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SubjectRe: Initcall / device model meltdown?
On Fri, 17 Jan 2003, Russell King wrote:

> Both the input core and the multifunction chip drivers are using
> module_init(), the order in which these are initialised is link-order
> specific, and it happens that drivers/input is initialised really late
> during boot, after drivers/misc.
> Since the device model requires any object to be initialised before it
> is used, this causes an oops from devclass_add_driver().
> We appear to have two conflicting requirements here:
> 1. the device model requires a certain initialisation order.
> 2. modules need to use module_init() which means the initialisation order
> is link-order dependent, despite our multi-level initialisation system.

I think there's basically two ways to overcome the current fragility:

o Get the init order right. Well, doing it by hand is obviously fragile,
so it needs to be done automatically. I think rusty had patches floating
about which would ensure proper ordering depending on the exported
interfaces. Example: The pci code exports pci_register_driver(), so we
make sure that every (built-in) module which uses pci_register_driver()
runs only after the the module which defines pci_register_driver() has
finished its initcall.

Note that this is how things are done today for actual modules, you
cannot load a module which depends on symbols defined in another module
which has not yet been loaded, thus not yet initialized.

However, it relies on sufficient modularization, which is true for much
of the driver business, but e.g. pci isn't modularized and thus
a bad example (USB, ISDN, etc are better ones). This method however
does not help with early arch init etc., where I think explicit ordering
is a better idea, anyway.

o Make the init order not matter. That is, make sure that the registration
routines ("pci_register_driver()") can be run safely even before
the corresponding __initcall() has executed. E.g. have
pci_register_driver() only add the driver to a (statically initialized)
list of drivers. Then, when pci_init() gets executed, walk the list of
registered drivers, call ->probe() etc.


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