1 files changed, 40 insertions, 0 deletions

diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt
index 1c0db652b366..7e77f93634ea 100644
--- a/Documentation/kbuild/modules.txt
+++ b/Documentation/kbuild/modules.txt
@@ -18,6 +18,7 @@ In this document you will find information about:
         === 5. Include files
            --- 5.1 How to include files from the kernel include dir
            --- 5.2 External modules using an include/ dir
+           --- 5.3 External modules using several directories
         === 6. Module installation
            --- 6.1 INSTALL_MOD_PATH
            --- 6.2 INSTALL_MOD_DIR
@@ -344,6 +345,45 @@ directory and therefore needs to deal with this in their kbuild file.
         Note that in the assignment there is no space between -I and the path.
         This is a kbuild limitation:  there must be no space present.
 
+--- 5.3 External modules using several directories
+
+        If an external module does not follow the usual kernel style but
+        decide to spread files over several directories then kbuild can
+        support this too.
+
+        Consider the following example:
+        
+        |
+        +- src/complex_main.c
+        |   +- hal/hardwareif.c
+        |   +- hal/include/hardwareif.h
+        +- include/complex.h
+        
+        To build a single module named complex.ko we then need the following
+        kbuild file:
+
+        Kbuild:
+                obj-m := complex.o
+                complex-y := src/complex_main.o
+                complex-y += src/hal/hardwareif.o
+
+                EXTRA_CFLAGS := -I$(src)/include
+                EXTRA_CFLAGS += -I$(src)src/hal/include
+
+
+        kbuild knows how to handle .o files located in another directory -
+        although this is NOT reccommended practice. The syntax is to specify
+        the directory relative to the directory where the Kbuild file is
+        located.
+
+        To find the .h files we have to explicitly tell kbuild where to look
+        for the .h files. When kbuild executes current directory is always
+        the root of the kernel tree (argument to -C) and therefore we have to
+        tell kbuild how to find the .h files using absolute paths.
+        $(src) will specify the absolute path to the directory where the
+        Kbuild file are located when being build as an external module.
+        Therefore -I$(src)/ is used to point out the directory of the Kbuild
+        file and any additional path are just appended.
 
 === 6. Module installation