Radix cross Linux Build System

Index: PRINCIPLE.md
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+
+The fundamental principle of the build system
+=============================================
+
+Assume that we need to build the program or alienated package for working
+on the three devices with names **ci20**, **bt01** and **dm64**. The first two devices
+(**ci20**, **bt01**) are based on the **MIPS** architecture, and the third device (**dm64**)
+is built on **ARM**-based processor. Toolchains for building our program, for
+simplicity, let's call **mips** and **arm**, respectively.
+
+The build script of the source program is the same for each of our devices
+and is written on **GNU Make**.
+
+If we present all available combinations of command line calls, required for
+building the program for our devices, we get:
+
+```bash
+ $ TOOLCHAIN=mips HARDWARE=ci20 make
+ $ TOOLCHAIN=mips HARDWARE=bt01 make
+ $ TOOLCHAIN=arm  HARDWARE=dm64 make
+```
+
+or (in case when the **TOOLCHAIN-HARDWARE** pairs are transmitted as arguments):
+
+```bash
+ $ make TOOLCHAIN=mips HARDWARE=ci20
+ $ make TOOLCHAIN=mips HARDWARE=bt01
+ $ make TOOLCHAIN=arm  HARDWARE=dm64
+```
+
+Thus, the **build system** must receive a **TOOLCHAIN-HARDWARE** pair, and then the
+**build system** has to determine which toolchain must be used for a particular
+device.
+
+Let us now consider how to organize the sequence of command calls (on the
+**build system** level) in such way that the user can do these actions by
+applying only one call:
+
+```bash
+ $ make
+```
+
+without specifying additional arguments which are responsible for selection
+of the target device and applicable toolchain.
+
+If we describe the list of valid terget devices at the beginning of our script,
+for example, as follows:
+
+```make
+COMPONENT_TARGETS  = $(HARDWARE_CI20)
+COMPONENT_TARGETS += $(HARDWARE_BT01)
+COMPONENT_TARGETS += $(HARDWARE_DM64)
+```
+
+then the **build system** can automatically construct a list of possible
+**TOOLCHAIN-HARDWARE** combinations for a given build script, which will looks
+like following:
+
+```make
+  targets = target_mips_ci20 target_mips_bt01 target_arm_dm64
+```
+
+With such list, the **build system** can restore arguments which are needed for
+each of three our calls. It is very simple to do. On the **GNU Make** language we can
+do it as shown by following lines:
+
+```make
+target_%: TOOLCHAIN = $(shell echo $(word 2, $(subst _, , $@)))
+target_%: HARDWARE = $(shell echo $(word 3, $(subst _, , $@)))
+target_%:
+	$(MAKE) TOOLCHAIN=$(TOOLCHAIN) HARDWARE=$(HARDWARE)
+```
+
+Thus, if we call the **Make** utility without arguments then **TOOLCHAIN** and **HARDWARE**
+variables will be undefined. In this case the **build system** starts to collect the
+targets list. When the **targets** list will be complete the **build system** can do the
+call
+
+```make
+	$(MAKE) TOOLCHAIN=$(TOOLCHAIN) HARDWARE=$(HARDWARE)
+```
+
+with valid arguments.
+
+When (at the next call) the system will make sure that the **TOOLCHAIN** and
+**HARDWARE** variables are defined, the control of the build process will be passed
+to our build script without additional calculations.
+
+The described mechanism is directly derived from the **GNU Make** documentation.
+
+
+References
+----------
+1. <http://www.gnu.org/software/make/manual/>
+2. <https://radix.pro/build-system/>
+