頂層目錄下的config.mk文件主要完成如下功能的配置：
1、確定生成可執行文件過程中需要的各種工具，如編譯器(arm-linux-gcc)、連接器(arm-linux-ld)、反匯編器(arm-linux-objdump)等
2、確定CPU、板相關的配置文件，存在於各個目錄下的config.mk
3、確定編譯、鏈接、轉換等過程的操作選項

4、根據步驟3確定的編譯連接選項生成需要的文件

config.mk完整內容及必要註釋如下
註：config.mk文件註釋符改為/* 註釋內容 */

  1 ifneq ($(OBJTREE),$(SRCTREE))
  2     ifeq ($(CURDIR),$(SRCTREE))
 
  3         dir :=
  4     else
  5         dir := $(subst $(SRCTREE)/,,$(CURDIR))
  6     endif
  7 
  8     obj := $(if $(dir),$(OBJTREE)/$(dir)/,$(OBJTREE)/)
  9     src := $(if $(dir),$(SRCTREE)/$(dir)/,$(SRCTREE)/)
 10 
 11     $(shell mkdir -p $(obj))
 12 else
 13     obj :=
 14     src :=
 15 endif
 16
 
 /* obj = 空，src = 空
 17  * dir = 空
 18  */
 19  
 20 /* clean the slate ... */
 21 PLATFORM_RELFLAGS =
 22 PLATFORM_CPPFLAGS =
 23 PLATFORM_LDFLAGS =
 24 
 25 /* HOSTCFLAGS = -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
 26  * -Wall: 打印出編譯時所有的錯誤或警告信息
 27  * -Wstrict-prototypes: 編譯時，若產生與數據類型不相符的問題，打印出提示或警告信息。當在不同體系結構間移植時，加上該選項可避免很多錯誤
 
