1. kernel version:

2. Building kernel out of tree
   http://www.crashcourse.ca/wiki/index.php/Building_kernel_out_of_tree

   2 options:
    * constantly add the option O=${KERNEL_OUT} to every make command, or
    * set the environment variable of KBUILD_OUTPUT to ${KERNEL_OUT}

   $(MAKE) -j1 $(KERNEL_VERBOSE) O=$(KERNEL_OUT) ARCH=$(ARCH) KCONFIG_ALLCONFIG=$(KERNEL_OUT)/.merged.config alldefconfig
    * set KERNEL_VERBOSE: to "V=1" will show more compiling logs
    * O=${KERNEL_OUT}: set to generate the result files under $KERNEL_OUT instead of the kernel source tree
    * ARCH=$(ARCH): sepcify the target platform arch
    * KCONFIG_ALLCONFIG:  used to specify a config file which contains (usually a subset of all) preset config symbols.
                          These variable settings are still subject to normal dependency checks.
                          Enables to create "miniature" config (miniconfig) or custom config files containing just the config symbols that you are interested in.
                          Then the kernel config system generates the full .config file, including symbols of your miniconfig file.
      see Documentation/kbuild/kconfig.txt

    * alldefconfig: New config with all symbols set to default
                    Fills in any missing symbols with Kconfig default
      see ./scripts/kconfig/Makefile for other options like oldconfig, allnoconfig

3. when set kernel config value
   if not have "# CONFIG_AIO is not set" specified, then might have 'CONFIG_AIO=y' generated in .config
   if has "# CONFIG_AIO is not set" specified, then will "# CONFIG_AIO is not set" generated in .config

   in ./init/Kconfig, we have AIO config defined as following:
   config AIO
       bool "Enable AIO support" if EXPERT
       default y
       help
         This option enables POSIX asynchronous I/O which may by used
         by some high performance threaded applications. Disabling
         this option saves about 7k.

   in ./drivers/tty/Kconfig, config VT is defined as following:
   config VT
       bool "Virtual terminal" if EXPERT
       depends on !S390 && !UML
       select INPUT
       default y
       ---help---
         If you say Y here, you will get support for terminal devices with
         display and keyboard devices. These are called "virtual" because you
         can run several virtual terminals (also called virtual consoles) on
         one physical terminal. This is rather useful, for example one
         virtual terminal can collect system messages and warnings, another
         one can be used for a text-mode user session, and a third could run
         an X session, all in parallel. Switching between virtual terminals
         is done with certain key combinations, usually Alt-<function key>.

         The setterm command ("man setterm") can be used to change the
         properties (such as colors or beeping) of a virtual terminal. The
         man page console_codes(4) ("man console_codes") contains the special
         character sequences that can be used to change those properties
         directly. The fonts used on virtual terminals can be changed with
         the setfont ("man setfont") command and the key bindings are defined
         with the loadkeys ("man loadkeys") command.

         You need at least one virtual terminal device in order to make use
         of your keyboard and monitor. Therefore, only people configuring an
         embedded system would want to say N here in order to save some
         memory; the only way to log into such a system is then via a serial
         or network connection.

         If unsure, say Y, or else you won't be able to do much with your new
         shiny Linux system :-)
  when we have CONFIG_VT enabled, we will have following configs enabled by default:
  (Will not have these settings if we have "# CONFIG_VT is not set")
    CONFIG_VT=y
    CONFIG_CONSOLE_TRANSLATIONS=y
    CONFIG_VT_CONSOLE=y
    CONFIG_VT_CONSOLE_SLEEP=y
    CONFIG_HW_CONSOLE=y
    CONFIG_VT_HW_CONSOLE_BINDING=y

    #
    # Console display driver support
    #
    CONFIG_DUMMY_CONSOLE=y
    CONFIG_FRAMEBUFFER_CONSOLE=y
    CONFIG_FRAMEBUFFER_CONSOLE_DETECT_PRIMARY=y
    CONFIG_FRAMEBUFFER_CONSOLE_ROTATION=y

    # Speakup console speech
    #
    # CONFIG_SPEAKUP is not set

    CONFIG_FONT_SUPPORT=y
    # CONFIG_FONTS is not set
    CONFIG_FONT_8x8=y
    CONFIG_FONT_8x16=y

4. when CONFIG_MODULES not enabled, compiling module will cause problems like this:
  /SATA3/nougat/out/target/product/am57xevm/target/kbuild/pvrsrvkm.mod.c:8:1: error: variable '__this_module' has initializer but incomplete type
   __attribute__((section(".gnu.linkonce.this_module"))) = {
    ^
    /SATA3/nougat/out/target/product/am57xevm/target/kbuild/pvrsrvkm.mod.c:9:2: error: unknown field 'name' specified in initializer
      .name = KBUILD_MODNAME,
        ^
   For this problem, please check if CONFIG_MODULES is enabled first
5. when specify dirctory path for OUTPUT or anything variants, please remove the last "/",
   otherwise it would cause problem during compiling. Like could not found rules for compiling some ko files