Documentation: update kbuild loadable modules goals & examples

Update section 3.3 Loadable module goals - obj-m, from $(<module_name>-objs)
to $(<module_name>-y) for easier addition of conditional objects to the
module. The examples are also updated to reflect the current state of
each Makefile used.

Signed-off-by: matt mooney <mfm@muteddisk.com>
Reviewed-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/kbuild/makefiles.txt

@@ -187,34 +187,35 @@ more details, with real examples.
 	Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
 
 	If a kernel module is built from several source files, you specify
-	that you want to build a module in the same way as above.
+	that you want to build a module in the same way as above; however,
-
+	kbuild needs to know which object files you want to build your
-	Kbuild needs to know which the parts that you want to build your
+	module from, so you have to tell it by setting a $(<module_name>-y)
-	module from, so you have to tell it by setting an
+	variable.
-	$(<module_name>-objs) variable.
 
 	Example:
 		#drivers/isdn/i4l/Makefile
-		obj-$(CONFIG_ISDN) += isdn.o
+		obj-$(CONFIG_ISDN_I4L) += isdn.o
-		isdn-objs := isdn_net_lib.o isdn_v110.o isdn_common.o
+		isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o
 
 	In this example, the module name will be isdn.o. Kbuild will
-	compile the objects listed in $(isdn-objs) and then run
+	compile the objects listed in $(isdn-y) and then run
 	"$(LD) -r" on the list of these files to generate isdn.o.
 
-	Kbuild recognises objects used for composite objects by the suffix
+	Due to kbuild recognizing $(<module_name>-y) for composite objects,
-	-objs, and the suffix -y. This allows the Makefiles to use
+	you can use the value of a CONFIG_ symbol to optionally include an
-	the value of a CONFIG_ symbol to determine if an object is part
+	object file as part of a composite object.
-	of a composite object.
 
 	Example:
 		#fs/ext2/Makefile
-	        obj-$(CONFIG_EXT2_FS)        += ext2.o
+	        obj-$(CONFIG_EXT2_FS) += ext2.o
-		ext2-y                       := balloc.o bitmap.o
+		ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \
-	        ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
+			  namei.o super.o symlink.o
+	        ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \
+						xattr_trusted.o
 
-	In this example, xattr.o is only part of the composite object
+	In this example, xattr.o, xattr_user.o and xattr_trusted.o are only
-	ext2.o if $(CONFIG_EXT2_FS_XATTR) evaluates to 'y'.
+	part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR)
+	evaluates to 'y'.
 
 	Note: Of course, when you are building objects into the kernel,
 	the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,