DRIVER_UNIDENTIFY

[Warner Losh]

> How about creating a new driver_if.m entry point that is the complement to 
> DRIVER_IDENTIFY.  I would call it DRIVER_UNIDENTIFY() and would change 
> bus_generic_detach() to call it on each driver similar to how 
> bus_generic_probe() currently calls DRIVER_IDENTIFY() for each driver.  This 
> would allow drivers that create devices in DRIVER_IDENTIFY() have a place to 
> remove the device when they are unloaded.

I have mixed feelings about this.

First, the identify routine has identified a hunk of hardware and has
placed it into the tree.  The driver disappearing doesn't change the
fact that the hardware is still there.  Adding this new function would
further blur the lines between attached drivers, the hardware and
nodes in the device tree.  Device nodes are expected to be there
unattached.  Once the device driver has detached from the device node,
there's no harm in leaving the node arround as there are no dangling
references.

Second, the bus may be the one that decides what hardware is there.  I
have a SOC chip that has a number of different children of its nexus
that are always there, and will always be there.  I do not want the
children drivers to know anything of their location, etc, since they
cannot know because a different SOC will have the children at a
different location.  Of course, the easy thing here is to never call
the idenfify routine at all for this bus, but this would require
changes to the code.

However, we do currently have a assymetrical arrangement.  There's a
way to add the device, but too many drivers are 'stupid' in how they
add the device.  They neglect to check to make sure that the device
hasn't already been added, which is what causes the grief.

Maybe it would be better to have a better way to add instances such
that if the idenify routine used this better way that it could be
called many times (eg, make it idempotent).  Right now we recommend
that driver writers do the following (which is idempotent):

static void
tscnmi_identify( driver_t* driver, device_t parent )
{
    devclass_t dc;

    dc = devclass_find(DRIVERNAME);
    if (devclass_get_device(dc, 0) == NULL) {
        if (BUS_ADD_CHILD(parent, 0, DRIVERNAME, 0) == 0)
            panic("failed to add " DRIVERNAME);
    }
}

I think a better solution would be:

static void
tscnmi_identify( driver_t* driver, device_t parent )
{
	device_add_child_once(parent, DRIVERNAME);
}

where device_add_child_once would look like the following (run through
style(9)izer):

int
device_add_child_once(device_t parent, char *name)
{
    devclass_t dc;

    dc = devclass_find(name);
    if (devclass_get_device(dc, 0) == NULL)
    	return BUS_ADD_CHILD(parent, 0, name, 0);
    return 0;
}