write 1, 2, and 4 byte data items on busses
which support those access sizes.
.Ss Fn bus_space_read_1 space handle offset
.Ss Fn bus_space_read_2 space handle offset
.Ss Fn bus_space_read_4 space handle offset
The
.Fn bus_space_read_N
family of functions reads a 1, 2, or 4 byte data item from
the offset specified by
.Fa offset
into the region specified by
.Fa handle
of the bus space specified by
.Fa space .
The location being read must lie within the bus space region specified by
.Fa handle .
.Pp
For portability, the starting address of the region specified by
.Fa handle
plus the offset should be a multiple of the size of data item being read.
On some systems, not obeying this requirement may cause incorrect data to
be read, on others it may cause a system crash.
.Pp
Read operations done by the
.Fn bus_space_read_N
functions may be executed out
of order with respect to other pending read and write operations unless
order is enforced by use of the
.Fn bus_space_barrier
function.
.Pp
These functions will never fail.
If they would fail (e.g.\& because of an
argument error), that indicates a software bug which should cause a
panic.
In that case, they will never return.
.Ss Fn bus_space_write_1 space handle offset value
.Ss Fn bus_space_write_2 space handle offset value
.Ss Fn bus_space_write_4 space handle offset value
The
.Fn bus_space_write_N
family of functions writes a 1, 2, or 4 byte data item to the offset
specified by
.Fa offset
into the region specified by
.Fa handle
of the bus space specified by
.Fa space .
The location being written must lie within
the bus space region specified by
.Fa handle .
.Pp
For portability, the starting address of the region specified by
.Fa handle
plus the offset should be a multiple of the size of data item being
written.
On some systems, not obeying this requirement may cause
incorrect data to be written, on others it may cause a system crash.
.Pp
Write operations done by the
.Fn bus_space_write_N
functions may be executed
out of order with respect to other pending read and write operations
unless order is enforced by use of the
.Fn bus_space_barrier
function.
.Pp
These functions will never fail.
If they would fail (e.g.\& because of an
argument error), that indicates a software bug which should cause a
panic.
In that case, they will never return.
.Sh BARRIERS
In order to allow high-performance buffering implementations to avoid bus
activity on every operation, read and write ordering should be specified
explicitly by drivers when necessary.
The
.Fn bus_space_barrier
function provides that ability.
.Ss Fn bus_space_barrier space handle offset length flags
The
.Fn bus_space_barrier
function enforces ordering of bus space read and write operations
for the specified subregion (described by the
.Fa offset
and
.Fa length
parameters) of the region named by
.Fa handle
in the space named by
.Fa space .
.Pp
The
.Fa flags
argument controls what types of operations are to be ordered.
Supported flags are:
.Bl -tag -width ".Dv BUS_SPACE_BARRIER_WRITE"
.It Dv BUS_SPACE_BARRIER_READ
Synchronize read operations.
.It Dv BUS_SPACE_BARRIER_WRITE
Synchronize write operations.
.El
.Pp
Those flags can be combined (or-ed together) to enforce ordering on both
read and write operations.
.Pp
All of the specified type(s) of operation which are done to the region
before the barrier operation are guaranteed to complete before any of the
specified type(s) of operation done after the barrier.
.Pp
Example: Consider a hypothetical device with two single-byte ports, one
write-only input port (at offset 0) and a read-only output port (at
offset 1).
Operation of the device is as follows: data bytes are written
to the input port, and are placed by the device on a stack, the top of
which is read by reading from the output port.
The sequence to correctly
write two data bytes to the device then read those two data bytes back
would be:
.Bd -literal
/*
 * t and h are the tag and handle for the mapped device's
 * space.
