path: root/static/netbsd/man9/driver.9

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.Dd April 30, 2017
.Dt DRIVER 9
.Os
.Sh NAME
.Nm driver
.Nd description of a device driver
.Sh SYNOPSIS
.In sys/param.h
.In sys/device.h
.In sys/errno.h
.Ft static int
.Fn foo_match "device_t parent" "cfdata_t match" "void *aux"
.Ft static void
.Fn foo_attach "device_t parent" "device_t self" "void *aux"
.Ft static int
.Fn foo_detach "device_t self" "int flags"
.Ft static int
.Fn foo_activate "device_t self" "enum devact act"
.Sh DESCRIPTION
This page briefly describes the basic
.Nx
autoconfiguration interface used by device drivers.
For a detailed overview of the autoconfiguration framework see
.Xr autoconf 9 .
.Pp
Each device driver must present to the system a standard
autoconfiguration interface.
This interface is provided by the
.Vt cfattach
structure.
The interface to the driver is constant and is defined
statically inside the driver.
For example, the interface to driver
.Ql foo
is defined with:
.Bd -literal -offset indent
CFATTACH_DECL_NEW(foo,                  /* driver name */
        sizeof(struct foo_softc),       /* size of instance data */
        foo_match,                      /* match/probe function */
        foo_attach,                     /* attach function */
        foo_detach,                     /* detach function */
        foo_activate);                  /* activate function */
.Ed
.Pp
For each device instance controlled by the driver, the
autoconfiguration framework allocates a block of memory to record
device-instance-specific driver variables.
The size of this memory block is specified by the second argument in the
.Dv CFATTACH_DECL
family of macros.
The memory block is referred to as the driver's
.Vt softc
structure.
The
.Vt softc
structure is only accessed within the driver, so its definition is
local to the driver.
Nevertheless, the
.Vt softc
structure should adopt the standard
.Nx
configuration and naming conventions.
For example, the
.Vt softc
structure for driver
.Ql foo
is defined with:
.Bd -literal -offset indent
struct foo_softc {
        device_t sc_dev;                /* generic device info */
        /* device-specific state */
};
.Ed
.Pp
If a driver has character device interfaces accessed from userland, the driver
must define the
.Vt cdevsw
structure.
The structure is constant and is defined inside the driver.
For example, the
.Vt cdevsw
structure for driver
.Ql foo
can be defined with:
.Bd -literal -offset indent
const struct cdevsw foo_cdevsw {
        .d_open = fooopen,
        .d_close = nullclose,
        .d_read = fooread,
        .d_write = nowrite,
        .d_ioctl = fooioctl,
        .d_stop = nostop,
        .d_tty = notty,
        .d_poll = foopoll,
        .d_mmap = nommap,
        .d_kqfilter = nokqfilter,
        .d_discard = nodiscard,
        .d_flag = D_OTHER | D_MPSAFE,
};
.Ed
.Pp
The structure variable must be named
.Va foo_cdevsw
by appending the letters
.Ql _cdevsw
to the driver's base name.
This convention is mandated by the autoconfiguration framework.
.Pp
If the driver
.Ql foo
has also block device interfaces, the driver must define the
.Vt bdevsw
structure.
The structure is constant and is defined inside the driver.
For example, the
.Vt bdevsw
structure for driver
.Ql foo
is defined with:
.Bd -literal -offset indent
const struct bdevsw foo_bdevsw {
        .d_open = fooopen,
        .d_close = fooclose,
        .d_strategy = foostrategy,
        .d_ioctl = fooioctl,
        .d_dump = nodump,
        .d_psize = nosize,
        .d_flag = D_DISK | D_MPSAFE,
};
.Ed
.Pp
The structure variable must be named
.Va foo_bdevsw
by appending the letters
.Ql _bdevsw
to the driver's base name.
This convention is mandated by the autoconfiguration framework.
.Pp
During system bootstrap, the autoconfiguration framework searches the
system for devices.
For each device driver, its match function is called
.Po
via its
.Vt cfattach
structure
.Pc
to match the driver with a device instance.
The match function is called with three arguments.
This first argument
.Fa parent
is a pointer to the driver's parent device structure.
The second argument
.Fa match
is a pointer to a data structure describing the autoconfiguration
framework's understanding of the driver.
Both the
.Fa parent
and
.Fa match
arguments are ignored by most drivers.
The third argument
.Fa aux
contains a pointer to a structure describing a potential
device-instance.
It is passed to the driver from the parent.
The match function would type-cast the
.Fa aux
argument to its appropriate attachment structure and use its contents
to determine whether it supports the device.
