Documented the compilation stages
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Michael Brown
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/** @page build_sys Build system
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@section overview Overview
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Building an Etherboot image consists of two stages:
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-# @ref compilation : Compiling all the source files into object files
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-# @ref linking : Building a particular image from select object files
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Though this is a remarkably complex process, it is important to note
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that it all happens automatically. Whatever state your build tree is
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in, you can always type, for example
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@code
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make bin/rtl8139.dsk
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@endcode
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and know that you will get a floppy disk image with an RTL8139 driver
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built from the current sources.
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@section compilation Compilation
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@subsection comp_general Overview
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Each source file (a @c .c or a @c .S file) is compiled into a @c .o
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file in the @c bin/ directory. Etherboot makes minimal use of
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conditional compilation (see @ref ifdef_harmful), and so you will find
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that all objects get built, even the objects that correspond to
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features that you are not intending to include in your image. For
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example, all network card drivers will be compiled even if you are
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just building a ROM for a 3c509 card. This is a deliberate design
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decision; please do @b not attempt to "fix" the build system to avoid
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doing this.
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Source files are defined to be any @c .c or @c .S files found in a
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directory listed in the Makefile variable #SRCDIRS. You therefore do
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@b not need to edit the Makefile just because you have added a new
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source file (although you will need to edit the Makefile if you have
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added a new source directory). To see a list of all source
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directories and source files that the build system currently knows
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about, you can use the commands
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@code
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make srcdirs
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make srcs
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@endcode
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Rules for compiling @c .c and @c .S files are defined in the Makefile
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variables #RULE_c and #RULE_S. Makefile rules are automatically
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generated for each source file using these rules. The generated rules
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can be found in the @c .d file corresponding to each source file;
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these are located in <tt>bin/deps/</tt>. For example, the rules
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generated for <tt>drivers/net/rtl8139.c</tt> can be found in
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<tt>bin/deps/drivers/net/rtl8139.c.d</tt>. These rules allow you to
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type, for example
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@code
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make bin/rtl8139.o
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@endcode
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and have <tt>rtl8139.o</tt> be built from
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<tt>drivers/net/rtl8139.c</tt> using the generic rule #RULE_c for
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compiling @c .c files.
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You can see the full list of object files that will be built using
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@code
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make bobjs
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@endcode
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@subsection comp_custom Customising compilation
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The Makefile rules for a particular object can be customised to a
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certain extent by defining the Makefile variable CFLAGS_@<object@>.
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For example, if you were to set
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@code
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CFLAGS_rtl8139 = -DFOO
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@endcode
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then <tt>bin/rtl8139.o</tt> would be compiled with the additional
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flags <tt>-DFOO</tt>. To see the flags that will be used when
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compiling a particular object, you can use e.g.
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@code
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make bin/rtl8139.flags
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@endcode
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If you need more flexibility than the CFLAGS_@<object@> mechanism
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provides, then you can exclude source files from the automatic rule
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generation process by listing them in the Makefile variable
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#NON_AUTO_SRCS. The command
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@code
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make autosrcs
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@endcode
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will show you which files are currently part of the automatic rule
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generation process.
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@subsection comp_multiobj Multiple objects
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A single source file can be used to generate multiple object files.
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This is used, for example, to generate the decompressing and the
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non-decompressing prefixes from the same source files.
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By default, a single object will be built from each source file. To
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override the list of objects for a source file, you can define the
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Makefile variable OBJS_@<object@>. For example, the
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<tt>arch/i386/prefix/dskprefix.S</tt> source file is built into two
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objects, <tt>bin/dskprefix.o</tt> and <tt>zdskprefix.o</tt> by
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defining the Makefile variable
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@code
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OBJS_dskprefix = dskprefix zdskprefix
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@endcode
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Since there would be little point in building two identical objects,
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customised compilation flags (see @ref comp_custom) are defined as
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@code
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CFLAGS_zdskprefix = -DCOMPRESS
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@endcode
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Thus, <tt>arch/i386/prefix/dskprefix.S</tt> is built into @c
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dskprefix.o using the normal set of flags, and into @c zdskprefix.o
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using the normal set of flags plus <tt>-DCOMPRESS</tt>.
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@subsection comp_debug Special debugging targets
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In addition to the basic rules #RULE_c and #RULE_S for compiling
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source files into object files, there are various other rules that can
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be useful for debugging.
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@subsubsection comp_debug_c_to_c Preprocessed C
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You can see the results of preprocessing a @c .c file (including the
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per-object flags defined via CFLAGS_@<object@> if applicable) using
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e.g.
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@code
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make bin/rtl8139.c
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@endcode
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and examining the resulting file (<tt>bin/rtl8139.c</tt> in this
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case).
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@subsubsection comp_debug_x_to_s Assembler
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You can see the results of assembling a @c .c file, or of
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preprocessing a @c .S file, using e.g.
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@code
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make bin/rtl8139.s
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make bin/zdskprefix.s
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@endcode
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@subsubsection comp_debug_dbg Debugging-enabled targets
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You can build targets with debug messages (DBG()) enabled using e.g.
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@code
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make bin/rtl8139.dbg.o
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make bin/rtl8139.dbg2.o
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@endcode
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You will probably not need to use these targets directly, since a
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mechanism exists to select debugging levels at link-time; see @ref
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link_debug.
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@section linking Linking
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@subsection link_debug Debugging-enabled builds
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*/
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