bitbake: bitbake-user-manual: Added clarifying note to BBCLASSEXTEND variable.

Fixes [YOCTO #9909]

Added a note that talks about how the BBCLASSEXTEND mechanism
adds recipe variants.

(Bitbake rev: 185c9c5fc205fe9c9daf5238f11b92bd0954f5c6)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 21 insertions, 0 deletions

diff --git a/bitbake/doc/bitbake-user-manual/bitbake-user-manual-ref-variables.xml b/bitbake/doc/bitbake-user-manual/bitbake-user-manual-ref-variables.xml
index 4d06ff950c..e26422a421 100644
--- a/bitbake/doc/bitbake-user-manual/bitbake-user-manual-ref-variables.xml
+++ b/bitbake/doc/bitbake-user-manual/bitbake-user-manual-ref-variables.xml
@@ -986,6 +986,27 @@
      BBCLASSEXTEND =+ "native nativesdk"
      BBCLASSEXTEND =+ "multilib:<replaceable>multilib_name</replaceable>"
                     </literallayout>
+                    <note>
+                        <para>
+                        Internally, the <filename>BBCLASSEXTEND</filename>
+                        mechanism generates recipe variants by rewriting
+                        variable values and applying overrides such as
+                        <filename>_class-native</filename>.
+                        For example, to generate a native version of a recipe
+                        named "foo", a
+                        <link linkend='var-DEPENDS'><filename>DEPENDS</filename></link>
+                        on "foo" is rewritten to a <filename>DEPENDS</filename>
+                        on "foo-native".
+                        </para>
+
+                        <para>
+                        No reparsing, which adds some limitations, of the
+                        recipe is done for each variant.
+                        For example, it is not possible to "include" a
+                        different file depending on the variant, since
+                        "include's" are processed when the recipe is parsed.
+                        </para>
+                    </note>
                 </para>
              </glossdef>
         </glossentry>