DRIVER(9)

NAME
driver - description of a device driver

SYNOPSIS
#include <sys/param.h>
#include <sys/device.h>
#include <sys/errno.h>
static int
foo_match(device_t parent, cfdata_t match, void *aux);
static void
foo_attach(device_t parent, device_t self, void *aux);
static int
foo_detach(device_t self, int flags);
static int
foo_activate(device_t self, enum devact act);

DESCRIPTION
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_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 */

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 {
    	device_t sc_dev;		/* generic device info */
    	/* device-specific state */
    };

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_flag;
    };

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_flag;
    };

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. The device's softc structure can be obtained from it using the device_private() function (see disk(9)). 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 activate function is used by some buses to notify drivers from interrupt context when detachment is imminent, with act set to DVACT_DEACTIVATE. Currently only pcmcia(9) and cardbus(9) use this. If the action is not supported the activate function should return EOPNOTSUPP.

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:

    foo*	at bar?

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).

SEE ALSO
config(1), autoconf(9), config(9), devsw(9), pmf(9)