techpack: add tech package support

Enable tech packages to have kernel drivers that are physically
located in the kernel source tree but not present in the kernel
repository. This is done via a manifest line item to fetch a
techpackage-module into $KERNEL/techpack/$techpackage-module.
$KERNEL/techpack/Kbuild will automatically detect any
subdirectories and link them to the kernel's kbuild system.

The resulting layout of techpack within kernel source would be as
follows :-

    kernel/
    ├── techpack/
        ├── Kbuild
        └── stub
        │    ├── Makefile
        │    ├── include
        │    │   └── uapi
        │    │       └── Kbuild
             └── stub.c
        ├── techpackage-module
            ├── Makefile
            ├── include
            │   └── uapi
            │       ├── Kbuild

$KERNEL/techpack only contains Kbuild (no Makefile) at the
toplevel and this Kbuild takes care of both compiling the
subdirectories and of exporting the needed header files therein.
The reason for having only Kbuild at the top is that Kbuild
and Makefile cannot exist together in same directory and Makefile
doesn't cater to uapi header installation.

stub is an empty techpackage-module which serves both as a sample
layout and satisfies the requirement of kernel build system, by
providing necessary buit-in.o, when no other techpackage-module has
been pulled under techpack/.

$KERNEL/techpack/techpackage-module should have a Makefile at the
top and Kbuild under $KERNEL/techpack/techpackage-module/include/uapi
directory. The uapi headers pertaining to a techpackage-module
should reside under $KERNEL/techpack/techpackage-module/include/uapi
and associated Kbuild should have necessary rules to export it.

Change-Id: I0d0ced38566907d2074831edde0934833f666eff
Signed-off-by: Bryan Huntsman <bryanh@codeaurora.org>
Signed-off-by: Imran Khan <kimran@codeaurora.org>
[satyap@codeaurora.org: fix trivial merge conflict and add SPDX-License-Identifier]
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>

Makefile

@@ -585,7 +585,7 @@ export KBUILD_MODULES KBUILD_BUILTIN
 ifeq ($(KBUILD_EXTMOD),)
 # Objects we will link into vmlinux / subdirs we need to visit
 init-y		:= init/
-drivers-y	:= drivers/ sound/ firmware/
+drivers-y	:= drivers/ sound/ firmware/ techpack/
 net-y		:= net/
 libs-y		:= lib/
 core-y		:= usr/
@@ -1238,6 +1238,7 @@ headers_install: __headers
 	  $(error Headers not exportable for the $(SRCARCH) architecture))
 	$(Q)$(MAKE) $(hdr-inst)=include/uapi dst=include
 	$(Q)$(MAKE) $(hdr-inst)=arch/$(SRCARCH)/include/uapi $(hdr-dst)
+	$(Q)$(MAKE) $(hdr-inst)=techpack
 
 PHONY += headers_check_all
 headers_check_all: headers_install_all
@@ -1247,6 +1248,7 @@ PHONY += headers_check
 headers_check: headers_install
 	$(Q)$(MAKE) $(hdr-inst)=include/uapi dst=include HDRCHECK=1
 	$(Q)$(MAKE) $(hdr-inst)=arch/$(SRCARCH)/include/uapi $(hdr-dst) HDRCHECK=1
+	$(Q)$(MAKE) $(hdr-inst)=techpack HDRCHECK=1
 
 # ---------------------------------------------------------------------------
 # Kernel selftest

techpack/.gitignore

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
+# ignore all subdirs except stub
+!/stub/

techpack/Kbuild

@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: GPL-2.0-only
+techpack-dirs := $(shell find $(srctree)/$(src) -maxdepth 1 -mindepth 1 -type d -not -name ".*")
+obj-y += stub/ $(addsuffix /,$(subst $(srctree)/$(src)/,,$(techpack-dirs)))
+
+techpack-header-dirs := $(shell find $(srctree)/techpack -maxdepth 1 -mindepth 1 -type d -not -name ".*")
+header-y += $(addsuffix /include/uapi/,$(subst $(srctree)/techpack/,,$(techpack-header-dirs)))

techpack/stub/Makefile

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
+ccflags-y := -Wno-unused-function
+obj-y := stub.o

techpack/stub/include/uapi/Kbuild

@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only WITH Linux-syscall-note
+#Stub place holder

techpack/stub/stub.c

@@ -0,0 +1,5 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+static void _techpack_stub(void)
+{
+}