 28  * -O: 編譯代碼時的優化等級，共有五種：-O0、-O1、-O2、-O3和-Os
 29  * -fomit-frame-pointer: 對於不需要幀指針的函數,不要在寄存器中保存幀指針
 30  * 代碼優化時打開-fomit-frame-pointer，函數調用時不保存frame指針，也就不能用backtrace()來查看函數棧調用
 31  * backtrace()系列函數見[http://blog.csdn.net/u013686019/article/details/42128771](Linux中backtrace()系列函數的應用實例)
 32  */
 33 HOSTCFLAGS    = -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer  34           $(HOSTCPPFLAGS)
 35 /* HOSTSTRIP = strip 
 36  * strip能清除執行文件中不必要的標示符及調試信息，可減小文件大小而不影響正常使用，、
 37  * 與壓縮不同的是，文件一旦strip後就不能恢復原樣
 38  * strip後的文件不包含調試信息
 39  */
 40 HOSTSTRIP    = strip
 41 
 42 /*
 43  * Mac OS X / Darwin‘s C preprocessor is Apple specific.  It
 44  * generates numerous errors and warnings.  We want to bypass it
 45  * and use GNU C‘s cpp.  To do this we pass the -traditional-cpp
 46  * option to the compiler.  Note that the -traditional-cpp flag
 47  * DOES NOT have the same semantics as GNU C‘s flag, all it does
 48  * is invoke the GNU preprocessor in stock ANSI/ISO C fashion.
 49  *
 50  * Apple‘s linker is similar, thanks to the new 2 stage linking
 51  * multiple symbol definitions are treated as errors, hence the
 52  * -multiply_defined suppress option to turn off this error.
 53  */
 54 ifeq ($(HOSTOS),darwin)
 55     ......
 56 else
 57     HOSTCC        = gcc
 58 endif
 59 
 60 ifeq ($(HOSTOS),cygwin)
 61     ......
 62 endif
 63 
 64 /* We build some files with extra pedantic flags to try to minimize things
 65  * that won‘t build on some weird host compiler -- though there are lots of
 66  * exceptions for files that aren‘t complaint.
 67  */
 68 HOSTCFLAGS_NOPED = $(filter-out -pedantic,$(HOSTCFLAGS))
 69 /* -pedantic: 當GCC在編譯不符合ANSI/ISO C語言標準的源代碼時，如果在編譯指令中加上了-pedantic選項
 70  * 那麽源程序中使用了擴展語法的地方將產生相應的警告信息
 71  */
 72 HOSTCFLAGS    += -pedantic
 73 
 74 #########################################################################
 75 /* Option checker (courtesy linux kernel) to ensure
 76  * only supported compiler options are used
 77  * cc-option變量保存了一個測試編譯選項的命令，其他地方會經常用call函數來調用它，測試編譯選項
 78  * if $(CC) $(CFLAGS) $(1) -S -o /dev/null -xc /dev/null > /dev/null 2>&1;then
 79         echo "$(1)";
 80    else
 81         echo "$(2)";
 82    fi;
 83  * -S：編譯後立即結束，不進行<a href="http://www.it165.net/pro/yysam/" target="_blank" class="keylink">匯編</a>等操作
 84  * -o /dev/null : 生成文件到/dev/null，即不生成任何編譯結果,要編譯的文件也為空
 85  * -xc: 指定按c語言編譯
 86  * 用此語句如：call cc-option,-a,-b 則如果支持-a選項則返回-a否則返回-b
 87  */
 88 cc-option = $(shell if $(CC) $(CFLAGS) $(1) -S -o /dev/null -xc /dev/null  89         > /dev/null 2>&1; then echo "$(1)"; else echo "$(2)"; fi ;)
 90 
 91 /* Include the make variables (CC, etc...) */
 92 AS    = $(CROSS_COMPILE)as    /* 匯編工具 */
 93 LD    = $(CROSS_COMPILE)ld    /* 鏈接工具 */
 94 CC    = $(CROSS_COMPILE)gcc    /* 編譯工具 */
 95 CPP    = $(CC) -E                /* 預處理   */
 96 AR    = $(CROSS_COMPILE)ar    /* 歸檔工具 */
 97 NM    = $(CROSS_COMPILE)nm    /* 列出object文件中的符號 */
 98 LDR    = $(CROSS_COMPILE)ldr
 99 STRIP    = $(CROSS_COMPILE)strip
100 OBJCOPY = $(CROSS_COMPILE)objcopy /* 轉換可執行文件格式工具 */
101 OBJDUMP = $(CROSS_COMPILE)objdump /* 反匯編工具 */
