xref: /src/doc/TODO.modules

/* $NetBSD: TODO.modules,v 1.1 2016/08/04 10:45:52 pgoyette Exp $ */

Some notes on the limitations of our current (as of 7.99.35) module
subsystem.  This list was triggered by an Email exchange between
christos and pgoyette.

1. Builtin drivers can't depend on modularized drivers (the modularized
   drivers are attempted to load as builtins).

	The assumption is that dependencies are loaded before those
	modules which depend on them.  At load time, a module's
	undefined global symbols are resolved;  if any symbols can't
	be resolved, the load fails.  Similarly, if a module is
	included in (built-into) the kernel, all of its symbols must
	be resolvable by the linker, otherwise the link fails.

	There are ways around this (such as, having the parent
	module's initialization command recursively call the module
	load code), but they're gross hacks.

2. Currently, config(1) has no way to "no define" drivers

3. It is not always obvious by their names which drivers/options
   correspond to which modules.

4. Right now critical drivers that would need to be pre-loaded (ffs,
   exec_elf64) are still built-in so that we don't need to alter the boot
   blocks to boot.

	This was a conscious decision by core@ some years ago.  It is
	not a requirement that ffs or exec_* be built-in.  The only
	requirement is that the root file-system's module must be
	available when the module subsystem is initialized, in order
	to load other modules.  This can be accomplished by having the
	boot loader "push" the module at boot time.  (It used to do
	this in all cases; currently the "push" only occurs if the
	booted filesystem is not ffs.)

5. Not all parent bus drivers are capable of rescan, so some drivers
   just have to be built-in.

6. Many (most?) drivers are not yet modularized

7. There's currently no provisions for autoconfig to figure out which
   modules are needed, and thus to load the required modules.

	In the "normal" built-in world, autoconfigure can only ask
	existing drivers if they're willing to manage (ie, attach) a
	device.  Removing the built-in drivers tends to limit the
	availability of possible managers.  There's currently no
	mechanism for identifying and loading drivers based on what
	devices might be found.