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Date:   Mon,  2 Aug 2021 11:39:10 -0700
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Subject: [PATCH v6 3/3] Documentation/llvm: update CROSS_COMPILE inferencing
From:   Nick Desaulniers <ndesaulniers@google.com>
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     Miguel Ojeda <ojeda@kernel.org>, Fangrui Song <maskray@google.com>,
        Michal Marek <michal.lkml@markovi.net>,
        Arnd Bergmann <arnd@kernel.org>, linux-kernel@vger.kernel.org,
        linux-kbuild@vger.kernel.org, clang-built-linux@googlegroups.com,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Christoph Hellwig <hch@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Nick Desaulniers <ndesaulniers@google.com>
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Precedence: bulk

As noted by Masahiro, document how we can generally infer CROSS_COMPILE
(and the more specific details about --target and --prefix) based on
ARCH.

Change use of env vars to command line parameters.

Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Fangrui Song <maskray@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
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.
 
+Omitting CROSS_COMPILE
+----------------------
+
+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
 -----------------------
 
-- 
2.32.0.554.ge1b32706d8-goog