 */
bus_space_write_1(t, h, 0, data0);
bus_space_barrier(t, h, 0, 1, BUS_SPACE_BARRIER_WRITE);  /* 1 */
bus_space_write_1(t, h, 0, data1);
bus_space_barrier(t, h, 0, 2,
    BUS_SPACE_BARRIER_READ|BUS_SPACE_BARRIER_WRITE);     /* 2 */
ndata1 = bus_space_read_1(t, h, 1);
bus_space_barrier(t, h, 1, 1, BUS_SPACE_BARRIER_READ);   /* 3 */
ndata0 = bus_space_read_1(t, h, 1);
/* data0 == ndata0, data1 == ndata1 */
.Ed
.Pp
The first barrier makes sure that the first write finishes before the
second write is issued, so that two writes to the input port are done
in order and are not collapsed into a single write.
This ensures that
the data bytes are written to the device correctly and in order.
.Pp
The second barrier makes sure that the writes to the output port finish
before any of the reads to the input port are issued, thereby making sure
that all of the writes are finished before data is read.
This ensures
that the first byte read from the device really is the last one that was
written.
.Pp
The third barrier makes sure that the first read finishes before the
second read is issued, ensuring that data is read correctly and in order.
.Pp
The barriers in the example above are specified to cover the absolute
minimum number of bus space locations.
It is correct (and often
easier) to make barrier operations cover the device's whole range of bus
space, that is, to specify an offset of zero and the size of the
whole region.
.Sh REGION OPERATIONS
Some devices use buffers which are mapped as regions in bus space.
Often, drivers want to copy the contents of those buffers to or from
memory, e.g.\& into mbufs which can be passed to higher levels of the
system or from mbufs to be output to a network.
In order to allow
drivers to do this as efficiently as possible, the
.Fn bus_space_read_region_N
and
.Fn bus_space_write_region_N
families of functions are provided.
.Pp
Drivers occasionally need to copy one region of a bus space to another,
or to set all locations in a region of bus space to contain a single
value.
The
.Fn bus_space_copy_region_N
family of functions and the
.Fn bus_space_set_region_N
family of functions allow drivers to perform these operations.
.Ss Fn bus_space_read_region_1 space handle offset datap count
.Ss Fn bus_space_read_region_2 space handle offset datap count
.Ss Fn bus_space_read_region_4 space handle offset datap count
The
.Fn bus_space_read_region_N
family of functions reads
.Fa count
1, 2, or 4 byte data items from bus space
starting at byte offset
.Fa offset
in the region specified by
.Fa handle
of the bus space specified by
.Fa space
and writes them into the array specified by
.Fa datap .
Each successive data item is read from an offset
1, 2, or 4 bytes after the previous data item (depending on which
function is used).
All locations being read must lie within the bus
space region specified by
.Fa handle .
.Pp
For portability, the starting address of the region specified by
.Fa handle
plus the offset should be a multiple of the size of data items being
read and the data array pointer should be properly aligned.
On some
systems, not obeying these requirements may cause incorrect data to be
read, on others it may cause a system crash.
.Pp
Read operations done by the
.Fn bus_space_read_region_N
functions may be executed in any order.
They may also be executed out
of order with respect to other pending read and write operations unless
order is enforced by use of the
.Fn bus_space_barrier
function.
There is no way to insert barriers between reads of
individual bus space locations executed by the
.Fn bus_space_read_region_N
functions.
.Pp
These functions will never fail.
If they would fail (e.g.\& because of an
argument error), that indicates a software bug which should cause a
panic.
In that case, they will never return.
.Ss Fn bus_space_write_region_1 space handle offset datap count
.Ss Fn bus_space_write_region_2 space handle offset datap count
.Ss Fn bus_space_write_region_4 space handle offset datap count
The
.Fn bus_space_write_region_N
family of functions reads
.Fa count
1, 2, or 4 byte data items from the array
specified by
.Fa datap
and writes them to bus space starting at byte offset
.Fa offset
in the region specified by
.Fa handle
of the bus space specified
by
.Fa space .
Each successive data item is written to an offset 1, 2, or 4
bytes after the previous data item (depending on which function is
used).
