/* $NetBSD: TODO.modules,v 1.11 2017/01/26 04:24:20 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 often gross hacks.

	Another alternative (which is used by ppp) is to provide a
	"registration" mechanism for the "child" modules, and then when
	the need for a specific child module is encountered, use
	module_autoload() to load the child module.  Of course, this
	requires that the parent module know about all potentially
	loadable children.

 2. Currently, config(1) has no way to "no define" drivers
	XXX: I don't think this is true anymore. I think we can
	undefine drivers now, see MODULAR in amd64, which does
	no ath* and no select sppp*

 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.

 8. Even for existing modules, there are "surprise" dependencies with
    code that has not yet been modularized.

	For example, even though the bpf code has been modularized,
	there is some shared code in bpf_filter.c which is needed by
	both ipfilter and ppp.  ipf is already modularized, but ppp
	is not.  Thus, even though bpf_filter is modular, it MUST be
	included as a built-in module if you also have ppp in your
	configuration.

	Another example is sysmon_taskq module.  It is required by
	other parts of the sysmon subsystem, including the
	"sysmon_power" module.  Unfortunately, even though the
	sysmon_power code is modularized, it is referenced by the
	acpi code which has not been modularized.  Therefore, if your
	configuration has acpi, then you must include the "sysmon_power"
	module built-in the kernel.  And therefore your also need to
	have "sysmon_taskq" and "sysmon" built-in since "sysmon_power"
	rerefences them.

 9. As a corollary to #8 above, having dependencies on modules from code
    which has not been modularized makes it extremely difficult to test
    the module code adequately.  Testing of module code should include
    both testing-as-a-built-in module and testing-as-a-loaded-module, and
    all dependencies need to be identified.

10. The current /stand/$ARCH/$VERSION/modules/ hierarchy won't scale as
    we get more and more modules.  There are hundreds of potential device
    driver modules.

11. There currently isn't any good way to handle attachment-specific
    modules.  The build infrastructure (ie, sys/modules/Makefile) doesn't
    readily lend itself to bus-specific modules irrespective of $ARCH,
    and maintaining distrib/sets/lists/modules/* is awkward at best.

    Furthermore, devices such as ld(4), which can attach to a large set
    of parent devices, need to be modified.  The parent devices need to
    provide a common attribute (for example, ld_bus), and the ld driver
    should attach to that attribute rather than to each parent.  But
    currently, config(1) doesn't handle this - it doesn't allow an
    attribute to be used as the device tree's pseudo-root. The current
    directory structure where driver foo is split between ic/foo.c
    and bus1/foo_bus1.c ... busn/foo_busn.c is annoying. It would be
    better to switch to the FreeBSD model which puts all the driver
    files in one directory.

12. Item #11 gets even murkier when a particular parent can provide more
    than one attribute.

13. It seems that we might want some additional sets-lists "attributes"
    to control contents of distributions.  As an example, many of our
    architectures have PCI bus capabilities, but not all.  It is rather
    painful to need to maintain individual architectures' modules/md_*
    sets lists, especially when we already have to conditionalize the
    build of the modules based on architecture.  If we had a single
    "attribute" for PCI-bus-capable, the same attribute could be used to
    select which modules to build and which modules from modules/mi to
    include in the release.  (This is not limited to PCI;  recently we
    encounter similar issues with spkr aka spkr_synth module.)

14. As has been pointed out more than once, the current method of storing
    modules in a version-specific subdirectory of /stand is sub-optimal
    and leads to much difficulty and/or confusion.  A better mechanism of
    associating a kernel and its modules needs to be developed.  Some
    have suggested having a top-level directory (say, /netbsd) with a
    kernel and its modules at /netbsd/kernel and /netbsd/modules/...
    Whatever new mechanism we arrive at will probably require changes to
    installation procedures and bootstrap code, and will need to handle
    both the new and old mechanisms for compatability.

    One additional option mentioned is to be able to specify, at boot
    loader time, an alternate value for the os-release portion of the
    default module path,  i.e. /stand/$MACHINE/$ALT-RELEASE/modules/