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libbsd.txt: Move initialization details
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Sebastian Huber
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139
CONTRIBUTING.rst
139
CONTRIBUTING.rst
@ -530,3 +530,142 @@ the priority inheritance protocol.
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* `SYSINIT(9) <http://www.freebsd.org/cgi/man.cgi?query=sysinit&sektion=9>`_: A framework for dynamic kernel initialization
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* `TASKQUEUE(9) <http://www.freebsd.org/cgi/man.cgi?query=taskqueue&sektion=9>`_: Asynchronous task execution
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* `UMA(9) <http://www.freebsd.org/cgi/man.cgi?query=uma&sektion=9>`_: General-purpose kernel object allocator
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LibBSD Initialization Details
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=============================
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The initialization of LibBSD is based on the FreeBSD
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`SYSINIT(9) <http://www.freebsd.org/cgi/man.cgi?query=sysinit&sektion=9>`_
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infrastructure. The key to initializing a system is to ensure that the desired
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device drivers are explicitly pulled into the linked application. This plus
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linking against the LibBSD (``libbsd.a``) will pull in the necessary FreeBSD
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infrastructure.
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The FreeBSD kernel is not a library like the RTEMS kernel. It is a bunch of
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object files linked together. If we have a library, then creating the
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executable is simple. We begin with a start symbol and recursively resolve all
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references. With a bunch of object files linked together we need a different
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mechanism. Most object files don't know each other. Lets say we have a driver
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module. The rest of the system has no references to this driver module. The
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driver module needs a way to tell the rest of the system: Hey, kernel I am
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here, please use my services!
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This registration of independent components is performed by SYSINIT(9) and
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specializations
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The SYSINIT(9) uses some global data structures that are placed in a certain
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section. In the linker command file we need this:
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.. code-block:: none
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.rtemsroset : {
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KEEP (*(SORT(.rtemsroset.*)))
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}
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.rtemsrwset : {
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KEEP (*(SORT(.rtemsrwset.*)))
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}
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This results for example in this executable layout:
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.. code-block:: none
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[...]
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*(SORT(.rtemsroset.*))
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.rtemsroset.bsd.modmetadata_set.begin
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0x000000000025fe00 0x0 libbsd.a(rtems-bsd-init.o)
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0x000000000025fe00 _bsd__start_set_modmetadata_set
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.rtemsroset.bsd.modmetadata_set.content
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0x000000000025fe00 0x8 libbsd.a(rtems-bsd-nexus.o)
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.rtemsroset.bsd.modmetadata_set.content
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0x000000000025fe08 0x4 libbsd.a(kern_module.o)
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[...]
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.rtemsroset.bsd.modmetadata_set.content
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0x000000000025fe68 0x4 libbsd.a(mii.o)
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.rtemsroset.bsd.modmetadata_set.content
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0x000000000025fe6c 0x4 libbsd.a(mii_bitbang.o)
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.rtemsroset.bsd.modmetadata_set.end
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0x000000000025fe70 0x0 libbsd.a(rtems-bsd-init.o)
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0x000000000025fe70 _bsd__stop_set_modmetadata_set
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[...]
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.rtemsrwset 0x000000000030bad0 0x290
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*(SORT(.rtemsrwset.*))
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.rtemsrwset.bsd.sysinit_set.begin
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0x000000000030bad0 0x0 libbsd.a(rtems-bsd-init.o)
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0x000000000030bad0 _bsd__start_set_sysinit_set
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.rtemsrwset.bsd.sysinit_set.content
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0x000000000030bad0 0x4 libbsd.a(rtems-bsd-nexus.o)
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.rtemsrwset.bsd.sysinit_set.content
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0x000000000030bad4 0x8 libbsd.a(rtems-bsd-thread.o)
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.rtemsrwset.bsd.sysinit_set.content
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0x000000000030badc 0x4 libbsd.a(init_main.o)
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[...]
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.rtemsrwset.bsd.sysinit_set.content
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0x000000000030bd54 0x4 libbsd.a(frag6.o)
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.rtemsrwset.bsd.sysinit_set.content
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0x000000000030bd58 0x8 libbsd.a(uipc_accf.o)
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.rtemsrwset.bsd.sysinit_set.end
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0x000000000030bd60 0x0 libbsd.a(rtems-bsd-init.o)
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0x000000000030bd60 _bsd__stop_set_sysinit_set
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[...]
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Here you can see, that some global data structures are collected into
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continuous memory areas. This memory area can be identified by start and stop
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symbols. This constructs a table of uniform items.
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The low level FreeBSD code calls at some time during the initialization the
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mi_startup() function (machine independent startup). This function will sort
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the SYSINIT(9) set and call handler functions which perform further
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initialization. The last step is the scheduler invocation.
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The SYSINIT(9) routines are run in ``mi_startup()`` which is called by
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``rtems_bsd_initialize()``. This is also explained in "The Design and
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Implementation of the FreeBSD Operating System" section 14.3 "Kernel
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Initialization".
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In RTEMS, we have a library and not a bunch of object files. Thus we need a
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way to pull-in the desired services out of the libbsd. Here the
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``rtems-bsd-sysinit.h`` comes into play. The SYSINIT(9) macros have been
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modified and extended for RTEMS in ``<sys/kernel.h>``:
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.. code-block:: none
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#ifndef __rtems__
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#define C_SYSINIT(uniquifier, subsystem, order, func, ident) \
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static struct sysinit uniquifier ## _sys_init = { \
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subsystem, \
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order, \
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func, \
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(ident) \
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}; \
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DATA_SET(sysinit_set,uniquifier ## _sys_init)
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#else /* __rtems__ */
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#define SYSINIT_ENTRY_NAME(uniquifier) \
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_bsd_ ## uniquifier ## _sys_init
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#define SYSINIT_REFERENCE_NAME(uniquifier) \
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_bsd_ ## uniquifier ## _sys_init_ref
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#define C_SYSINIT(uniquifier, subsystem, order, func, ident) \
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struct sysinit SYSINIT_ENTRY_NAME(uniquifier) = { \
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subsystem, \
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order, \
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func, \
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(ident) \
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}; \
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RWDATA_SET(sysinit_set,SYSINIT_ENTRY_NAME(uniquifier))
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#define SYSINIT_REFERENCE(uniquifier) \
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extern struct sysinit SYSINIT_ENTRY_NAME(uniquifier); \
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static struct sysinit const * const \
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SYSINIT_REFERENCE_NAME(uniquifier) __used \
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= &SYSINIT_ENTRY_NAME(uniquifier)
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#define SYSINIT_MODULE_REFERENCE(mod) \
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SYSINIT_REFERENCE(mod ## module)
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#define SYSINIT_DRIVER_REFERENCE(driver, bus) \
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SYSINIT_MODULE_REFERENCE(driver ## _ ## bus)
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#define SYSINIT_DOMAIN_REFERENCE(dom) \
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SYSINIT_REFERENCE(domain_add_ ## dom)
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#endif /* __rtems__ */
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Here you see that the SYSINIT(9) entries are no longer static. The
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``*_REFERENCE()`` macros will create references to the corresponding modules
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which are later resolved by the linker. The application has to provide an
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object file with references to all required FreeBSD modules.
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