All locations being written must lie within the bus space region
specified by
.Fa handle .
.Pp
For portability, the starting address of the region specified by
.Fa handle
plus the offset should be a multiple of the size of data items being
written and the data array pointer should be properly aligned.
On some
systems, not obeying these requirements may cause incorrect data to be
written, on others it may cause a system crash.
.Pp
Write operations done by the
.Fn bus_space_write_region_N
functions may be
executed in any order.
They may also be executed out of order with
respect to other pending read and write operations unless order is
enforced by use of the
.Fn bus_space_barrier
function.
There is no way to insert barriers between writes of
individual bus space locations executed by the
.Fn bus_space_write_region_N
functions.
.Pp
These functions will never fail.
If they would fail (e.g.\& because of an
argument error), that indicates a software bug which should cause a
panic.
In that case, they will never return.
.Ss Fn bus_space_copy_region_1 space srchandle srcoffset dsthandle \
dstoffset count
.Ss Fn bus_space_copy_region_2 space srchandle srcoffset dsthandle \
dstoffset count
.Ss Fn bus_space_copy_region_4 space srchandle srcoffset dsthandle \
dstoffset count
The
.Fn bus_space_copy_region_N
family of functions copies
.Fa count
1, 2, or 4 byte data items in bus space
from the area starting at byte offset
.Fa srcoffset
in the region specified by
.Fa srchandle
of the bus space specified by
.Fa space
to the area starting at byte offset
.Fa dstoffset
in the region specified by
.Fa dsthandle
in the same bus space.
Each successive data item read or written has
an offset 1, 2, or 4 bytes after the previous data item (depending
on which function is used).
All locations being read and written must
lie within the bus space region specified by their respective handles.
.Pp
For portability, the starting addresses of the regions specified by the
each handle plus its respective offset should be a multiple of the size
of data items being copied.
On some systems, not obeying this
requirement may cause incorrect data to be copied, on others it may cause
a system crash.
.Pp
Read and write operations done by the
.Fn bus_space_copy_region_N
functions may be executed in any order.
They may also be executed out
of order with respect to other pending read and write operations unless
order is enforced by use of the
.Fn bus_space_barrier
function .
There is no way to insert barriers between reads or writes of
individual bus space locations executed by the
.Fn bus_space_copy_region_N
functions.
.Pp
Overlapping copies between different subregions of a single region
of bus space are handled correctly by the
.Fn bus_space_copy_region_N
functions.
.Pp
These functions will never fail.
If they would fail (e.g.\& because of an
argument error), that indicates a software bug which should cause a
panic.
In that case, they will never return.
.Ss Fn bus_space_set_region_1 space handle offset value count
.Ss Fn bus_space_set_region_2 space handle offset value count
.Ss Fn bus_space_set_region_4 space handle offset value count
The
.Fn bus_space_set_region_N
family of functions writes the given
.Fa value
to
.Fa count
1, 2, or 4 byte
data items in bus space starting at byte offset
.Fa offset
in the region specified by
.Fa handle
of the bus space specified by
.Fa space .
Each successive data item has an offset 1, 2, or 4 bytes after the
previous data item (depending on which function is used).
All
locations being written must lie within the bus space region specified
by
.Fa handle .
.Pp
For portability, the starting address of the region specified by
.Fa handle
plus the offset should be a multiple of the size of data items being
written.
On some systems, not obeying this requirement may cause
incorrect data to be written, on others it may cause a system crash.
.Pp
Write operations done by the
.Fn bus_space_set_region_N
functions may be
executed in any order.
They may also be executed out of order with
respect to other pending read and write operations unless order is
enforced by use of the
.Fn bus_space_barrier
function.
There is no way to insert barriers between writes of
individual bus space locations executed by the
.Fn bus_space_set_region_N
functions.
.Pp
These functions will never fail.
If they would fail (e.g.\& because of an
argument error), that indicates a software bug which should cause a
panic.