Depending on the device hardware, the contents of the attachment
structure may contain
.Dq locators
to locate the device instance so that the driver can probe it for its
identity.
If the probe process identifies additional device properties, it may
modify the members of the attachment structure.
For these devices, the
.Nx
convention is to
call the match routine
.Fn foo_probe
instead of
.Fn foo_match
to make this distinction clear.
Either way, the match function returns a nonzero integer indicating the
confidence of supporting this device and a value of 0 if the driver
doesn't support the device.
Generally, only a single driver exists for a device, so the match
function returns 1 for a positive match.
.Pp
The autoconfiguration framework will call the attach function
.Po
via its
.Vt cfattach
structure
.Pc
of the driver which returns the highest value from its match function.
The attach function is called with three arguments.
The attach function performs the necessary process to initialise the
device for operation.
The first argument
.Fa parent
is a pointer to the driver's parent device structure.
The second argument
.Fa self
is a pointer to the driver's device structure.
The device's
.Vt softc
structure can be obtained from it using the
.Fn device_private
function
.Po see
.Xr disk 9
.Pc .
The third argument
.Fa aux
is a pointer to the attachment structure.
The
.Fa parent
and
.Fa aux
arguments are the same as passed to the match function.
.Pp
The driver's attach function is called before system interrupts are
enabled.
If interrupts are required during initialisation, then the attach
function should make use of
.Fn config_interrupts
.Po
see
.Xr autoconf 9
.Pc .
.Pp
Some devices can be removed from the system without requiring a system
reboot.
The autoconfiguration framework calls the driver's detach function
.Po
via its
.Vt cfattach
structure
.Pc
during device detachment.
If the device does not support detachment, then the driver does not
have to provide a detach function.
The detach function is used to relinquish resources allocated to the
driver which are no longer needed.
The first argument
.Fa self
is a pointer to the driver's device structure.
It is the same structure as passed to the attach function.
The second argument
.Fa flags
contains detachment flags.
Valid values are
.Dv DETACH_FORCE
.Pq force detachment; hardware gone
and
.Dv DETACH_QUIET
.Pq do not print a notice .
.Pp
The activate function is used by some buses to notify drivers from
interrupt context when detachment is imminent, with
.Fa act
set to
.Dv DVACT_DEACTIVATE .
Currently only
.Xr pcmcia 9
and
.Xr cardbus 9
use this.
If the action is not supported the activate function should return
.Er EOPNOTSUPP .
.Pp
Most drivers will want to make use of interrupt facilities.
Interrupt locators provided through the attachment structure should be
used to establish interrupts within the system.
Generally, an interrupt interface is provided by the parent.
The interface will require a handler and a driver-specific argument
to be specified.
This argument is usually a pointer to the device-instance-specific
softc structure.
When a hardware interrupt for the device occurs the handler is called
with the argument.
Interrupt handlers should return 0 for
.Dq interrupt not for me ,
1 for
.Dq I took care of it ,
or \-1 for
.Do
I guess it was mine, but I wasn't expecting it
.Dc .
.Pp
For a driver to be compiled into the kernel,
.Xr config 1
must be aware of its existence.
This is done by including an entry in files.<bus> in the
directory containing the driver.
For example, the driver
.Ar foo
attaching to bus
.Ar bar
with dependency on kernel module
.Ar baz
has the entry:
.Bd -literal -offset indent
.Cd device Ar foo Ns Cd \&: Ar baz
.Cd attach Ar foo Cm at Ar bar
.Cd file Pa dev/ Ns Ar bar Ns Pa / Ns Ar foo Ns Pa \&.c Ar foo
.Ed
.Pp
An entry can now be added to the machine description file:
.Bd -ragged -offset indent
.Ar foo\^ Ns Cm \&* Cm at Ar bar\^ Ns Cm \&?
.Ed
.Pp
For device interfaces of a driver to be compiled into the kernel,
.Xr config 1
must be aware of its existence.
This is done by including an entry in
.Pa majors. Ns Aq Ar arch .
For example, the driver
.Ar foo
with character device interfaces, a character major device number
.Ar cmaj ,
block device interfaces, a block device major number
.Ar bmaj
and dependency on kernel module
.Ar baz
has the entry:
.Bd -ragged -offset indent
.Cd device-major Ar foo Cm char Ar cmaj Cm block Ar bmaj Ar baz
.Ed
.Pp
For a detailed description of the machine description file and the
.Dq device definition
language see
.Xr config 9 .
.Sh SEE ALSO
.Xr config 1 ,
.Xr autoconf 9 ,
.Xr config 9 ,
.Xr devsw 9 ,
.Xr pmf 9