[OE-core] why conditional assignment of DEPLOY_DIR_IMAGE in bitbake.conf?

[Robert P. J. Day]

On Tue, 22 Jul 2014, Christopher Larson wrote:

>
> On Tue, Jul 22, 2014 at 9:36 AM, Robert P. J. Day <rpjday at crashcourse.ca> wrote:
>         quite possibly another silly question, but i'm perusing poky's
>       version of bitbake.conf and i see this:
>
>       DEPLOY_DIR ?= "${TMPDIR}/deploy"
>       DEPLOY_DIR_TAR = "${DEPLOY_DIR}/tar"
>       DEPLOY_DIR_IPK = "${DEPLOY_DIR}/ipk"
>       DEPLOY_DIR_RPM = "${DEPLOY_DIR}/rpm"
>       DEPLOY_DIR_DEB = "${DEPLOY_DIR}/deb"
>       DEPLOY_DIR_IMAGE ?= "${DEPLOY_DIR}/images/${MACHINE}"
>       DEPLOY_DIR_TOOLS = "${DEPLOY_DIR}/tools"
>
>         now, what is the value of using "?=" to set DEPLOY_DIR_IMAGE, rather
>       than just "=". i know what "?=" represents, but i normally expect to
>       see it in a context where someone might have set it earlier to some
>       other value, but this is in bitbake.conf, before any of the "include"
>       or "require" directives to pull in any of the other .conf files. so in
>       the midst of all those other DEPLOY_DIR_* hard assignments, why is the
>       images directory a conditional install?
>
>
> s/install/define/

  um, quite so. :-P

> If there's a ?= done before any includes in bitbake.conf, either its
> position in the file has changed, or it's set that way to allow the
> user to add those variables to the env whitelist and set them in the
> environment, as the env is set before bitbake.conf is parsed.

  ah, gotcha. so the obvious question is, who decides which variables
merit this sort of behaviour and which don't? why two variables out of
the DEPLOY_DIR_* variables and not the rest? seems sort of arbitrary.

rday

-- 

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Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

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