Openwrt的makefile

Question

I'm new with OpenWrt and I'm trying to cross compile a new kernel. To have a clue, I checked one file in /source/package/kernel/linux/modules called video.mk

I can see there are some variables, like $(VIDEO_MENU) which I can clearly see it was defined, but what is $(LINUX_DIR)? I though it was a env variable, but when I go to the terminal and enter echo $LINUX_DIR I get nothing

#
# Copyright (C) 2009 David Cooper <dave@kupesoft.com>
# Copyright (C) 2006-2010 OpenWrt.org
#
# This is free software, licensed under the GNU General Public License v2.
# See /LICENSE for more information.
#

VIDEO_MENU:=Video Support

V4L2_DIR=v4l2-core
V4L2_USB_DIR=usb

#
# Video Display
#

define KernelPackage/backlight
        SUBMENU:=$(VIDEO_MENU)
        TITLE:=Backlight support
        DEPENDS:=@DISPLAY_SUPPORT
        HIDDEN:=1
        KCONFIG:=CONFIG_BACKLIGHT_CLASS_DEVICE \
                CONFIG_BACKLIGHT_LCD_SUPPORT=y \
                CONFIG_LCD_CLASS_DEVICE=n \
                CONFIG_BACKLIGHT_GENERIC=n \
                CONFIG_BACKLIGHT_ADP8860=n \
                CONFIG_BACKLIGHT_ADP8870=n \
                CONFIG_BACKLIGHT_OT200=n \
                CONFIG_BACKLIGHT_PM8941_WLED=n
        FILES:=$(LINUX_DIR)/drivers/video/backlight/backlight.ko
        AUTOLOAD:=$(call AutoProbe,video backlight)
endef

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Answer 1

LINUX_DIR and kernel related variable and generic build functions are written inside kernel.mk, which is part of source/include directory.

If you want to print the value of any of these variable, you can use Makefile log functionality as described in GNU make