DEVICE_IDENTIFY.9: Modernize description and use cases

Mention adding devices based on firmware tables and software-only
pseudo-devices as use cases for identify methods as those are more
common than reading random I/O ports to identify a legacy ISA device.

Describe how device_find_chid can be used to avoid duplicates.  While
here, explicitly note that devices added in identify methods typically
use a fixed device name.

Trim the cross-references a bit.

Reviewed by:	ziaee, imp
Differential Revision:	https://reviews.freebsd.org/D48367

share/man/man9/DEVICE_IDENTIFY.9

@@ -26,44 +26,46 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd January 15, 2017
+.Dd January 9, 2025
 .Dt DEVICE_IDENTIFY 9
 .Os
 .Sh NAME
 .Nm DEVICE_IDENTIFY
-.Nd identify a device, register it
+.Nd identify new child devices and register them
 .Sh SYNOPSIS
 .In sys/param.h
 .In sys/bus.h
 .Ft void
 .Fn DEVICE_IDENTIFY "driver_t *driver" "device_t parent"
 .Sh DESCRIPTION
-The identify function for a device is only needed for devices on buses
-that cannot identify their children independently, e.g.\& the ISA bus.
-It is used to recognize the device (usually done by accessing non-ambiguous
-registers in the hardware) and to tell the kernel about it and thus
-creating a new device instance.
+The identify method of a device driver is used to add devices that cannot be
+enumerated by the standard method on a bus device.
+Devices can be enumerated in various ways including accessing non-ambiguous
+device registers and parsing firmware tables.
+Software-only pseudo devices are also often enumerated via identify methods.
 .Pp
+For each newly identified device,
+a new device instance should be created by invoking the
 .Xr BUS_ADD_CHILD 9
-is used to register the device as a child of the bus.
-The device's resources (such as IRQ and I/O ports) are registered
-with the kernel by calling
-.Fn bus_set_resource
-for each resource (refer to
+method.
+If the identify method is able to discover other properties about the new
+device, those should also be set.
+For example, device resources should be added to the device by calling
 .Xr bus_set_resource 9
-for more information).
+for each resource.
 .Pp
-Since the device tree and the device driver tree are disjoint, the
-.Fn DEVICE_IDENTIFY
-routine needs to take this into account.
-If you load and unload your device driver that has the identify
-routine, the child node has the potential for adding the same node
-multiple times unless specific measure are taken to preclude that
-possibility.
+An identify method might be invoked multiple times.
+If a device driver is unloaded and loaded,
+the identify method will be called a second time after being reloaded.
+As a result, the identify method should avoid duplicate devices.
+Devices added by identify methods typically use a fixed device name
+in which case
+.Xr device_find_child 9
+can be used to detect existing devices.
 .Sh EXAMPLES
 The following pseudo-code shows an example of a function that
 probes for a piece of hardware and registers it and its resource
-(an I/O port) with the kernel.
+(an I/O port) with the parent bus device.
 .Bd -literal
 void
 foo_identify(driver_t *driver, device_t parent)
@@ -72,7 +74,7 @@ foo_identify(driver_t *driver, device_t parent)
 
 	retrieve_device_information;
 	if (devices matches one of your supported devices &&
-	    not already in device tree) {
+	    device_get_child(parent, "foo", DEVICE_UNIT_ANY) == NULL) {
 		child = BUS_ADD_CHILD(parent, 0, "foo", DEVICE_UNIT_ANY);
 		bus_set_resource(child, SYS_RES_IOPORT, 0, FOO_IOADDR, 1);
 	}
@@ -82,11 +84,7 @@ foo_identify(driver_t *driver, device_t parent)
 .Xr BUS_ADD_CHILD 9 ,
 .Xr bus_set_resource 9 ,
 .Xr device 9 ,
-.Xr device_add_child 9 ,
-.Xr DEVICE_ATTACH 9 ,
-.Xr DEVICE_DETACH 9 ,
-.Xr DEVICE_PROBE 9 ,
-.Xr DEVICE_SHUTDOWN 9
+.Xr device_find_child 9
 .Sh AUTHORS
 This manual page was written by
 .An Alexander Langer Aq Mt alex@FreeBSD.org .