102 RANLIB    = $(CROSS_COMPILE)RANLIB  /* 產生歸檔文件索引 */
103 
104 #########################################################################
105 /* Load generated board configuration */
106 /* sinclude:
107  * 在Makefile中可使用"sinclude"代替"include"，用來忽略由於包含文件不存在或者無法創建時的錯誤
108  */
109 sinclude $(OBJTREE)/include/autoconf.mk
110 
111 /* Some architecture config.mk files need to know what CPUDIR is set to,
112  * so calculate CPUDIR before including ARCH/SOC/CPU config.mk files.
113  * Check if arch/$ARCH/cpu/$CPU exists, otherwise assume arch/$ARCH/cpu contains
114  * CPU-specific code.
115  */
116 CPUDIR=arch/$(ARCH)/cpu/$(CPU)
117 ifneq ($(SRCTREE)/$(CPUDIR),$(wildcard $(SRCTREE)/$(CPUDIR)))
118 CPUDIR=arch/$(ARCH)/cpu
119 endif
120 /* CPUDIR=arch/arm/cpu/arm920t */
121 
122 /* include architecture dependend rules: arch/arm/config.mk */
123 sinclude $(TOPDIR)/arch/$(ARCH)/config.mk
124 /* include  CPU    specific rules: arch/arm/cpu/arm920t/config.mk */
125 sinclude $(TOPDIR)/$(CPUDIR)/config.mk
126 ifdef    SOC
127     /* include  SoC    specific rules: arch/arm/cpu/arm920t/s3c24x0/config.mk */
128     sinclude $(TOPDIR)/$(CPUDIR)/$(SOC)/config.mk
129 endif
130 ifdef    VENDOR
131     BOARDDIR = $(VENDOR)/$(BOARD)
132 else
133     BOARDDIR = $(BOARD)
134 endif
135 /* BOARDDIR = samsung/smdk2410 */
136 
137 ifdef    BOARD
138     /* include board specific rules: board/samsung/smdk2410/config.mk */
139     sinclude $(TOPDIR)/board/$(BOARDDIR)/config.mk
140 endif
141 
142 #########################################################################
143 ifneq (,$(findstring s,$(MAKEFLAGS)))
144 ARFLAGS = cr
145 else
146 ARFLAGS = crv
147 endif
148 RELFLAGS= $(PLATFORM_RELFLAGS)
149 DBGFLAGS= -g # -DDEBUG
150 OPTFLAGS= -Os #-fomit-frame-pointer
151 
152 /* LDSCRIPT = arch/arm/cpu/arm920t/u-boot.lds，在在文件arch/arm/config.mk中賦值 */
153 ifndef LDSCRIPT
154     #LDSCRIPT := $(TOPDIR)/board/$(BOARDDIR)/u-boot.lds.debug
155     ifeq ($(CONFIG_NAND_U_BOOT),y)
156         LDSCRIPT := $(TOPDIR)/board/$(BOARDDIR)/u-boot-nand.lds
157     else
158         LDSCRIPT := $(TOPDIR)/board/$(BOARDDIR)/u-boot.lds
159     endif
160 endif
161 /* 段之間的空隙用0xff填充 */
162 OBJCFLAGS += --gap-fill=0xff
163 
164 gccincdir := $(shell $(CC) -print-file-name=include)
165 
166 /* CPPFLAGS變量綜合了DBGFLAGS，OPTFLAGS，RELFLAGS編譯選項，並定義了__KERBEL__
167  * -D: 設置宏定義__KERNEL__
168  */
169 CPPFLAGS := $(DBGFLAGS) $(OPTFLAGS) $(RELFLAGS)        170     -D__KERNEL__
171 ifneq ($(TEXT_BASE),)
172 CPPFLAGS += -DTEXT_BASE=$(TEXT_BASE)
173 endif
174 
175 ifneq ($(RESET_VECTOR_ADDRESS),)
176 CPPFLAGS += -DRESET_VECTOR_ADDRESS=$(RESET_VECTOR_ADDRESS)
177 endif
178 
179 ifneq ($(OBJTREE),$(SRCTREE))
180 CPPFLAGS += -I$(OBJTREE)/include2 -I$(OBJTREE)/include
181 endif
182 
183 CPPFLAGS += -I$(TOPDIR)/include
184 CPPFLAGS += -fno-builtin -ffreestanding -nostdinc    185     -isystem $(gccincdir) -pipe $(PLATFORM_CPPFLAGS)
186 
187 ifdef BUILD_TAG
188 CFLAGS := $(CPPFLAGS) -Wall -Wstrict-prototypes 189     -DBUILD_TAG=‘"$(BUILD_TAG)"‘
190 else
191 CFLAGS := $(CPPFLAGS) -Wall -Wstrict-prototypes
192 endif
193 
