Documentation: KUnit: add note about mrproper in start.rst

The "Getting Started" guide should be beginner-friendly, therefore
add a note about the requirement of a clean source tree when running
kunit_tool for the first time, and its related error.

Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

diff --git a/Documentation/dev-tools/kunit/start.rst b/Documentation/dev-tools/kunit/start.rst
index 2e31350a85e17595e1914b06ba3eb8bb7cd3dc9f..9beec7d6ac4b2daf360cfa79b7af69bede2424ae 100644 (file)
--- a/Documentation/dev-tools/kunit/start.rst
+++ b/Documentation/dev-tools/kunit/start.rst
@@ -19,6 +19,22 @@ can run kunit_tool:
 
        ./tools/testing/kunit/kunit.py run
 
+.. note ::
+       You may see the following error:
+       "The source tree is not clean, please run 'make ARCH=um mrproper'"
+
+       This happens because internally kunit.py specifies ``.kunit``
+       (default option) as the build directory in the command ``make O=output/dir``
+       through the argument ``--build_dir``.  Hence, before starting an
+       out-of-tree build, the source tree must be clean.
+
+       There is also the same caveat mentioned in the "Build directory for
+       the kernel" section of the :doc:`admin-guide </admin-guide/README>`,
+       that is, its use, it must be used for all invocations of ``make``.
+       The good news is that it can indeed be solved by running
+       ``make ARCH=um mrproper``, just be aware that this will delete the
+       current configuration and all generated files.
+
 If everything worked correctly, you should see the following:
 
 .. code-block::