In that case, they will never return.
.Sh READING AND WRITING A SINGLE LOCATION MULTIPLE TIMES
Some devices implement single locations in bus space which are to be read
or written multiple times to communicate data, e.g.\& some ethernet
devices' packet buffer FIFOs.
In order to allow drivers to manipulate
these types of devices as efficiently as possible, the
.Fn bus_space_read_multi_N ,
.Fn bus_space_set_multi_N ,
and
.Fn bus_space_write_multi_N
families of functions are provided.
.Ss Fn bus_space_read_multi_1 space handle offset datap count
.Ss Fn bus_space_read_multi_2 space handle offset datap count
.Ss Fn bus_space_read_multi_4 space handle offset datap count
The
.Fn bus_space_read_multi_N
family of functions reads
.Fa count
1, 2, or 4 byte data items from bus space
at byte offset
.Fa offset
in the region specified by
.Fa handle
of the bus space specified by
.Fa space
and writes them into the array specified by
.Fa datap .
Each successive data item is read from the same location in bus
space.
The location being read must lie within the bus space region
specified by
.Fa handle .
.Pp
For portability, the starting address of the region specified by
.Fa handle
plus the offset should be a multiple of the size of data items being
read and the data array pointer should be properly aligned.
On some
systems, not obeying these requirements may cause incorrect data to be
read, on others it may cause a system crash.
.Pp
Read operations done by the
.Fn bus_space_read_multi_N
functions may be
executed out of order with respect to other pending read and write
operations unless order is enforced by use of the
.Fn bus_space_barrier
function.
Because the
.Fn bus_space_read_multi_N
functions read the same bus space location multiple times, they
place an implicit read barrier between each successive read of that bus
space location.
.Pp
These functions will never fail.
If they would fail (e.g.\& because of an
argument error), that indicates a software bug which should cause a
panic.
In that case, they will never return.
.Ss Fn bus_space_write_multi_1 space handle offset datap count
.Ss Fn bus_space_write_multi_2 space handle offset datap count
.Ss Fn bus_space_write_multi_4 space handle offset datap count
The
.Fn bus_space_write_multi_N
family of functions reads
.Fa count
1, 2, or 4 byte data items from the array
specified by
.Fa datap
and writes them into bus space at byte offset
.Fa offset
in the region specified by
.Fa handle
of the bus space specified by
.Fa space .
Each successive data item is written to the same location in
bus space.
The location being written must lie within the bus space
region specified by
.Fa handle .
.Pp
For portability, the starting address of the region specified by
.Fa handle
plus the offset should be a multiple of the size of data items being
written and the data array pointer should be properly aligned.
On some
systems, not obeying these requirements may cause incorrect data to be
written, on others it may cause a system crash.
.Pp
Write operations done by the
.Fn bus_space_write_multi_N
functions may be executed out of order with respect to other pending
read and write operations unless order is enforced by use of the
.Fn bus_space_barrier
function.
Because the
.Fn bus_space_write_multi_N
functions write the same bus space location multiple times, they
place an implicit write barrier between each successive write of that
bus space location.
.Pp
These functions will never fail.
If they would fail (e.g.\& because of an
argument error), that indicates a software bug which should cause a
panic.
In that case, they will never return.
.Ss Fn bus_space_set_multi_1 space handle offset value count
.Ss Fn bus_space_set_multi_2 space handle offset value count
.Ss Fn bus_space_set_multi_4 space handle offset value count
The
.Fn bus_space_set_multi_N
writes
.Fa value
into bus space at byte offset
.Fa offset
in the region specified by
.Fa handle
of the bus space specified by
.Fa space ,
.Fa count
times.
The location being written must lie within the bus space
region specified by
.Fa handle .
.Pp
For portability, the starting address of the region specified by
.Fa handle
plus the offset should be a multiple of the size of data items being
written and the data array pointer should be properly aligned.
