ref-manual: variables: provide no-match example for COMPATIBLE_MACHINE

COMPATIBLE_MACHINE is used to forbid the use of a recipe or its packages
for a specific set of machines.

In some cases, it may make more sense to have the logic inverted and
have the recipe always forbidden except for hand-picked machines. Such
could be the case for pieces of software that only support some
architectures. In that scenario, it is sometimes a bit easier on the eye
and for maintenance to use the OVERRIDES mechanism but for that, a
default should be set.

COMPATIBLE_MACHINE:aarch64 = "^(aarch64)$"
COMPATIBLE_MACHINE:mips64 = "^(mips64)$"

wouldn't do much because if COMPATIBLE_MACHINE isn't set, the recipe is
assumed compatible and therefore, if no default is provided we enter
that case.

Hence, we need to add

COMPATIBLE_MACHINE = "^$"

as default so that it only matches the empty string, which isn't
possible for MACHINEOVERRIDES.

Cc: Quentin Schulz <foss+yocto@0leil.net>
(From yocto-docs rev: 051f868d1da1cbcd5726eb4c1e3f25946ce690e3)

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

1 files changed, 20 insertions, 1 deletions

diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index 0e11ab2f68..abd361bf63 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -1362,13 +1362,32 @@ system and gives an overview of their function and contents.
       speed since the build system skips parsing recipes not compatible
       with the current machine.
 
+      If one wants to have a recipe only available for some architectures
+      (here ``aarch64`` and ``mips64``), the following can be used::
+
+         COMPATIBLE_MACHINE = "^$"
+         COMPATIBLE_MACHINE:arch64 = "^(aarch64)$"
+         COMPATIBLE_MACHINE:mips64 = "^(mips64)$"
+
+      The first line means "match all machines whose :term:`MACHINEOVERRIDES`
+      contains the empty string", which will always be none.
+
+      The second is for matching all machines whose :term:`MACHINEOVERRIDES`
+      contains one override which is exactly ``aarch64``.
+
+      The third is for matching all machines whose :term:`MACHINEOVERRIDES`
+      contains one override which is exactly ``mips64``.
+
+      The same could be achieved with::
+
+         COMPATIBLE_MACHINE = "^(aarch64|mips64)$"
+
       .. note::
 
          When :term:`COMPATIBLE_MACHINE` is set in a recipe inherits from
          native, the recipe is always skipped. All native recipes must be
          entirely target independent and should not rely on :term:`MACHINE`.
 
-
    :term:`COMPLEMENTARY_GLOB`
       Defines wildcards to match when installing a list of complementary
       packages for all the packages explicitly (or implicitly) installed in