194 CFLAGS += $(call cc-option,-fno-stack-protector)
195 
196 /* $(CPPFLAGS) sets -g, which causes gcc to pass a suitable -g<format> */
197 /* option to the assembler. */
198 AFLAGS_DEBUG :=
199 
200 AFLAGS := $(AFLAGS_DEBUG) -D__ASSEMBLY__ $(CPPFLAGS)
201 
202 LDFLAGS += -Bstatic -T $(obj)u-boot.lds $(PLATFORM_LDFLAGS)
203 ifneq ($(TEXT_BASE),)
204 LDFLAGS += -Ttext $(TEXT_BASE)
205 endif
206 /* LDFLAGS = -Bstatic -T u-boot.lds -Ttext 0x33F80000 */
207 /* CFLAGS = -g  -Os   -fno-common -ffixed-r8 -msoft-float  -D__KERNEL__ -DTEXT_BASE=0x33F80000 -I/u-boot-2010.06/include -fno-builtin -ffreestanding -nostdinc -isystem /usr/local/arm/4.2.2-eabi/usr/bin-ccache/../lib/gcc/arm-unknown-linux-gnueabi/4.2.2/include -pipe  -DCONFIG_ARM -D__ARM__ -marm  -mabi=aapcs-linux -mno-thumb-interwork -march=armv4 -Wall -Wstrict-prototypes -fno-stack-protector */
208 
209 /* Location of a usable BFD library, where we define "usable" as
210  * "built for ${HOST}, supports ${TARGET}".  Sensible values are
211  * - When cross-compiling: the root of the cross-environment
212  * - Linux/ppc (native): /usr
213  * - NetBSD/ppc (native): you lose ... (must extract these from the
214  *   binutils build directory, plus the native and U-Boot include
215  *   files don‘t like each other)
216  *
217  * So far, this is used only by tools/gdb/Makefile.
218  */
219 ifeq ($(HOSTOS),darwin)
220     BFD_ROOT_DIR =        /usr/local/tools
221 else
222     ifeq ($(HOSTARCH),$(ARCH))
223         /* native */
224         BFD_ROOT_DIR =        /usr
225     else
226         /* BFD_ROOT_DIR =        /LinuxPPC/CDK        # Linux/i386 */
227         /* BFD_ROOT_DIR =        /usr/pkg/cross        # NetBSD/i386 */
228         BFD_ROOT_DIR =        /opt/powerpc
229     endif
230 endif
231 
232 #########################################################################
233 export    HOSTCC HOSTCFLAGS HO<a href="http://www.it165.net/pro/" target="_blank" class="keylink">STL</a>DFLAGS PEDCFLAGS HOSTSTRIP CROSS_COMPILE 234     AS LD CC CPP AR NM STRIP OBJCOPY OBJDUMP MAKE
235 export    TEXT_BASE PLATFORM_CPPFLAGS PLATFORM_RELFLAGS CPPFLAGS CFLAGS AFLAGS
236 
237 #########################################################################
238 /* 下面幾行規定了各種文件的編譯時用到的編譯選項 */
239 /* Allow boards to use custom optimize flags on a per dir/file basis */
240 BCURDIR = $(subst $(SRCTREE)/,,$(CURDIR:$(obj)%=%))
241 /* BCURDIR = 頂層目錄 */
242 $(obj)%.s:    %.S
243     $(CPP) $(AFLAGS) $(AFLAGS_$(BCURDIR)/$(@F)) $(AFLAGS_$(BCURDIR)) 244         -o [email protected] $<
245 $(obj)%.o:    %.S
246     $(CC)  $(AFLAGS) $(AFLAGS_$(BCURDIR)/$(@F)) $(AFLAGS_$(BCURDIR)) 247         -o [email protected] $< -c
248 $(obj)%.o:    %.c
249     $(CC)  $(CFLAGS) $(CFLAGS_$(BCURDIR)/$(@F)) $(CFLAGS_$(BCURDIR)) 250         -o [email protected] $< -c
251 $(obj)%.i:    %.c
252     $(CPP) $(CFLAGS) $(CFLAGS_$(BCURDIR)/$(@F)) $(CFLAGS_$(BCURDIR)) 253         -o [email protected] $< -c
254 $(obj)%.s:    %.c
255     $(CC)  $(CFLAGS) $(CFLAGS_$(BCURDIR)/$(@F)) $(CFLAGS_$(BCURDIR)) 256     -o [email protected] $< -c -S

頂層目錄下的config.mk文件分析