On some
systems, not obeying these requirements may cause incorrect data to be
written, on others it may cause a system crash.
.Pp
Write operations done by the
.Fn bus_space_set_multi_N
functions may be executed out of order with respect to other pending
read and write operations unless order is enforced by use of the
.Fn bus_space_barrier
function.
Because the
.Fn bus_space_set_multi_N
functions write the same bus space location multiple times, they
place an implicit write barrier between each successive write of that
bus space location.
.Pp
These functions will never fail.
If they would fail (e.g.\& because of an
argument error), that indicates a software bug which should cause a
panic.
In that case, they will never return.
.Sh STREAM FUNCTIONS
Most of the
.Nm
functions imply a host byte-order and a bus byte-order and take care of
any translation for the caller.
In some cases, however, hardware may map a FIFO or some other memory region
for which the caller may want to use multi-word, yet untranslated access.
Access to these types of memory regions should be with the
.Fn bus_space_*_stream_N
functions.
.Pp
.Bl -tag -compact
.It Fn bus_space_read_stream_1
.It Fn bus_space_read_stream_2
.It Fn bus_space_read_stream_4
.It Fn bus_space_read_multi_stream_1
.It Fn bus_space_read_multi_stream_2
.It Fn bus_space_read_multi_stream_4
.It Fn bus_space_read_region_stream_1
.It Fn bus_space_read_region_stream_2
.It Fn bus_space_read_region_stream_4
.It Fn bus_space_write_stream_1
.It Fn bus_space_write_stream_2
.It Fn bus_space_write_stream_4
.It Fn bus_space_write_multi_stream_1
.It Fn bus_space_write_multi_stream_2
.It Fn bus_space_write_multi_stream_4
.It Fn bus_space_write_region_stream_1
.It Fn bus_space_write_region_stream_2
.It Fn bus_space_write_region_stream_4
.It Fn bus_space_copy_region_stream_1
.It Fn bus_space_copy_region_stream_2
.It Fn bus_space_copy_region_stream_4
.It Fn bus_space_set_multi_stream_1
.It Fn bus_space_set_multi_stream_2
.It Fn bus_space_set_multi_stream_4
.It Fn bus_space_set_region_stream_1
.It Fn bus_space_set_region_stream_2
.It Fn bus_space_set_region_stream_4
.El
.Pp
These functions are defined just as their non-stream counterparts,
except that they provide no byte-order translation.
.Sh COMPATIBILITY
The current
.Nx
version of the
.Nm
interface specification differs slightly from the original
specification that came into wide use and
.Fx
adopted.
A few of the function names and arguments have changed
for consistency and increased functionality.
.Sh SEE ALSO
.Xr bus_dma 9
.Sh HISTORY
The
.Nm
functions were introduced in a different form (memory and I/O spaces
were accessed via different sets of functions) in
.Nx 1.2 .
The functions were merged to work on generic
.Dq spaces
early in the
.Nx 1.3
development cycle, and many drivers were converted to use them.
This document was written later during the
.Nx 1.3
development cycle, and the specification was updated to fix some
consistency problems and to add some missing functionality.
.Pp
The manual page was then adapted to the version of the interface that
.Fx
imported for the CAM SCSI drivers, plus subsequent evolution.
The
.Fx
.Nm
version was imported in
.Fx 3.0 .
.Sh AUTHORS
.An -nosplit
The
.Nm
interfaces were designed and implemented by the
.Nx
developer
community.
Primary contributors and implementors were
.An Chris Demetriou ,
.An Jason Thorpe ,
and
.An Charles Hannum ,
but the rest of the
.Nx
developers and the user community played a significant role in development.
.Pp
.An Justin Gibbs
ported these interfaces to
.Fx .
.Pp
.An Chris Demetriou
wrote this manual page.
.Pp
.An Warner Losh
modified it for the
.Fx
implementation.
.Sh BUGS
This manual may not completely and accurately document the interface,
and many parts of the interface are unspecified.