xref: /src/usr.bin/config/TODO

o Call module as module.

  Until now, everything is called as attribute.  Separate module from it:

	- Module is a collection of code (*.[cSo]), and provides a function.
	  Module can depend on other modules.

	- Attribute provides metadata for modules.  One module can have
	  multiple attributes.  Attribute doesn't generate a module (*.o,
	  *.ko).

o Emit everything (ioconf.*, Makefile, ...) per-attribute.

  config(9) related metadata (cfdriver, cfattach, cfdata, ...) should be
  collected using linker.  Create ELF sections like
  .{rodata,data}.config.{cfdriver,cfattach,cfdata}.  Provide reference
  symbols (e.g. cfdriverinit[]) using linker script.  Sort entries by name
  to lookup entries by binary search in kernel.

o Generate modular(9) related information.  Especially module dependency.

  At this moment modular(9) modules hardcode dependency in *.c using the
  MODULE() macro:

	MODULE(MODULE_CLASS_DRIVER, hdaudio, "pci");

  This information already exists in config(5) definitions (files.*).
  Extend config(5) to be able to specify module's class.

  Ideally these module metadata are kept somewhere in ELF headers, so that
  loaders (e.g. boot(8)) can easily read.  One idea is to abuse DYNAMIC
  sections to record dependency, as shared library does.  (Feasibility
  unknown.)

o Rename "interface attribute" to "bus".

  Instead of

	define	audiobus {}
	attach	audio at audiobus

  Do like this

	defbus	audiobus {}
	attach	audio at audiobus

o Retire "attach foo at bar with foo_bar.c"

  Most of these should be rewritten by defining a common interface attribute
  "foobus", instead of writing multiple attachments.  com(4), ld(4), ehci(4)
  are typical examples.  For ehci(4), EHCI-capable controller drivers implement
  "ehcibus" interface, like:

	defne	ehcibus {}
	device	imxehci: ehcibus

  These drivers' attach functions call config_found() to attach ehci(4) via
  the "ehcibus" interface attribute, instead of calling ehci_init() directly.
  Same for com(4) (com_attach_subr()) and ld(4) (ldattach()).

o Sort objects in more reasonable order.

  Put machdep.ko in the lowest address.  uvm.ko and kern.ko follow.

  Kill alphabetical sort (${OBJS:O} in sys/conf/Makefile.inc.kern.

  Use ldscript.  Do like this

	.text :
	AT (ADDR(.text) & 0x0fffffff)
	{
	  *(.text.machdep.locore.entry)
	  *(.text.machdep.locore)
	  *(.text.machdep)
	  *(.text)
	  *(.text.*)
	  :

  Kill linker definitions in sys/conf/Makefile.inc.kern.

o Differentiate "options" and "flags"/"params".

  "options" enables features by adding *.c files (via attributes).

  "flags" and "params" are to change contents of *.c files.  These don't add
  *.c files to the result kernel, or don't build attributes (modules).

o Make flags/params per attributes (modules).

  Basically flags and params are cpp(1) #define's generated in opt_*.h.  Make
  them local to one attributes (modules).  Flags/params which affects files
  across attributes (modules) are possible, but should be discouraged.

o Generate things only by definitions.

  In the ideal dynamically modular world, "selection" will be done not at
  compile time but at runtime.  Users select their wanted modules, by
  dynamically loading them.

  This means that the system provides all choices; that is, build all modules
  in the source tree.  Necessary information is defined in the "definition"
  part.

o Split cfdata.

  cfdata is a set of pattern matching rules to enable devices at runtime device
  auto-configuration.  It is pure data and can (should) be generated separately
  from the code.

o Allow easier adding and removing of options.

  It should be possible to add or remove options, flags, etc.,
  without regard to whether or not they are already defined.
  For example, a configuration like this:

	include GENERIC
	options FOO
	no options BAR

  should work regardless of whether or not options FOO and/or
  options BAR were defined in GENERIC.  It should not give
  errors like "options BAR was already defined" or "options FOO
  was not defined".

o Introduce "class".

  Every module should be classified as at least one class, as modular(9)
  modules already do.  For example, file systems are marked as "vfs", network
  protocols are "netproto".

  Consider to merge "devclass" into "class".

  For syntax clarity, class names could be used as a keyword to select the
  class's instance module:

	# Define net80211 module as netproto class
	class netproto
	define net80211: netproto

	# Select net80211 to be builtin
	netproto net80211

  Accordingly device/attach selection syntax should be revisited.

o Support kernel constructor/destructor (.kctors/.kdtors)

  Initialization and finalization should be called via constructors and
  destructors.  Don't hardcode those sequences as sys/kern/init_main.c:main()
  does.

