Re: [PATCH v6 3/3] Documentation/llvm: update CROSS_COMPILE inferencing

From: Nathan Chancellor
Date: Mon Aug 02 2021 - 15:16:25 EST

On 8/2/2021 11:39 AM, 'Nick Desaulniers' via Clang Built Linux wrote:
As noted by Masahiro, document how we can generally infer CROSS_COMPILE
(and the more specific details about --target and --prefix) based on

Change use of env vars to command line parameters.

Suggested-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

Thanks for the update, I think this is much easier for non-CBL folks to understand.

Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>

Changes v5 -> v6:
* Pick up Fangrui's RB tag.
* Change use of env vars to command line parameters for consistency.

Documentation/kbuild/llvm.rst | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
index b18401d2ba82..f8a360958f4c 100644
--- a/Documentation/kbuild/llvm.rst
+++ b/Documentation/kbuild/llvm.rst
@@ -38,7 +38,7 @@ Cross Compiling
A single Clang compiler binary will typically contain all supported backends,
which can help simplify cross compiling. ::
- ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
+ make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu-
``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
@@ -63,6 +63,23 @@ They can be enabled individually. The full list of the parameters: ::
Currently, the integrated assembler is disabled by default. You can pass
``LLVM_IAS=1`` to enable it.
+As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
+Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive
+``--prefix=<path>`` to search for the GNU assembler and linker.
+If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
+from ``ARCH``.
+That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
+For example, to cross-compile the arm64 kernel::
+ make ARCH=arm64 LLVM=1 LLVM_IAS=1
Supported Architectures