device_attach(9) and driver initialization

Hello,
there seems to be a problem with device attach sequence offered by newbus.
Basically, when device attach method is executing, device is not fully
initialized yet. Also the device state in the newbus part of the world
is DS_ALIVE. There is definitely no shattering news in the statements,
but drivers that e.g. create devfs node to communicate with consumers
are prone to a race.

If /dev node is created inside device attach method, then usermode
can start calling cdevsw methods before device fully initialized itself.
Even more, if device tries to use newbus helpers in cdevsw methods,
like device_busy(9), then panic occurs "called for unatteched device".
I get reports from users about this issues, to it is not something
that only could happen.

I propose to add DEVICE_AFTER_ATTACH() driver method, to be called
from newbus right after device attach finished and newbus considers
the device fully initialized. Driver then could create devfs node
in the after_attach method instead of attach. Please see the patch below.

diff --git a/sys/kern/device_if.m b/sys/kern/device_if.m
index eb720eb..9db74e2 100644
--- a/sys/kern/device_if.m
+++ b/sys/kern/device_if.m
_at__at_ -43,6 +43,10 _at__at_ INTERFACE device;
 # Default implementations of some methods.
 #
 CODE {
+	static void null_after_attach(device_t dev)
+	{
+	}
+
 	static int null_shutdown(device_t dev)
 	{
 	    return 0;
_at__at_ -199,6 +203,21 _at__at_ METHOD int attach {
 };
 
 /**
+ * _at_brief Notify the driver that device is in attached state
+ *
+ * Called after driver is successfully attached to the device and
+ * corresponding device_t is fully operational. Driver now may expose
+ * the device to the consumers, e.g. create devfs nodes.
+ *
+ * _at_param dev		the device to probe
+ *
+ * _at_see DEVICE_ATTACH()
+ */
+METHOD void after_attach {
+	device_t dev;
+} DEFAULT null_after_attach;
+
+/**
  * _at_brief Detach a driver from a device.
  *
  * This can be called if the user is replacing the
diff --git a/sys/kern/subr_bus.c b/sys/kern/subr_bus.c
index d485b9f..6d849cb 100644
--- a/sys/kern/subr_bus.c
+++ b/sys/kern/subr_bus.c
_at__at_ -2743,6 +2743,7 _at__at_ device_attach(device_t dev)
 	dev->state = DS_ATTACHED;
 	dev->flags &= ~DF_DONENOMATCH;
 	devadded(dev);
+	DEVICE_AFTER_ATTACH(dev);
 	return (0);
 }