Re: newbus flaw

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To: Dag-Erling Smørgrav <des@des.no>
Date: Tue, 11 May 2004 16:07:50 +0100

On Tue, 2004-05-11 at 15:39, Dag-Erling Smørgrav wrote:
> I've found what I believe is a serious flaw in newbus.
>
> When a driver that has a DEVICE_IDENTIFY method is loaded, the
> identify method is called. If it finds supported hardware, it uses
> BUS_ADD_CHILD to notify the parent bus of the presence of that
> hardware. At some later point, during a bus rescan, the attach
> routine is called for each device that was identified in this manner.
>
> When the driver is unloaded, the device is detached, but it remains on
> the bus's list of child devices. The next time the module is loaded,
> its DEVICE_IDENTIFY method is called again, and incorrectly adds a
> second child device to the bus, because it does not know that one
> already exists.
>
> There is no way for DEVICE_IDENTIFY to check if a matching child
> already exists on the bus, or for the module's event handler to unlist
> the child when unloading.
>
> The first time you load the module, you get foo0; the second time, you
> get foo0 *and* foo1 referencing the same physical device; the third
> time, you get foo0, foo1, and foo2, etc.
>
> I've also seen something similar happen when multiple ndis drivers are
> loaded; the first one re-attaches to the hardware when the second one
> is loaded.

This is a known problem. The 'right' solution is to add a new static
method to the device interface which is the opposite of IDENTIFY (e.g.
UNIDENTIFY). One way to get around the problem is to do something like:

void foo_identify(driver_t *driver, device_t parent)
{
        device_t child;
        child = device_find_child(parent, "foo", 0);
        if (!child)
                BUS_ADD_CHILD(parent, 0, "foo", 0);
}

Alternatively, you can add a module handler:

static device_t foo;

void foo_identify(driver_t *driver, device_t parent)
{
        foo = BUS_ADD_CHILD(parent, 0, "foo", -1);
}

...

int foo_module_handler(struct module *mod, int what, void *arg)
{
        if (what == MOD_UNLOAD && foo)
                device_delete_child(device_get_parent(foo), foo);
}

...

DRIVER_MODULE(foo, bar, foo_driver, foo_devclass, foo_module_handler,
0);

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