[yocto] Extending images

[Christopher Larson]

On Fri, Jul 18, 2014 at 9:54 AM, Gary Thomas <gary at mlbassoc.com> wrote:

> Most likely the image defined its own IMAGE_INSTALL using ?=, so defining
>> it yourself in the configuration data overrode its default definition,
>> since ?= is "set only if unset". If
>> the recipe didn't use ?=, then your IMAGE_INSTALL += would have had no
>> effect at all. To append to IMAGE_INSTALL directly without using
>> CORE_IMAGE_EXTRA_INSTALL you could have used
>> IMAGE_INSTALL_append, since that's a postponed operation that happens at
>> the end of the recipe parsing.
>>
>
> That makes sense, thanks.  I'll stick to using CORE_IMAGE_EXTRA_INSTALL
> from now on!


That's best, indeed. This is a case where there's a real difference between
variables intended for the user to override from configuration, and those
which are not. IMAGE_INSTALL is a detail of how images are constructed,
CORE_IMAGE_EXTRA_INSTALL is an explicit hook provided to the user to
non-destructively add packages to an image, so we don't have to try to
guess at what the recipe will do with it -- it has explicit semantics, so
we know this. There might be value in differentiating between cases like
this in the documentation, if we don't already.
-- 
Christopher Larson
clarson at kergoth dot com
Founder - BitBake, OpenEmbedded, OpenZaurus
Maintainer - Tslib
Senior Software Engineer, Mentor Graphics
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