doc: move Clang documentation to HTML

* Move README.clang to doc/build/clang.rst and reformat as
  reStructeredText.
* Indicate that -ffixed-r9 and -ffixed-x18 are used to reserve registers
  for gd.
* Minor editing.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

2 files changed, 77 insertions, 0 deletions

diff --git a/doc/build/clang.rst b/doc/build/clang.rst
new file mode 100644
index 00000000000..1d35616eb5e
--- /dev/null
+++ b/doc/build/clang.rst
@@ -0,0 +1,76 @@
+Building with Clang
+===================
+
+The biggest problem when trying to compile U-Boot with Clang is that almost all
+archs rely on storing gd in a global register and the Clang 3.5 user manual
+states: "Clang does not support global register variables; this is unlikely to
+be implemented soon because it requires additional LLVM backend support."
+
+The ARM backend can be instructed not to use the r9 and x18 registers using
+-ffixed-r9 or -ffixed-x18 respectively. As global registers themselves are not
+supported inline assembly is needed to get and set the r9 or x18 value. This
+leads to larger code then strictly necessary, but at least works.
+
+**NOTE:** target compilation only work for _some_ ARM boards at the moment.
+Also AArch64 is not supported currently due to a lack of private libgcc
+support. Boards which reassign gd in c will also fail to compile, but there is
+in no strict reason to do so in the ARM world, since crt0.S takes care of this.
+These assignments can be avoided by changing the init calls but this is not in
+mainline yet.
+
+
+Debian based
+------------
+
+Required packages can be installed via apt, e.g.
+
+.. code-block:: bash
+
+    sudo apt-get install clang
+
+Note that we still use binutils for some tools so we must continue to set
+CROSS_COMPILE. To compile U-Boot with Clang on Linux without IAS use e.g.
+
+.. code-block:: bash
+
+    make HOSTCC=clang rpi_2_defconfig
+    make HOSTCC=clang CROSS_COMPILE=arm-linux-gnueabi- \
+         CC="clang -target arm-linux-gnueabi" -j8
+
+It can also be used to compile sandbox:
+
+.. code-block:: bash
+
+    make HOSTCC=clang sandbox_defconfig
+    make HOSTCC=clang CC=clang -j8
+
+
+FreeBSD 11
+----------
+
+Since llvm 3.4 is currently in the base system, the integrated assembler as
+is incapable of building U-Boot. Therefore gas from devel/arm-gnueabi-binutils
+is used instead. It needs a symlink to be picked up correctly though:
+
+.. code-block:: bash
+
+    ln -s /usr/local/bin/arm-gnueabi-freebsd-as /usr/bin/arm-freebsd-eabi-as
+
+The following commands compile U-Boot using the Clang xdev toolchain.
+
+**NOTE:** CROSS_COMPILE and target differ on purpose!
+
+.. code-block:: bash
+
+    export CROSS_COMPILE=arm-gnueabi-freebsd-
+    gmake rpi_2_defconfig
+    gmake CC="clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd" -j8
+
+Given that U-Boot will default to gcc, above commands can be
+simplified with a simple wrapper script - saved as
+/usr/local/bin/arm-gnueabi-freebsd-gcc - listed below:
+
+.. code-block:: bash
+
+    #!/bin/sh
+    exec clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd "$@"
diff --git a/doc/build/index.rst b/doc/build/index.rst
index e4e34114af3..e0072afb5e8 100644
--- a/doc/build/index.rst
+++ b/doc/build/index.rst
@@ -6,4 +6,5 @@ Build U-Boot
 .. toctree::
    :maxdepth: 2
 
+   clang
    tools