meta-arm-toolchain: Add README

Document usage details for GNU Arm toolchains which could either be built
from source or pre-built.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm-toolchain/README.md

@@ -0,0 +1,113 @@
+meta-arm-toolchain Yocto Layer
+==============================
+
+This layer contains recipes for GNU Arm toolchains which could either be built
+from source or pre-built toolchain binaries.
+
+Information regarding contributing, reporting bugs, etc can be found in the
+top-level meta-arm readme file.
+
+Source Arm toolchain for Linux development
+------------------------------------------
+
+Recipes for GNU Arm toolchain built from source are provided under
+``recipes-devtools/gcc/``. In order to use Arm toolchain instead of OE core
+toolchain, one just needs to override ``GCCVERSION`` in corresponding distro
+conf file.
+
+-  Eg. to use GNU Arm toolchain version ``9.2``
+   GCCVERSION = "arm-9.2"
+
+Pre-built Arm toolchain for Linux development
+---------------------------------------------
+
+Recipes for pre-built GNU Arm toolchain for Linux development are provided under
+``recipes-devtools/external-arm-toolchain/``.
+
+external-arm-toolchain.bb
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This recipe provides support for pre-built GNU toolchains targeting processors
+from the Arm Cortex-A family and implementing the Arm A-profile architecture.
+
+Usage
+^^^^^
+
+In order to use any of pre-built Arm toolchain versions (8.2, 8.3, 9.2 and so
+on), a user needs to download and untar tool-set on host machine at a particular
+installation path eg: ``/opt/toolchain/``. Then user needs to specify following
+in ``conf/local.conf`` in order to replace OE toolchain with pre-built GNU-A
+toolchain:
+
+TCMODE = "external-arm"
+EXTERNAL_TOOLCHAIN = "<path-to-the-toolchain>"
+
+-  Eg. for AArch64 (eg. qemuarm64 machine in poky distro)
+   EXTERNAL_TOOLCHAIN = "\
+     <installation-path>/gcc-arm-9.2-2019.12-x86_64-aarch64-none-linux-gnu \
+   "
+
+-  Eg. for AArch32 (eg. qemuarm machine in poky distro)
+   EXTERNAL_TOOLCHAIN = "\
+     <installation-path>/gcc-arm-9.2-2019.12-x86_64-arm-none-linux-gnueabihf \
+   "
+
+Supported distros and machines
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Since this pre-built GNU-A tool-set simply replaces OE toolchain, so it is
+meant to be distro and machine agnostic as long as one is cross-compiling for
+Arm A-profile architecture.
+
+Tested distro and machines (for zeus stable release):
+1.  Distro: poky; machines: qemuarm and qemuarm64 (build and boot tested)
+2.  Distro: RPB; machines: dragonboard-410c (build and boot tested)
+3.  Distro: world; machines: qemuarm and qemuarm64. Build tested for following
+    layers:
+    - poky/meta
+    - poky/meta-poky
+    - poky/meta-yocto-bsp
+    - meta-openembedded/meta-oe
+    - meta-openembedded/meta-python
+    - meta-openembedded/meta-networking
+
+SDK support
+^^^^^^^^^^^
+
+Pre-built toochain provides support to build OE SDK which has been tested using
+following commands:
+
+$ bitbake core-image-base -c populate_sdk
+$ bitbake core-image-base -c testsdk
+
+Note: Currently generated SDK only uses glibc provided by pre-built toolchain.
+      The cross compiler, binutils, gdb/gdbserver etc. are built from source.
+      This is something we would like to improve in future in order to package
+      most of the components from pre-built toolchain instead.
+
+Pre-built Arm toolchain for bare-metal development
+--------------------------------------------------
+
+Recipes for pre-built GNU Arm toolchain for bare-metal development are provided
+under ``recipes-devtools/external-arm-toolchain/``.
+
+gcc-arm-none-eabi_<version>.bb
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This recipe provides support for pre-built GNU Arm Embedded toolchain for
+bare-metal software development on devices based on 32-bit Arm Cortex-A,
+Cortex-R and Cortex-M processors.
+
+Supported version: 9-2019-q4-major
+
+gcc-aarch64-none-elf_<version>.bb
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This recipe provides support for pre-built GNU Arm toolchain for bare-metal
+software development on devices based on 64-bit Arm Cortex-A processors.
+
+Supported version: 9.2-2019.12
+
+Layer maintainer(s)
+-------------------
+* Sumit Garg <sumit.garg@linaro.org>