This page briefly describes the basic
NetBSD autoconfiguration interface used by device drivers. For a detailed overview of the autoconfiguration framework see
autoconf(9).
Each device driver must present to the system a standard autoconfiguration interface. This interface is provided by the
cfattach structure. The interface to the driver is constant and is defined statically inside the driver. For example, the interface to driver “foo” is defined with:
CFATTACH_DECL(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 */
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
CFATTACH_DECL macro. The memory block is referred to as the driver's
softc structure. The
softc structure is only accessed within the driver, so its definition is local to the driver. Nevertheless, the
softc structure should adopt the standard
NetBSD configuration and naming conventions. For example, the
softc structure for driver “foo” is defined with:
struct foo_softc {
struct device sc_dev; /* generic device info */
/* device-specific state */
};
The autoconfiguration framework mandates that the first member of the
softc structure must be the driver-independent
struct device. Probably its most useful aspect to the driver is that it contains the device-instance name
dv_xname.
If a driver has character device interfaces accessed from userland, the driver must define the
cdevsw structure. The structure is constant and is defined inside the driver. For example, the
cdevsw structure for driver “foo” is defined with:
const struct cdevsw foo_cdevsw {
int (*d_open)(dev_t, int, int, struct lwp *);
int (*d_close)(dev_t, int, int, struct lwp *);
int (*d_read)(dev_t, struct uio *, int);
int (*d_write)(dev_t, struct uio *, int);
int (*d_ioctl)(dev_t, u_long, void *, int, struct lwp *);
void (*d_stop)(struct tty *, int);
struct tty *(*d_tty)(dev_t);
int (*d_poll)(dev_t, int, struct lwp *);
paddr_t (*d_mmap)(dev_t, off_t, int);
int (*d_kqfilter)(dev_t, struct knote *);
int d_type;
};
The structure variable must be named foo_cdevsw by appending the letters “_cdevsw” to the driver's base name. This convention is mandated by the autoconfiguration framework.
If the driver “foo” has also block device interfaces, the driver must define the
bdevsw structure. The structure is constant and is defined inside the driver. For example, the
bdevsw structure for driver “foo” is defined with:
const struct bdevsw foo_bdevsw {
int (*d_open)(dev_t, int, int, struct lwp *);
int (*d_close)(dev_t, int, int, struct lwp *);
void (*d_strategy)(struct buf *);
int (*d_ioctl)(dev_t, u_long, void *, int, struct lwp *);
int (*d_dump)(dev_t, daddr_t, void *, size_t);
int (*d_psize)(dev_t);
int d_type;
};
The structure variable must be named foo_bdevsw by appending the letters “_bdevsw” to the driver's base name. This convention is mandated by the autoconfiguration framework.
During system bootstrap, the autoconfiguration framework searches the system for devices. For each device driver, its match function is called (via its
cfattach structure) to match the driver with a device instance. The match function is called with three arguments. This first argument
parent is a pointer to the driver's parent device structure. The second argument
match is a pointer to a data structure describing the autoconfiguration framework's understanding of the driver. Both the
parent and
match arguments are ignored by most drivers. The third argument
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
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 “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
NetBSD convention is to call the match routine
foo_probe() instead of
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.
The autoconfiguration framework will call the attach function (via its
cfattach structure) 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
parent is a pointer to the driver's parent device structure. The second argument
self is a pointer to the driver's device structure. It is also a pointer to our
softc structure since the device structure is its first member. The third argument
aux is a pointer to the attachment structure. The
parent and
aux arguments are the same as passed to the match function.
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
config_interrupts() (see
autoconf(9)).
Some devices can be removed from the system without requiring a system reboot. The autoconfiguration framework calls the driver's detach function (via its
cfattach structure) 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
self is a pointer to the driver's device structure. It is the same structure as passed to the attach function. The second argument
flags contains detachment flags. Valid values are
DETACH_FORCE (force detachment; hardware gone) and
DETACH_QUIET (do not print a notice).
The autoconfiguration framework may call the driver's activate function to notify the driver of a change in the resources that have been allocated to it. For example, an Ethernet driver has to be notified if the network stack is being added or removed from the kernel. The first argument to the activate function
self is a pointer to the driver's device structure. It is the same argument as passed to the attach function. The second argument
act describes the action. Valid actions are
DVACT_ACTIVATE (activate the device) and
DVACT_DEACTIVATE (deactivate the device). If the action is not supported the activate function should return
EOPNOTSUPP. The
DVACT_DEACTIVATE call will only be made if the
DVACT_ACTIVATE call was successful. The activate function is called in interrupt context.
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 “interrupt not for me”, 1 for “I took care of it”, or -1 for “I guess it was mine, but I wasn't expecting it”.
For a driver to be compiled into the kernel,
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 “foo” attaching to bus “bar” with dependency on kernel module “baz” has the entry:
device foo: baz
attach foo at bar
file dev/bar/foo.c foo
An entry can now be added to the machine description file:
For device interfaces of a driver to be compiled into the kernel,
config(1) must be aware of its existence. This is done by including an entry in majors.<arch>. For example, the driver “foo” with character device interfaces, a character major device number “cmaj”, block device interfaces, a block device major number “bmaj” and dependency on kernel module “baz” has the entry:
device-major foo char cmaj block bmaj baz
For a detailed description of the machine description file and the “device definition” language see
config(9).