[xen master] build: introduce documentation for xen Makefiles

[patchbot@xxxxxxx]

commit 16055cb50cbb9b0b9eaf9937696a5c1d1f6fd71f
Author:     Anthony PERARD <anthony.perard@xxxxxxxxxx>
AuthorDate: Fri Apr 24 15:00:37 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Apr 24 15:00:37 2020 +0200

    build: introduce documentation for xen Makefiles
    
    This start explainning the variables that can be used in the many
    Makefiles in xen/.
    
    Most of the document copies and modifies text from Linux v5.4 document
    linux.git/Documentation/kbuild/makefiles.rst. Modification are mostly
    to avoid mentioning kbuild. Thus I've added the SPDX tag which was
    only in index.rst in linux.git.
    
    Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 docs/misc/xen-makefiles/makefiles.rst | 87 +++++++++++++++++++++++++++++++++++
 xen/Rules.mk                          |  4 ++
 2 files changed, 91 insertions(+)

diff --git a/docs/misc/xen-makefiles/makefiles.rst 
b/docs/misc/xen-makefiles/makefiles.rst
new file mode 100644
index 0000000000..a86e3a612d
--- /dev/null
+++ b/docs/misc/xen-makefiles/makefiles.rst
@@ -0,0 +1,87 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+Xen Makefiles
+=============
+
+Documentation for the build system of Xen, found in xen.git/xen/.
+
+Makefile files
+==============
+
+Description of the syntax that can be used in most Makefiles named
+'Makefile'. ('xen/Makefile' isn't part of the description.)
+
+'Makefile's are consumed by 'Rules.mk' when building.
+
+Goal definitions
+----------------
+
+       Goal definitions are the main part (heart) of the Makefile.
+       These lines define the files to be built, any special compilation
+       options, and any subdirectories to be entered recursively.
+
+       The most simple makefile contains one line:
+
+       Example::
+
+               obj-y += foo.o
+
+       This tells the build system that there is one object in that
+       directory, named foo.o. foo.o will be built from foo.c or foo.S.
+
+       The following pattern is often used to have object selected
+       depending on the configuration:
+
+       Example::
+
+               obj-$(CONFIG_FOO) += foo.o
+
+       $(CONFIG_FOO) can evaluates to y.
+       If CONFIG_FOO is not y, then the file will not be compiled nor linked.
+
+Descending down in directories
+------------------------------
+
+       A Makefile is only responsible for building objects in its own
+       directory. Files in subdirectories should be taken care of by
+       Makefiles in these subdirs. The build system will automatically
+       invoke make recursively in subdirectories, provided you let it know of
+       them.
+
+       To do so, obj-y is used.
+       acpi lives in a separate directory, and the Makefile present in
+       drivers/ tells the build system to descend down using the following
+       assignment.
+
+       Example::
+
+               #drivers/Makefile
+               obj-$(CONFIG_ACPI) += acpi/
+
+       If CONFIG_ACPI is set to 'y'
+       the corresponding obj- variable will be set, and the build system
+       will descend down in the apci directory.
+       The build system only uses this information to decide that it needs
+       to visit the directory, it is the Makefile in the subdirectory that
+       specifies what is modular and what is built-in.
+
+       It is good practice to use a `CONFIG_` variable when assigning directory
+       names. This allows the build system to totally skip the directory if the
+       corresponding `CONFIG_` option is 'y'.
+
+Compilation flags
+-----------------
+
+    CFLAGS-y and AFLAGS-y
+       These two flags apply only to the makefile in which they
+       are assigned. They are used for all the normal cc, as and ld
+       invocations happening during a recursive build.
+
+       $(CFLAGS-y) is necessary because the top Makefile owns the
+       variable $(XEN_CFLAGS) and uses it for compilation flags for the
+       entire tree. And the variable $(CFLAGS) is modified by Config.mk
+       which evaluated in every subdirs.
+
+       CFLAGS-y specifies options for compiling with $(CC).
+       AFLAGS-y specifies assembler options.
diff --git a/xen/Rules.mk b/xen/Rules.mk
index 8e4b9e3a4a..9d1e1ebf5e 100644
--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -1,3 +1,7 @@
+#
+# See docs/misc/xen-makefiles/makefiles.rst on variables that can be used in
+# Makefile and are consumed by Rules.mk
+#
 
 -include $(BASEDIR)/include/config/auto.conf
 
--
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