  The order of .kctors/.kdtors is resolved by dependency.  The difference from
  userland is that in kernel depended ones are located in lower addresses;
  "machdep" module is the lowest.  Thus the lowest entry in .ctors must be
  executed the first.

  The .kctors/.kdtors entries are executed by kernel's main() function, unlike
  userland where start code executes .ctors/.dtors before main().  The hardcoded
  sequence of various subsystem initializations in init_main.c:main() will be
  replaced by an array of .kctors invocations, and #ifdef's there will be gone.

o Hide link-set in the final kernel.

  Link-set is used to collect references (pointers) at link time.  It relys on
  the ld(1) behavior that it automatically generates `__start_X' and `__stop_X'
  symbols for the section `X' to reduce coding.

  Don't allow kernel subsystems create random ELF sections.

  Pre-define all the available link-set names and pre-generate a linker script
  to merge them into .rodata.

  (For modular(9) modules, `link_set_modules' is looked up by kernel loader.
  Provide only it.)

  Provide a way for 3rd party modules to declare extra link-set.

o Shared kernel objects.

  Since NetBSD has not established a clear kernel ABI, every single kernel
  has to build all the objects by their own.  As a result, similar kernels
  (e.g. evbarm kernels) repeatedly compile similar objects, that is waste of
  energy & space.

  Share them if possible.  For evb* ports, ideally everything except machdep.ko
  should be shared.

  While leaving optimizations as options (CPU specific optimizations, inlined
  bus_space(9) operations, etc.) for users, the official binaries build
  provided by TNF should be as portable as possible.

o Control ELF sections using linker script.

  Now kernel is linked and built directly from object files (*.o).  Each port
  has an MD linker script, which does everything needed to be done at link
  time.  As a result, they do from MI alignment restriction (read_mostly,
  cacheline_aligned) to load address specification for external boot loaders.

  Make this into multiple stages to make linkage more structural.  Especially,
  reserve the final link for purely MD purpose.  Note that in modular build,
  *.ko are shared between build of kernel and modular(9) modules (*.kmod).

	Monolithic build:
		     *.o  ---> netbsd.ko	Generic MI linkage
		netbsd.ko ---> netbsd.ro	Kernel MI linkage
		netbsd.ro ---> netbsd		Kernel MD linkage

	Modular build (kernel):
		     *.o  --->      *.ko	Generic + Per-module MI linkage
		     *.ko ---> netbsd.ro	Kernel MI linkage
		netbsd.ro ---> netbsd		Kernel MD linkage

	Modular build (module):
		     *.o  --->      *.ko	Generic + Per-module MI linkage
		     *.ko --->      *.ro	Modular MI linkage
		     *.ro --->      *.kmod	Modular MD linkage

  Genric MI linkage is for processing MI linkage that can be applied generally.
  Data section alignment (.data.read_mostly and .data.cacheline_aligned) is
  processed here.

  Per-module MI linkage is for modules that want some ordering.  For example,
  machdep.ko wants to put entry code at the top of .text and .data.

  Kernel MI linkage is for collecting kernel global section data, that is what
  link-set is used for now.  Once they are collected and symbols to the ranges
  are assigned, those sections are merged into the pre-existing sections
  (.rodata) because link-set sections in "netbsd" will never be interpreted by
  external loaders.

  Kernel MD linkage is used purely for MD purposes, that is, how kernels are
  loaded by external loaders.  It might be possible that one kernel relocatable
  (netbsd.ro) is linked into multiple final kernel image (netbsd) for diferent
  load addresses.

  Modular MI linkage is to prepare a module to be loadable as modular(9).  This
  may add some extra sections and/or symbols.

  Modular MD linkage is again for pure MD purposes like kernel MD linkage.
  Adjustment and/or optimization may be done.

  Kernel and modular MI linkages may change behavior depending on existence
  of debug information.  In the future .symtab will be copied using linker
  during this stage.

o Redesign swapnetbsd.c (root/swap device specification)

  Don't build a whole kernel only to specify root/swap devices.

  Make these parameter re-configurable afterwards.

o Namespace.

  Investigate namespace of attributes/modules/options.  Figure out the hidden
  design about these, document it, then re-design it.

  At this moment, all of them share the single "selecttab", which means their
  namespaces are common, but they also have respective tables (attrtab,
  opttab, etc.).

  Selecting an option (addoption()), that is also a module name, works only if
  the module doesn't depend on anything, because addoption() doesn't select
  module and its dependencies (selectattr()).  In other words, an option is
  only safely converted to a module (define), only if it doesn't depend on
  anything.  (One example is DDB.)