Makefile: Support building with Clang and LLVM binutils

This is intended to mirror the Linux kernel. Building with CC=clang will
use Clang as the compiler but default to using the existing binutils.
Building with LLVM=1 will default to using Clang and LLVM binutils.

Whilst GCC will accept the -N linker option and forward it on to the
linker, Clang will not, and so in order to support both compilers we
must use -Wl, to forward it to the linker as is required for most other
linker options.

Note that there is currently a bug when using Clang as the compiler and
riscv64-linux-gnu-ld as the linker for FW_PIC=y. At first glance this
appears to be a bug in GNU binutils, but this could also be Clang or
OpenSBI at fault in some subtle way. Thus, for now, advise that this
combination be avoided.

Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Tested-by: Anup Patel <anup.patel@wdc.com>

1 files changed, 46 insertions, 2 deletions

diff --git a/README.md b/README.md
index 03c02fb..b296f00 100644
--- a/README.md
+++ b/README.md
@@ -96,8 +96,13 @@ Required Toolchain
 ------------------
 
 OpenSBI can be compiled natively or cross-compiled on a x86 host. For
-cross-compilation, you can build your own toolchain or just download
-a prebuilt one from the [Bootlin toolchain repository].
+cross-compilation, you can build your own toolchain, download a prebuilt one
+from the [Bootlin toolchain repository] or install a distribution-provided
+toolchain; if you opt to use LLVM/Clang, most distribution toolchains will
+support cross-compiling for RISC-V using the same toolchain as your native
+LLVM/Clang toolchain due to LLVM's ability to support multiple backends in the
+same binary, so is often an easy way to obtain a working cross-compilation
+toolchain.
 
 Please note that only a 64-bit version of the toolchain is available in
 the Bootlin toolchain repository for now.
@@ -202,6 +207,45 @@ export PLATFORM_RISCV_XLEN=32
 
 will generate 32-bit OpenSBI images. And vice vesa.
 
+Building with Clang/LLVM
+------------------------
+
+OpenSBI can also be built with Clang/LLVM. To build with just Clang but keep
+the default binutils (which will still use the *CROSS_COMPILE* prefix if
+defined), override the *CC* make variable with:
+```
+make CC=clang
+```
+
+To build with a full LLVM-based toolchain, not just Clang, enable the *LLVM*
+option with:
+```
+make LLVM=1
+```
+
+When using Clang, *CROSS_COMPILE* often does not need to be defined unless
+using GNU binutils with prefixed binary names. *PLATFORM_RISCV_XLEN* will be
+used to infer a default triple to pass to Clang, so if *PLATFORM_RISCV_XLEN*
+itself defaults to an undesired value then prefer setting that rather than the
+full triple via *CROSS_COMPILE*. If *CROSS_COMPILE* is nonetheless defined,
+rather than being used as a prefix for the executable name, it will instead be
+passed via the `--target` option with the trailing `-` removed, so must be a
+valid triple.
+
+These can also be mixed; for example using a GCC cross-compiler but LLVM
+binutils would be:
+```
+make CC=riscv64-unknown-elf-gcc LLVM=1
+```
+
+These variables must be passed for all the make invocations described in this
+document.
+
+NOTE: Using Clang with a `riscv*-linux-gnu` GNU binutils linker has been seen
+to produce broken binaries with missing relocations; it is therefore currently
+recommended that this combination be avoided or *FW_PIC=n* be used to disable
+building OpenSBI as a position-independent binary.
+
 Contributing to OpenSBI
 -----------------------